Question 1

In thermochemistry, during which type of change is energy absorbed from the surroundings by the system?

Accepted Answer

Introduction / Context: In thermochemistry and physical chemistry, chemical and physical changes are classified based on whether they release energy to the surroundings or absorb energy from the surroundings. These ideas are crucial for understanding reaction energetics, enthalpy changes, and everyday phenomena such as melting ice or burning fuel. This question asks you to identify the type of change in which energy is absorbed by the system from its surroundings. Given Data / Assumptions: The key terms are exothermic and endothermic changes. Exothermic processes release energy, often as heat, to the surroundings. Endothermic processes absorb energy, usually as heat, from the surroundings. We assume the usual convention where the system is the reacting or changing material. Concept / Approach: An exothermic change is one in which the system loses energy to the surroundings; examples include combustion reactions and many neutralization reactions. An endothermic change is one in which the system gains energy from the surroundings; examples include melting of ice, evaporation of water, and some decomposition reactions. The question specifically asks about energy being absorbed from the surroundings by the system, which matches the definition of an endothermic change. Therefore, the correct answer is endothermic change. Step-by-Step Solution: Step 1: Recall that in an exothermic process, heat flows from the system to the surroundings, making the surroundings warmer. Step 2: Recall that in an endothermic process, heat flows from the surroundings into the system, making the surroundings cooler. Step 3: Interpret the phrase “energy is absorbed from the surroundings by the system” as describing an endothermic situation. Step 4: Compare this interpretation with the definitions and confirm that exothermic processes do the opposite, releasing instead of absorbing energy. Step 5: Select “endothermic change” as the correct answer. Verification / Alternative check: Consider familiar examples: when ice melts, it feels cold to the touch because it absorbs heat from your hand and the environment; this is an endothermic process. When water boils, heat is absorbed from the stove or heater by the water, again indicating an endothermic change. On the other hand, when a candle burns, heat is released to the surroundings and the flame feels warm, showing an exothermic process. These real life examples reinforce the idea that the process which absorbs energy from the surroundings is endothermic, matching the option given in the question. Why Other Options Are Wrong: Exothermic changes release energy to the surroundings, so they are the opposite of what the question describes. The option claiming that both exothermic and endothermic changes absorb energy is incorrect because only endothermic processes absorb net energy from the surroundings, while exothermic processes release net energy. The suggestion that neither type of change absorbs energy ignores the definition of endothermic changes and is therefore incorrect. Only the endothermic option accurately matches the description provided in the question. Common Pitfalls: A common mistake is to confuse the terms exothermic and endothermic because they sound similar or to remember them based only on whether temperature seems to increase or decrease. Another pitfall is to forget that the key is the direction of energy transfer relative to the system, not just whether the surroundings feel hot or cold. To avoid confusion, remember the simple associations: “exo” means out (energy leaving the system), and “endo” means in (energy entering the system). This helps you quickly determine which kind of process is being described in exam questions. Final Answer: Energy is absorbed from the surroundings by the system during an endothermic change.

Question 2

For a gaseous carbon atom undergoing successive ionizations, which of the following ionization steps requires the greatest amount of ionization energy?

Accepted Answer

Introduction / Context: Ionization energy is the energy required to remove an electron from a gaseous atom or ion. For a given element, successive ionization energies increase as more electrons are removed, because the remaining electrons experience a stronger effective nuclear attraction. This question focuses on a carbon atom and asks which successive ionization step requires the most energy, that is, which step has the highest ionization energy among the options listed. Given Data / Assumptions: We consider carbon in the gaseous state undergoing successive ionizations. The steps are: C (g) -> C+ (g) + e-, C+ (g) -> C2+ (g) + e-, C2+ (g) -> C3+ (g) + e-, and C3+ (g) -> C4+ (g) + e-. Each step removes one more electron from an increasingly positively charged ion. We assume no major electronic shell changes occur that would reduce ionization energy as we proceed. Concept / Approach: For any element, the first ionization energy (removal of the first electron from a neutral atom) is lower than subsequent ionization energies, because after one electron is removed, the resulting cation holds the remaining electrons more tightly. As the charge on the ion increases (C+, C2+, C3+), the electrostatic attraction between the positively charged nucleus and the remaining electrons grows stronger, making it progressively more difficult to remove another electron. Therefore, the fourth ionization (removing an electron from C3+ to form C4+) requires the greatest amount of energy among the given steps, because it is removing an electron from the most positively charged ion listed. Step-by-Step Solution: Step 1: Identify that C (g) -> C+ (g) + e- is the first ionization, removing an electron from a neutral atom. Step 2: Recognise that C+ (g) -> C2+ (g) + e- is the second ionization, starting from a positively charged ion, which is already more difficult than the first. Step 3: Note that C2+ (g) -> C3+ (g) + e- is the third ionization, requiring even more energy due to the higher positive charge on the starting ion. Step 4: Observe that C3+ (g) -> C4+ (g) + e- is the fourth ionization, involving removal of an electron from a triply charged cation, where electrons are most strongly attracted to the nucleus. Step 5: Conclude that the fourth ionization step, C3+ (g) -> C4+ (g) + e-, has the highest ionization energy among the options and select it as the correct answer. Verification / Alternative check: Successive ionization energy data for atoms with a few electrons show a clear pattern: each additional electron removed requires more energy than the previous one, with occasional large jumps when a new electron shell is emptied. For carbon, as electrons are removed, the effective nuclear charge experienced by the remaining electrons increases, reducing shielding and making further removal more difficult. While the exact numerical values are not needed here, the trend of increasing ionization energy with increasing positive charge on the ion supports the conclusion that the fourth ionization is the largest among the four listed steps. Why Other Options Are Wrong: The first ionization step, C (g) -> C+ (g) + e-, starts from a neutral atom and, by definition, has the lowest ionization energy of the series. The second ionization, C+ (g) -> C2+ (g) + e-, has a higher energy requirement than the first but less than the third and fourth. The third ionization, C2+ (g) -> C3+ (g) + e-, is higher still but still does not exceed the difficulty of removing an electron from C3+, which is more strongly bound. Thus, each step requires more energy than the previous, and none of the earlier steps can surpass the fourth ionization in energy requirement. Common Pitfalls: Some students may mistakenly think that the first ionization energy is always the largest because it starts from the neutral atom or may confuse the trend with electron affinity trends. Another pitfall is not recognising that as positive charge builds up on the ion, the electrostatic attraction to the remaining electrons becomes stronger, so it becomes progressively harder, not easier, to remove electrons. To avoid these errors, remember that successive ionization energies generally increase with each electron removed, so the later ionization steps typically require more energy than earlier ones, unless a new shell is being accessed in a special way. Final Answer: The ionization process that requires the greatest amount of energy is C3+ (g) -> C4+ (g) + e-, the fourth ionization step of carbon.

Question 3

In solution chemistry, a concentrated solution is best described as one that contains what relative amounts of solute and solvent?

Accepted Answer

Introduction / Context: In basic chemistry, terms like dilute and concentrated are used to describe how much solute is present in a solution compared with the amount of solvent. These terms are qualitative but important for understanding everyday examples such as concentrated fruit juices, strong acids, or weak salt solutions. This question asks you to identify the correct description of a concentrated solution in terms of the relative amounts of solute and solvent. Given Data / Assumptions: A solution consists of a solute (the substance being dissolved) and a solvent (the substance doing the dissolving, usually present in larger amount). Concentrated and dilute are comparative, not absolute, terms. The options describe different possible combinations of solute and solvent amounts, including extremes like no solute or no solvent. We assume normal usage of these terms in school level chemistry. Concept / Approach: A concentrated solution contains a relatively large amount of solute dissolved in a given amount of solvent, while a dilute solution contains a relatively small amount of solute in the same amount of solvent. It is not necessary to specify exact numerical values; the key is the higher proportion of solute relative to solvent. A solution cannot exist without both solute and solvent present; if there is no solvent, the system is not a solution but just pure solute, and if there is no solute, the system is just pure solvent. Step-by-Step Solution: Step 1: Recall the definition of a solution as a homogeneous mixture of solute and solvent. Step 2: Understand that a concentrated solution has more solute per unit volume or per unit mass of solvent compared with a dilute solution. Step 3: Eliminate the option "no solute at all" because a solution without solute is simply pure solvent and not concentrated. Step 4: Eliminate the option "no solvent at all" because there must be a solvent present for a solution to exist. Step 5: Recognise that "a large amount of solute dissolved in a given amount of solvent" correctly describes a concentrated solution and select that option. Verification / Alternative check: Everyday examples support this interpretation. Concentrated acids, such as concentrated hydrochloric acid, contain a high proportion of HCl solute compared with water solvent. Concentrated fruit juices have less water relative to the amount of sugar and flavour compounds. When water is added, these solutions become more dilute because the amount of solute per unit solvent decreases. These examples illustrate that the term concentrated is always associated with a higher solute content in a fixed amount of solvent, confirming the correctness of the chosen definition. Why Other Options Are Wrong: A solution with no solute is just pure solvent and cannot be described as concentrated; it has zero solute. A solution with no solvent is not a solution at all, but simply pure solute. The option that describes a large amount of solvent and very little solute is essentially describing a dilute solution, not a concentrated one. These descriptions do not match the commonly accepted meaning of a concentrated solution and are therefore incorrect in this context. Common Pitfalls: Students sometimes confuse the absolute amount of solute with the relative concentration and think that any large amount of liquid must be concentrated, regardless of how much solute is actually dissolved. Another pitfall is forgetting that both solute and solvent must be present to use the terminology of solutions. To avoid these mistakes, focus on the ratio of solute to solvent rather than the absolute quantities and remember that concentrated means a higher ratio of solute to solvent compared with a dilute solution. Final Answer: A concentrated solution is best described as containing a large amount of solute dissolved in a given amount of solvent.

Question 4

In atomic structure, what determines the mass number (A) of an element for a given atom or nuclide?

Accepted Answer

Introduction / Context: In nuclear chemistry and atomic physics, each atom or nuclide is characterized by its atomic number and mass number. The mass number is important for distinguishing isotopes and for calculating nuclear properties. This question asks you to determine which particles inside the atom contribute to the mass number of an element and therefore define its value for a given atom. Given Data / Assumptions: We consider a neutral or ionized atom in general, focusing on its nucleus. Atomic number (Z) is defined as the number of protons in the nucleus. Mass number (A) is defined in standard nuclear notation. Electrons have negligible mass compared with protons and neutrons. Concept / Approach: The mass number A of a nuclide is defined as the total number of nucleons in the nucleus. Nucleons are the particles found in the nucleus: protons and neutrons. Electrons orbit the nucleus and contribute almost nothing to the mass because their mass is much smaller than that of protons and neutrons. Therefore, the mass number is the sum of the number of protons and the number of neutrons in the nucleus. Isotopes of the same element have the same number of protons but different numbers of neutrons, and hence different mass numbers. Step-by-Step Solution: Step 1: Recall that protons and neutrons together are called nucleons and reside in the nucleus. Step 2: Remember that the mass number A is defined as A = number of protons + number of neutrons. Step 3: Recognise that electrons, while important for chemical properties, do not significantly contribute to the atomic mass due to their very small mass. Step 4: Compare the given options and identify the one that matches the definition of mass number as the sum of protons and neutrons. Step 5: Select “number of protons and neutrons” as the correct answer. Verification / Alternative check: Consider a common isotope such as carbon 12. It has 6 protons and 6 neutrons, giving a mass number A of 12. Another isotope, carbon 14, has 6 protons and 8 neutrons, giving a mass number of 14. The atomic number (number of protons) remains 6 for both, but the mass number changes because the number of neutrons changes. This demonstrates that mass number depends on both protons and neutrons, and not on electrons. Similar reasoning applies to other elements and their isotopes, confirming the correctness of the answer. Why Other Options Are Wrong: The number of protons alone determines the atomic number, not the mass number, so option A is incomplete. The number of electrons and protons mainly determines the charge state of the atom and its chemical behavior, but electrons are too light to significantly affect mass number, making option B incorrect. The number of neutrons alone does not identify an element, because different elements can have the same number of neutrons. Furthermore, mass number must account for both types of nucleons. Option D therefore cannot be correct for the definition of mass number. Common Pitfalls: Students sometimes confuse atomic number with mass number or mistakenly think that electrons contribute significantly to the mass because they are often discussed alongside protons and neutrons. Another pitfall is to forget that isotopes have different mass numbers even though they share the same number of protons. To avoid confusion, remember the simple formulas: atomic number Z = number of protons; mass number A = number of protons + number of neutrons. Keeping these definitions clear helps in solving many problems in nuclear chemistry and physics. Final Answer: The mass number (A) of an element is determined by the number of protons and neutrons in the nucleus of that atom.

Question 5

In quantum chemistry, the principal quantum number n primarily indicates which property or properties of an electron in an atom?

Accepted Answer

Introduction / Context: Quantum numbers are used to describe the allowed states of electrons in atoms in quantum mechanics. The principal quantum number, symbolised by n, is one of the most important because it is directly related to the main energy levels or shells of an atom. Many exam questions test whether students understand what physical properties are associated with the principal quantum number. This question asks what property or properties of an electron are indicated by the value of n. Given Data / Assumptions: The principal quantum number n can take positive integer values (1, 2, 3, and so on). Electrons in higher n levels are generally farther from the nucleus and have higher energy in hydrogen like atoms. The options suggest that n might reflect energy, distance, both, or something unrelated such as the total number of subshells in the periodic table. We assume a basic understanding of quantum mechanical models of the atom. Concept / Approach: The principal quantum number n primarily defines the main energy level or shell that an electron occupies in an atom. In hydrogen like atoms (single electron systems), the energy of an electron depends mainly on n; as n increases, the energy becomes less negative and the electron is less tightly bound. Additionally, the average distance of the electron from the nucleus also increases with n, because higher n levels correspond to orbitals that extend farther from the nucleus. Therefore, n is related both to the energy of the electron and to its average distance from the nucleus. It does not directly specify the total number of subshells in the periodic table, although each shell with principal quantum number n contains n subshells. Step-by-Step Solution: Step 1: Recall that the principal quantum number n labels the main energy levels or shells (K, L, M, etc.) of an atom. Step 2: Recognise that as n increases, the energy level of the electron generally increases (becomes less negative), especially for hydrogen like atoms. Step 3: Understand that electrons in higher n shells are, on average, farther from the nucleus, so n also reflects the average distance from the nucleus. Step 4: Compare the options and see that options A and B each cover only one of these aspects, while option C correctly mentions both energy and average distance. Step 5: Recognise that option D, dealing with the number of subshells in the entire periodic table, is not the primary role of the principal quantum number in describing an individual electron. Verification / Alternative check: In the Bohr model of the hydrogen atom, the energy of an electron in the nth orbit is given by an expression proportional to 1 / n^2, clearly showing dependence on n. Quantum mechanical treatments also show that the radial distribution of electron probability depends strongly on n, with higher n orbitals having significant probability density farther from the nucleus. These results demonstrate that n is intimately connected with both the energy and the radial size of the electron's region of space. While it is true that each shell with quantum number n possesses n subshells (s, p, d, f, etc.), this is a consequence of the definition, not the primary physical meaning of n for a given electron. Why Other Options Are Wrong: Option A states that n indicates only the energy of the electron; this is incomplete because n is also closely related to the electron's average distance from the nucleus. Option B states that n indicates only the distance from the nucleus, again leaving out the important link with energy, especially in hydrogen like systems. Option D focuses on the number of subshells in the periodic table, which is an indirect and less fundamental point, and not the main physical property described by n for an individual electron. Therefore, none of these options fully captures the dual role of the principal quantum number. Common Pitfalls: Students sometimes treat quantum numbers as mere labels without connecting them to physical properties such as energy and distance. Another pitfall is to memorise isolated statements like “n gives the shell” without understanding what that implies in terms of electron energy and radial extent. To avoid confusion, remember that higher n means both a higher energy level (in hydrogen like atoms) and an orbital that extends farther from the nucleus on average. Recognising both aspects will help you deal with many quantum number questions more confidently. Final Answer: The principal quantum number n primarily indicates both the energy of the electron and its average distance from the nucleus in an atom.

Question 6

In organic chemistry, dry distillation of calcium formate Ca(HCOO)2 gives which main organic product?

Accepted Answer

Introduction / Context: Dry distillation of calcium salts of carboxylic acids is a classic reaction in basic organic chemistry. Different calcium salts give different organic products when heated strongly without any liquid solvent. Calcium formate, Ca(HCOO)2, is the calcium salt of formic acid. This question tests whether you remember the specific product obtained when calcium formate is subjected to dry distillation, a reaction that is often used in textbooks to illustrate thermal decomposition of salts and the formation of aldehydes. Given Data / Assumptions: The starting compound is calcium formate, Ca(HCOO)2. Dry distillation means strong heating of the solid without any added solvent. The options include several possible organic products: formic acid, acetic acid, formaldehyde and acetaldehyde, plus methanol as a plausible alcohol. We assume typical school level reaction conditions used to show decomposition of calcium formate. Concept / Approach: When calcium formate is heated strongly, it decomposes to give formaldehyde, calcium carbonate and hydrogen gas. The key point is that the formate ion HCOO minus is closely related to the aldehyde group. During dry distillation, the formate anion is converted into formaldehyde, HCHO, which is the simplest aldehyde. Acetic acid and acetaldehyde are formed from salts of acetic acid or other higher carboxylic acids, not from calcium formate. Formic acid itself is unstable on heating and does not distil unchanged in this reaction. Therefore, we look for the option that names formaldehyde as the main organic product. Step-by-Step Solution: Step 1: Recall that calcium salts of carboxylic acids can give characteristic products when heated. Step 2: For calcium formate, Ca(HCOO)2, the formate ion is the conjugate base of formic acid, the simplest carboxylic acid. Step 3: On dry distillation, calcium formate decomposes to produce formaldehyde (HCHO), calcium carbonate (CaCO3) and hydrogen gas (H2). Step 4: The main organic product that can be collected as a volatile compound is formaldehyde, HCHO. Step 5: Compare this with the options and select the one that names formaldehyde as the product. Verification / Alternative check: Many introductory organic chemistry books state this reaction in a word equation such as calcium formate on dry distillation gives formaldehyde, calcium carbonate and hydrogen. Balanced equations show that the carbon in the formate ion becomes the carbonyl carbon in formaldehyde. In the laboratory, the gas mixture evolved contains formaldehyde vapours that can be detected by their characteristic strong, irritating odour and reducing behaviour. There is no evidence that acetic acid or acetaldehyde is formed directly from calcium formate under these simple dry distillation conditions, which confirms that formaldehyde is the correct product to choose. Why Other Options Are Wrong: Option A, formic acid, is not the main product because the reaction involves decomposition of the formate salt rather than simple liberation of the acid. Option B, acetic acid, would be expected from calcium acetate, not calcium formate. Option D, acetaldehyde, is usually formed by reactions involving higher carboxylates or oxidation of ethanol, and not by dry distillation of calcium formate. Option E, methanol, is an alcohol and does not match the typical aldehyde product of this decomposition. Only option C correctly identifies formaldehyde (HCHO) as the main organic product of dry distillation of calcium formate. Common Pitfalls: Students often confuse formic acid and formaldehyde because their names sound similar and both contain one carbon atom. Another common mistake is to assume that any salt of a carboxylic acid will regenerate the same acid on heating, which is not true for calcium formate. It is helpful to remember that formate is closely linked to the aldehyde group and that calcium formate is a textbook example of thermal decomposition to formaldehyde. Keeping the specific association between each calcium salt and its decomposition product in mind will prevent confusion in similar questions. Final Answer: Dry distillation of calcium formate Ca(HCOO)2 gives Formaldehyde (HCHO) as the main organic product, along with inorganic by products such as calcium carbonate and hydrogen gas.

Question 7

From the following everyday materials, which one represents a pure substance rather than a mixture?

Accepted Answer

Introduction / Context: A very important idea in basic chemistry is the distinction between pure substances and mixtures. Pure substances have a fixed, definite composition and consistent properties, whereas mixtures contain two or more substances physically combined and can have variable composition. Everyday materials like lemonade, air and sea water may look uniform but are usually mixtures. This question asks you to identify which item in the list is a pure substance in the chemical sense, assuming that it is taken in a chemically pure form. Given Data / Assumptions: Sugar in this context refers to pure crystalline sucrose, not a mixture of different ingredients. Lemonade is prepared by mixing water, sugar and lemon juice, and may also contain other additives. Air is a mixture of several gases such as nitrogen, oxygen, carbon dioxide and traces of others. Sea water contains dissolved salts, gases and organic matter in water. Milk is a complex mixture or emulsion of water, fats, proteins, lactose and minerals. Concept / Approach: A pure substance can be either an element or a compound. It has the same composition throughout and can be represented by a single chemical formula, for example H2O for water or C12H22O11 for sucrose. Mixtures, even if they look uniform, contain more than one chemical substance and cannot be represented by a single fixed formula. The approach is to examine each option and decide whether it is a single compound or a combination of several substances. The one that is a single compound in pure form will be the pure substance among the options. Step-by-Step Solution: Step 1: Consider pure crystalline sugar. Chemically, table sugar is sucrose, a single compound with a definite formula C12H22O11. Step 2: Because sucrose has a fixed composition and uniform properties when pure, it qualifies as a pure substance. Step 3: Lemonade is made by mixing water, sugar and lemon juice, and sometimes flavours and colours. Its composition can vary from one glass to another, so it is a mixture. Step 4: Air is a physical mixture of gases like nitrogen, oxygen, argon and carbon dioxide in varying proportions, so it is not a pure substance. Step 5: Sea water contains water plus dissolved salts and other substances. It is also a mixture, not a pure substance. Step 6: Milk is an emulsion of fat droplets in water with proteins and other solutes. It clearly contains several different components and is a mixture. Step 7: Therefore, the only option that can be a pure substance in the strict chemical sense is pure crystalline sugar. Verification / Alternative check: Textbooks often use examples like distilled water, pure sodium chloride and pure sugar to illustrate pure substances. Analytical techniques show that a sample of pure sucrose yields only sucrose molecules on analysis, while lemonade and milk yield multiple different substances. Air and sea water have compositions that vary with location and conditions, which is another sign of mixtures. Only a chemically purified, single compound has the consistent composition and properties expected of a pure substance, confirming sugar as the correct answer here. Why Other Options Are Wrong: Option B, lemonade, is clearly a mixture of several ingredients and does not have a fixed formula. Option C, air, is a mixture of many gases and sometimes small particles of dust or pollutants. Option D, sea water, combines water with many dissolved ions and organic materials. Option E, milk, is a complex colloidal system with proteins, fats and sugars suspended or dissolved in water. None of these can be described by a single chemical formula, which means they are mixtures, not pure substances. Only option A can represent a pure substance when sugar is taken in a purified crystalline form. Common Pitfalls: A common mistake is to think that anything that looks uniform to the naked eye must be a pure substance. In reality, many homogeneous looking materials are mixtures, such as air and soft drinks. Another pitfall is to confuse everyday meanings of pure, like pure milk or pure air, with the strict chemical meaning. In chemistry, pure means chemically single component, not just free from visible dirt. Remember to ask whether the sample can be represented by one chemical formula. If the answer is yes, then it is likely a pure substance; if not, it is a mixture. Final Answer: The pure substance among the given options is Pure crystalline sugar (sucrose) only, which is a single compound with a definite chemical composition.

Question 8

In nuclear chemistry, isotones are nuclides that always have which feature in common?

Accepted Answer

Introduction / Context: In nuclear chemistry and atomic physics, several related terms are used to compare different nuclides, including isotopes, isobars and isotones. Each term highlights a different way in which nuclei can be similar or different. Isotopes have the same number of protons, isobars have the same mass number and isotones have the same number of neutrons. This question checks whether you know the precise definition of isotones and can distinguish it clearly from the definitions of isotopes and isobars. Given Data / Assumptions: We are comparing nuclides in terms of their numbers of protons, neutrons and electrons. Atomic number is equal to the number of protons in the nucleus. Mass number is equal to the total number of protons plus neutrons. The options describe different possible relationships between nuclides. Concept / Approach: The key is to recall the standard definitions: Isotopes: same number of protons, different numbers of neutrons. Isobars: same mass number, different atomic numbers. Isotones: same number of neutrons, different atomic numbers. Isotones therefore share the same neutron count but not the same proton count, which means their atomic numbers are different and they represent different elements. The approach is to identify which option states that nuclides have the same number of neutrons but different atomic numbers, which matches the definition of isotones. Step-by-Step Solution: Step 1: Recall that isotones are defined by having equal numbers of neutrons in their nuclei. Step 2: Because the number of protons is not restricted to be the same, the atomic numbers of isotones are different, so they are different elements. Step 3: Examine option A, which states that the number of protons is the same but neutrons differ. That is the definition of isotopes, not isotones. Step 4: Option B claims that both atomic number and mass number are the same, which would mean the nuclides are actually the same, not a special related pair. Step 5: Option C states that the nuclides have the same number of neutrons but different atomic numbers, which matches the definition of isotones. Step 6: Options D and E describe situations that do not match the standard definition of isotones, so they can be rejected. Verification / Alternative check: Consider an example. Oxygen 17 has 8 protons and 9 neutrons. Fluorine 18 has 9 protons and 9 neutrons. These two nuclides have the same neutron count, 9, but different atomic numbers, 8 and 9. They are isotones. In contrast, carbon 14 and nitrogen 14 both have mass number 14 but different neutron counts, so they are isobars, not isotones. Chlorine 35 and chlorine 37 have the same atomic number 17 but different neutron numbers, so they are isotopes. This set of examples shows clearly that isotones are defined by sharing neutron count while differing in proton count, as described in option C. Why Other Options Are Wrong: Option A describes isotopes, which have the same number of protons and different numbers of neutrons, not isotones. Option B would mean that both atomic number and mass number are the same, which actually describes identical nuclides rather than a special category. Option D again describes identical nuclides, because if both the proton and neutron numbers are the same, the nuclei are not distinct. Option E focuses on electrons in the outermost shell, which is more related to chemical behaviour and valence, not nuclear classification. Only option C correctly states that isotones have the same number of neutrons but different atomic numbers. Common Pitfalls: A common difficulty is mixing up the similar sounding terms isotopes, isobars and isotones. Students often remember isotopes but forget isotones, or they confuse isotones with isobars. A helpful memory aid is to focus on the last part of each word: topo suggests place, so isotopes have the same place in the periodic table, meaning same atomic number. Bar refers to weight or mass, so isobars have equal mass number. Tone can be linked to neutrons only, so isotones have equal neutron number. Keeping these associations in mind reduces confusion during nuclear chemistry questions. Final Answer: Isotones are nuclides that have Same number of neutrons but different atomic numbers, so they share neutron count but belong to different elements.

Question 9

A single proton is identical to which nuclear species among the following options?

Accepted Answer

Introduction / Context: In atomic structure, the proton is one of the fundamental particles that make up the nucleus. Hydrogen, the simplest element, has a special role because its most common isotope consists of a nucleus with just one proton and no neutrons, surrounded by one electron. This question asks you to relate the idea of a single proton to the nucleus of specific atoms or particles, including hydrogen, helium, alpha particles and beta particles. Understanding this relationship helps in nuclear chemistry, mass spectrometry and many basic models of the atom. Given Data / Assumptions: A proton is a positively charged particle with a single positive charge. The most common hydrogen isotope, sometimes called protium, has one proton and no neutrons in its nucleus. An alpha particle is the nucleus of a helium 4 atom, He2 plus, with two protons and two neutrons. A beta particle is a high speed electron or positron, not a proton. Concept / Approach: The nucleus of a hydrogen atom of mass number 1 consists of a single proton. When the electron is removed, what remains is simply a proton. Therefore, a proton can be described as a hydrogen 1 nucleus. It is not correct to equate a proton with an alpha particle or with the nucleus of helium, since those contain more than one nucleon. Beta particles are a different kind of particle entirely. The approach is to match the definition of a proton with the nucleus that contains exactly one proton and no neutrons, which is the hydrogen 1 nucleus. Step-by-Step Solution: Step 1: Recall that the hydrogen atom with mass number 1 has a nucleus consisting of one proton and no neutrons. Step 2: If the electron is removed from this hydrogen atom, the remaining particle is a bare proton. Step 3: Therefore, a single proton is identical to the nucleus of hydrogen 1, sometimes called the protium nucleus. Step 4: Consider helium 4, which has two protons and two neutrons; its nucleus is too large to be a single proton. Step 5: An alpha particle is a helium 4 nucleus and includes four nucleons, not one, so it is not identical to a proton. Step 6: A beta particle is usually an electron or a positron, which has a different mass, charge and identity from a proton. Verification / Alternative check: Chemical notation often writes the proton as 1H plus, which emphasises that the proton is the ion formed when a neutral hydrogen 1 atom loses its electron. Mass spectrometry and nuclear reaction equations also treat 1H plus and the proton as equivalent. In contrast, an alpha particle is written as 4He2 plus, clearly showing two protons and two neutrons. Beta particles are written as 0e minus or 0e plus, indicating electrons or positrons with almost zero mass number. These standard symbols confirm that the closest and correct identification of a proton is with the nucleus of a hydrogen 1 atom. Why Other Options Are Wrong: Option A describes the nucleus of helium 4, which contains two protons and two neutrons and is therefore much more massive than a single proton. Option C describes an alpha particle, which is the same as the helium 4 nucleus and again contains four nucleons. Option D describes a beta particle, which is either an electron or positron, not a proton, and has a different charge to mass ratio. Option E describes the nucleus of deuterium, hydrogen 2, which has one proton and one neutron, not just a single proton. Only option B correctly states that a proton is identical to the nucleus of a hydrogen atom of mass number 1. Common Pitfalls: Students sometimes confuse alpha particles with protons because both carry positive charge, but alpha particles carry two positive charges and have much greater mass. Another common confusion is between hydrogen 1 and deuterium; both are isotopes of hydrogen but their nuclei differ in neutron content. Remember that a proton on its own is the bare nucleus of hydrogen 1 only. Keeping track of mass number and atomic number for each particle or nucleus will help avoid errors in nuclear chemistry questions. Final Answer: A single proton is identical to The nucleus of a hydrogen atom of mass number 1 (one proton only), which is the protium nucleus with no neutrons.

Question 10

An alpha (α) particle emitted in radioactive decay is best described as which of the following?

Accepted Answer

Introduction / Context: Many radioactive elements, especially heavy nuclei such as uranium and radium, decay by emitting alpha particles. Understanding what an alpha particle really is helps explain why alpha radiation has relatively low penetrating power but high ionising power, and why it carries a specific positive charge and mass. This question asks you to choose the correct physical description of an alpha particle from several options that include hydrogen atoms, helium nuclei, protons and electrons. Given Data / Assumptions: Alpha particles are emitted from the nuclei of certain heavy radioactive elements. They carry a charge of plus two and have a relatively high mass compared with electrons. A helium 4 nucleus consists of two protons and two neutrons. A hydrogen atom consists of one proton in the nucleus and one electron around it. A beta particle is a high speed electron or positron, not an alpha particle. Concept / Approach: Experimental measurements show that alpha particles have a charge of plus two and a mass approximately four times that of a proton. These values match the properties of a helium 4 nucleus, which contains two protons and two neutrons and no electrons. When a heavy nucleus emits an alpha particle, its atomic number decreases by two and its mass number decreases by four, exactly as if a helium nucleus had been removed. The approach is to match this description with the correct option and to distinguish alpha particles from other nuclear or atomic species such as hydrogen atoms, single protons and electrons. Step-by-Step Solution: Step 1: Recall that an alpha particle carries a charge of plus two and a mass number of four. Step 2: A particle with mass number four and charge plus two must contain two protons and two neutrons with no electrons. Step 3: This composition is exactly that of a helium 4 nucleus, often written as 4He2 plus. Step 4: A hydrogen molecule, H2, consists of two neutral hydrogen atoms, not a charged nuclear particle, so it does not match the known properties of alpha particles. Step 5: A single proton has charge plus one and mass approximately one atomic mass unit, so it cannot account for the alpha particle mass and charge. Step 6: A high speed electron is a beta particle with very low mass and negative charge, not an alpha particle. Step 7: Therefore, the correct description of an alpha particle is that it is a helium nucleus with two protons and two neutrons. Verification / Alternative check: Alpha particle scattering experiments, such as the famous Rutherford scattering experiment, used alpha particles emitted from radioactive sources. When alpha particles strike thin metal foils, their deflection patterns match the behaviour expected for heavy, positively charged helium nuclei. When alpha emitting sources are allowed to neutralise, helium gas can be detected, further confirming the link between alpha particles and helium nuclei. These observations strongly support the conclusion that an alpha particle is essentially a helium 4 nucleus stripped of its electrons. Why Other Options Are Wrong: Option A, a hydrogen molecule, describes two whole hydrogen atoms and does not match the charge and mass of an alpha particle. Option C, a neutral hydrogen atom, has only one proton in the nucleus and mass number one, so it is much lighter than an alpha particle. Option D, a single proton, also has mass number one and charge plus one, which does not match the properties of alpha radiation. Option E, a high speed electron, describes a beta particle, which has negative charge and extremely low mass. Only option B correctly describes an alpha particle as a helium nucleus with two protons and two neutrons. Common Pitfalls: Students sometimes confuse alpha particles with protons because both are positively charged, but they differ in charge and mass. Another common error is to mix up alpha and beta radiation, especially when learning about radioactive decay types for the first time. To avoid confusion, remember the simple pattern: alpha equals helium nucleus with two protons and two neutrons, beta is an electron or positron, and gamma is high energy electromagnetic radiation. This pattern makes it easier to connect each type of radiation with its properties and behaviour. Final Answer: An alpha particle is best described as A helium nucleus with two protons and two neutrons, which carries a charge of plus two and has relatively high mass.

Question 11

During anaerobic digestion in a biogas plant, which of the following gases is generally not produced in significant amounts?

Accepted Answer

Introduction / Context: Biogas plants use anaerobic bacteria to decompose organic waste such as animal dung, kitchen refuse and agricultural residues. The result is a combustible gas mixture known as biogas, which can be used as a fuel for cooking, heating and electricity generation. Knowing the composition of biogas is important for understanding its energy content, safety considerations and environmental impact. This question asks which gas is generally not a component of biogas produced in a typical biogas plant. Given Data / Assumptions: Biogas is formed by anaerobic digestion of organic matter in the absence of oxygen. The main combustible gas in biogas is methane, CH4. Hydrogen sulfide, H2S, is often present in small amounts and is responsible for a foul smell. Carbon monoxide, CO, is usually associated with incomplete combustion processes, not with biological digestion. Concept / Approach: Typical biogas composition includes about 50 to 70 percent methane, 30 to 40 percent carbon dioxide and small amounts of hydrogen, hydrogen sulfide and sometimes nitrogen. Carbon monoxide is not a normal product of anaerobic bacterial metabolism in biogas plants. Instead, CO is formed when carbon containing fuels burn with insufficient oxygen, such as in faulty stoves, heaters or engines. The approach is to recall the standard composition of biogas and identify the gas that does not belong to this biologically produced mixture. Step-by-Step Solution: Step 1: Recognise that methane (CH4) is the main fuel component of biogas and is always present. Step 2: Carbon dioxide (CO2) is also produced in large amounts during anaerobic digestion and dilutes the biogas. Step 3: Hydrogen sulfide (H2S) is typically present in small quantities, giving biogas a rotten egg odour and requiring removal to protect metal parts. Step 4: Nitrogen or other inert gases may be present in trace amounts, especially if some air enters the system, but they are not major combustible components. Step 5: Carbon monoxide (CO), however, is not normally produced in biological anaerobic digestion; it is a typical product of incomplete combustion, not of microbial fermentation. Step 6: Therefore, among the options listed, CO is the gas that is generally not generated in a biogas plant. Verification / Alternative check: Data from biogas research and practical plant operation show typical compositions such as 60 percent methane, 38 percent carbon dioxide, 1 to 2 percent hydrogen sulfide and traces of nitrogen and hydrogen. Carbon monoxide is absent or present only in negligible trace amounts that are not characteristic of biogas. In contrast, synthesis gas from industrial gasification processes contains significant carbon monoxide, but that is a completely different technology involving high temperature reactions, not biological digestion. This comparison reinforces the conclusion that CO is not a characteristic gas of biogas plants. Why Other Options Are Wrong: Option A, hydrogen sulfide, is an unwelcome but common component of biogas and must often be removed to prevent corrosion. Option B, carbon dioxide, is present in large amounts and reduces the calorific value of the gas but is definitely part of biogas. Option C, methane, is the main combustible component and is essential to biogas. Option E, small amounts of nitrogen, can be present due to residual air in the system or from certain substrates. Only option D, carbon monoxide, does not normally appear in the composition of biogas from standard anaerobic digestion processes. Common Pitfalls: Students sometimes mix up biogas with other fuel gas mixtures such as coal gas or producer gas, which can contain significant carbon monoxide. They may also assume that any combustible gas mixture must include CO. To avoid confusion, remember that biogas is biological and mainly methane and carbon dioxide, whereas gases from high temperature industrial processes often include CO. Keeping the source process in mind makes it easier to recall which gas mixtures contain which components. Final Answer: The gas that is generally not generated in a biogas plant is Carbon monoxide (CO), which is typical of incomplete combustion processes rather than anaerobic digestion.

Question 12

Commercial soft drinks and sodas mainly contain which weak acid formed when carbon dioxide dissolves in water?

Accepted Answer

Introduction / Context: Soft drinks and carbonated beverages have a characteristic fizz and slightly sharp taste. This is due to the presence of dissolved carbon dioxide gas, which reacts with water under pressure to form a weak acid. Understanding which acid is produced helps connect simple equilibrium chemistry with everyday life. The question asks you to identify the main weak acid present in soft drinks because of dissolved carbon dioxide, distinguishing it from other organic acids found in foods and fruits. Given Data / Assumptions: Soft drinks are carbonated beverages containing dissolved CO2 under pressure. When CO2 dissolves in water, it forms a weak acid in equilibrium with the dissolved gas. Other acids like citric, tartaric and lactic acid occur naturally in fruits and fermented foods. The focus here is specifically on the acid produced from dissolved CO2, not on flavouring acids that may also be added. Concept / Approach: When carbon dioxide is dissolved in water, a reversible reaction occurs forming carbonic acid, H2CO3. This weak acid partially dissociates to release hydrogen ions, giving the drink a mildly acidic pH. The overall reaction can be written as CO2 plus H2O giving H2CO3, with an equilibrium between these species. Although some soft drinks may also contain citric or phosphoric acid for flavour and preservation, the acid directly formed from dissolved carbon dioxide is carbonic acid. The approach is to match the name of this acid with the correct option and not confuse it with other common food acids. Step-by-Step Solution: Step 1: Recall that soft drinks are often called carbonated beverages because they contain dissolved carbon dioxide gas. Step 2: When CO2 dissolves in water, a weak acid called carbonic acid, H2CO3, is formed in equilibrium with the dissolved gas. Step 3: Carbonic acid is responsible for part of the sharp taste and for the slightly acidic pH of most soft drinks. Step 4: Oxalic acid is found in certain vegetables such as spinach and is not the main acid from carbonation. Step 5: Tartaric acid is present in tamarind and some fruits, and citric acid is found in lemons and oranges. Step 6: Lactic acid is produced in sour milk and curd by bacterial fermentation, not by dissolving CO2 in water. Step 7: Therefore, the correct answer is carbonic acid, the weak acid formed when carbon dioxide dissolves in water. Verification / Alternative check: If you measure the pH of a freshly opened soft drink, it is typically acidic, often in the range of pH 3 to 4. This is due to both carbonic acid and any additional acids added for flavour. Laboratory studies of the CO2 water system describe the equilibrium CO2 plus H2O giving H2CO3 giving H plus plus HCO3 minus, which matches the chemistry of carbonic acid in solution. Labels on soft drink bottles may list other acids such as phosphoric or citric acid, but these are separate additives. The fundamental acid generated by carbonation is always carbonic acid, confirming option C as correct. Why Other Options Are Wrong: Option A, oxalic acid, is more associated with some green vegetables and certain cleaning uses, not with carbonation in soft drinks. Option B, tartaric acid, is naturally present in grapes and tamarind and is used in some baking powders, but not as the primary acid from dissolved CO2. Option D, citric acid, is widely used in lemon flavoured drinks and fruit juices but is not produced by dissolving carbon dioxide in water. Option E, lactic acid, is a product of fermentation in dairy products and muscles. Only option C correctly identifies carbonic acid as the weak acid formed from dissolved carbon dioxide in soft drinks. Common Pitfalls: Students sometimes focus on the brand labels of drinks and see citric or phosphoric acid listed, then assume these are the only acids involved. While those acids do contribute to flavour and preservation, the question here is specifically about the acid formed from dissolved CO2. Another confusion is between carbonic acid and carboxylic acids like acetic or oxalic acid due to similarity of names. To avoid mistakes, remember that carbonic acid is uniquely associated with carbon dioxide and water, whereas other common food acids arise from plant metabolism or fermentation. Final Answer: The weak acid mainly present in carbonated soft drinks due to dissolved CO2 is Carbonic acid, formed from dissolved carbon dioxide.

Question 13

In a covalent molecule containing halogen atoms, the property of attracting shared electrons towards the halogen atoms is called what?

Accepted Answer

Introduction / Context: Halogens such as fluorine, chlorine, bromine and iodine are well known for their strong tendency to attract electrons in chemical bonds. This tendency influences bond polarity, molecular shape and reactivity. Several related terms are used in chemistry, including electron affinity and electronegativity, and it is important to distinguish between them. This question asks for the name of the property that describes how strongly an atom in a molecule attracts shared electrons in a covalent bond, which is crucial for understanding polar covalent bonds and partial charges. Given Data / Assumptions: We are dealing with covalent molecules where atoms share electron pairs. Halogen atoms usually have high attraction for shared electrons in bonds. Electron affinity is a separate concept, referring to energy change when an isolated gaseous atom gains an electron. Electropositivity refers to the tendency of an atom to lose electrons and form cations. We assume basic knowledge of terms used in periodic trends. Concept / Approach: Electronegativity is defined as the ability of an atom in a molecule to attract the shared electron pair towards itself in a covalent bond. It is a relative scale without units, with fluorine as the most electronegative element. This property is responsible for bond polarity, where one end of a bond becomes partially negative and the other partially positive. Electron affinity, in contrast, is an energy change for an isolated atom, not specifically for a bonded atom in a molecule. The correct approach is to choose the term that matches attraction for shared electrons in a bond, which is electronegativity. Step-by-Step Solution: Step 1: Identify the property being described: attraction for electrons in a covalent bond, not energy change for an isolated atom. Step 2: Recall that electronegativity is defined as the tendency of an atom in a molecule to attract the shared pair of electrons toward itself. Step 3: Recognise that halogens are high in electronegativity because they are close to completing their valence octet. Step 4: Electron affinity refers to the energy released or absorbed when a gaseous atom gains an extra electron, which is a different concept from electronegativity. Step 5: Electropositivity is the tendency of an element, usually a metal, to lose electrons and become a cation, which is the opposite behaviour of halogens. Step 6: Electrochemistry and electrical conductivity are branches and properties related to charge transfer and flow, not directly to electron attraction in covalent bonds. Step 7: Thus the correct term for the electron attracting property of halogen atoms in a molecule is electronegativity. Verification / Alternative check: Periodic tables often list electronegativity values alongside atomic numbers. Fluorine has the highest electronegativity, followed by oxygen, chlorine and so on. Halogens consistently show high electronegativity values, reflecting their strong pull on shared electrons. Bond polarity diagrams, which show partial charges, are based on differences in electronegativity between atoms in a bond. In contrast, tables of electron affinities show energy changes for adding electrons to isolated atoms, which is related but not the same as behaviour in a bonded state. This confirms that electronegativity is the property needed in this question. Why Other Options Are Wrong: Option A, electron affinity, describes the energy change when an isolated gaseous atom accepts an electron and is not defined specifically for atoms in molecules. Option B, electropositivity, refers to the tendency to lose electrons and is high for alkali metals, not for halogens. Option C, electrochemistry, is a branch of chemistry studying redox reactions and cells, not a property of atoms. Option E, electrical conductivity, describes how well a substance conducts electric current, which is not the same as attraction for shared electrons. Only option D, electronegativity, correctly names the property by which halogen atoms attract shared electrons in a covalent bond. Common Pitfalls: Students often confuse electron affinity and electronegativity because both involve attraction for electrons and show similar periodic trends. The key distinction is that electron affinity is about isolated atoms and energy changes, while electronegativity is about atoms that are part of a molecule and their influence on shared electron pairs. Another pitfall is to equate electropositivity simply with low electronegativity, but they are used in different contexts. Making clear mental definitions for each term and linking electronegativity specifically to covalent bonds will help avoid mistakes. Final Answer: The property of attracting shared electrons towards halogen atoms in a molecule is called Electronegativity.

Question 14

Which of the following elements is generally considered non radioactive under normal conditions?

Accepted Answer

Introduction / Context: Elements can be broadly classified into those with stable nuclei and those that are radioactive. Radioactive elements have unstable nuclei that spontaneously emit radiation in the form of alpha, beta or gamma rays. Many heavy elements such as uranium, thorium and plutonium are well known for their radioactivity. In contrast, lighter elements such as silicon or germanium are typically stable and non radioactive under normal conditions. This question tests whether you can distinguish a generally stable, non radioactive element from a group of mostly radioactive elements. Given Data / Assumptions: Thorium, uranium and plutonium are actinide elements with large, unstable nuclei. Polonium is a heavy element that is highly radioactive. Germanium is a metalloid located in the p block, used in semiconductors. The question asks which element is generally non radioactive, meaning it has stable isotopes in nature. Concept / Approach: Radioactive elements often have no stable isotopes and undergo continuous radioactive decay. Uranium and thorium occur naturally as radioactive elements and are used as nuclear fuels. Plutonium is an artificial or semi artificial radioactive element produced in nuclear reactors. Polonium has no stable isotopes and is very radioactive. Germanium, on the other hand, is a lighter element that has several stable isotopes found in nature. The approach is to identify the element that belongs to the group of stable metalloids rather than to the group of heavy radioactive actinides or related elements. Step-by-Step Solution: Step 1: Recall that uranium (U) and thorium (Th) are well known radioactive elements found in ores and used as nuclear fuels. Step 2: Plutonium (Pu) is produced in nuclear reactors and is strongly radioactive, used in some nuclear weapons and fuel. Step 3: Polonium (Po) is another heavy element with no stable isotopes and strong radioactivity, historically used as a source of alpha particles. Step 4: Germanium (Ge) is a metalloid in the same group as silicon and has stable isotopes; it is used in semiconductors and optics. Step 5: Therefore, among the listed elements, germanium is generally considered non radioactive under normal conditions. Verification / Alternative check: Looking at isotope tables, germanium has several stable isotopes, including mass numbers such as 70, 72, 73, 74 and 76, which do not undergo radioactive decay. In contrast, uranium naturally exists as radioactive isotopes such as U 238 and U 235, thorium exists mainly as Th 232 and plutonium isotopes such as Pu 239 are created in reactors and are all radioactive. Polonium isotopes including Po 210 are also radioactive. Standard periodic table references and nuclear data summaries clearly classify uranium, thorium, plutonium and polonium as radioactive, while germanium is listed as having stable, non radioactive isotopes. Why Other Options Are Wrong: Option A, thorium, is radioactive and undergoes a decay series that eventually produces stable lead isotopes. Option B, polonium, is strongly radioactive and has been used as a source of alpha particles. Option D, plutonium, is a well known radioactive element associated with nuclear technology. Option E, uranium, is a naturally occurring radioactive element used as nuclear fuel. Only option C, germanium, represents an element that is generally non radioactive due to the presence of stable isotopes in nature. Common Pitfalls: Students sometimes assume that any heavy element is automatically radioactive and any lighter element is always stable, but there are exceptions at the boundaries. However, in this list the radioactive elements are well known and often mentioned in connection with nuclear energy or radiation hazards. To avoid confusion, it helps to remember that actinides such as uranium, thorium and plutonium are typically radioactive, while metalloids in the middle of the table such as germanium and silicon are usually stable. Keeping track of which elements commonly appear in discussions of radioactivity will also guide your choices in similar questions. Final Answer: The element that is generally non radioactive under normal conditions is Germanium (Ge), which has stable isotopes and is widely used as a semiconductor material.

Question 15

According to the law of definite composition, in a chemical compound the constituent elements are always present in definite proportions by which quantity?

Accepted Answer

Introduction / Context: One of the fundamental laws of classical chemistry is the law of definite proportions, also called the law of constant composition. It describes how elements combine to form a given compound. This law helps explain why water from different sources always has the same basic composition and why a pure compound has a fixed formula. The question asks what kind of proportion remains fixed for the constituent elements in a compound according to this law, distinguishing it from other quantities such as volume or size. Given Data / Assumptions: A chemical compound is formed when two or more elements chemically combine in fixed ratios. The law of definite proportions states that these ratios are constant for a given compound. Possible proportional quantities include mass, volume and number of particles. We assume conditions where the compound is pure and the formula is well defined. Concept / Approach: The law of definite proportions states that a chemical compound always contains the same elements in the same fixed proportion by mass, regardless of the sample size or source. For example, water always contains hydrogen and oxygen in a mass ratio of about 1 to 8. This is a mass ratio, not a volume ratio. While the number of moles of each element in a formula is also fixed, the law is traditionally stated in terms of mass because early chemists measured reactants and products by weighing them. The correct approach is to choose the quantity that is explicitly mentioned in the law, which is mass. Step-by-Step Solution: Step 1: Recall the statement of the law of definite proportions: a given compound always contains elements combined in the same proportion by mass. Step 2: Apply this to water, H2O. One mole of water has a mass of approximately 18 grams, with about 2 grams of hydrogen and 16 grams of oxygen, giving a fixed mass ratio of hydrogen to oxygen of 1 to 8. Step 3: No matter how much water you have, the ratio of mass of hydrogen to mass of oxygen remains the same. Step 4: Volume is not used in the law because the volume of gases or liquids can change significantly with temperature and pressure. Step 5: The size of atoms is not relevant to the law, and the number of moles is a different way of expressing composition that came later in chemistry history. Step 6: Therefore, the correct answer is that elements in a compound are always present in fixed proportions by mass. Verification / Alternative check: Consider carbon dioxide, CO2. Each mole of CO2 contains 12 grams of carbon and 32 grams of oxygen, giving a constant mass ratio of carbon to oxygen of 3 to 8. If you decompose CO2 from any source, such as respiration or combustion, you always obtain carbon and oxygen in this same mass ratio. Early quantitative experiments by chemists like Joseph Proust confirmed such constant mass ratios for many compounds, leading to the formulation of the law of definite proportions. This historical and experimental evidence supports mass as the correct quantity for the law. Why Other Options Are Wrong: Option B, volume, depends on physical conditions like temperature and pressure, especially for gases, so volume ratios are not constant for a given compound. Option C, physical size of atoms, is a microscopic property that does not directly define composition in the law. Option D, number of moles only, is related to the formula, but the classical statement of the law focuses on mass, and different formulas such as CO and CO2 have different mole ratios. Option E claims no option is correct, which contradicts the well established wording of the law. Only option A correctly states that elements in a compound are always present in definite proportions by mass. Common Pitfalls: A common confusion is mixing the law of definite proportions with the law of multiple proportions, which deals with different compounds formed by the same elements. Another pitfall is focusing on the chemical formula and mole ratios without connecting back to mass ratios, which were historically measured. To avoid errors, remember that definite proportions refer to constant mass ratios for a given compound, while mole ratios and subscripts in formulas are another, more modern way of expressing the same idea. Final Answer: According to the law of definite composition, in a compound the elements are always present in definite proportions by Mass.

Question 16

Among the following common metals, which one has the highest density and is therefore the heaviest metal?

Accepted Answer

Introduction / Context: The term heaviest metal in everyday chemistry questions usually refers to the metal with the greatest density among a set of familiar metals. Density is defined as mass per unit volume and determines how heavy a substance feels for a given size. Gold, silver, iron and copper are common metals with different densities and uses, and comparing them helps learners associate physical properties with elements on the periodic table. This question asks which of the listed metals is the heaviest in terms of density, not in terms of total mass of a given object. Given Data / Assumptions: The metals to be compared are gold, silver, iron, copper and aluminium. We are considering density at room temperature and normal pressure. Heaviest means highest density, so more mass is packed into the same volume. We assume standard physical data as found in school level tables. Concept / Approach: Typical densities of these metals at room temperature are roughly as follows: Gold: about 19.3 grams per cubic centimetre. Silver: about 10.5 grams per cubic centimetre. Copper: about 8.9 grams per cubic centimetre. Iron: about 7.9 grams per cubic centimetre. Aluminium: about 2.7 grams per cubic centimetre. From these values, gold is clearly the densest and thus the heaviest for equal volumes. The correct approach is to recall or estimate these densities and pick the metal with the largest value, which is gold. Step-by-Step Solution: Step 1: Recognise that aluminium is a very light metal, used where low weight is needed, so it has much lower density than the others. Step 2: Iron and copper are moderately dense structural metals, but their densities are still lower than those of the so called heavy metals like gold and silver. Step 3: Silver is denser than iron and copper, but its density of about 10.5 grams per cubic centimetre is still significantly less than that of gold. Step 4: Gold has a very high density of about 19.3 grams per cubic centimetre, making it much heavier than the same volume of the other listed metals. Step 5: Therefore, among the options provided, gold is the heaviest metal by density. Verification / Alternative check: Density tables in physics and chemistry handbooks list gold as one of the densest common metals, surpassed only by a few less familiar elements such as osmium and iridium. When jewellers compare weights of jewellery pieces, gold items feel much heavier than silver or copper items of the same apparent size. Experiments where students compare equal sized metal blocks also demonstrate that gold or lead blocks are heavier than iron or aluminium ones. These observations and reference data confirm that gold is the densest metal in the given list. Why Other Options Are Wrong: Option B, silver, is dense and heavy but has only about half the density of gold. Option C, iron, is less dense than both silver and gold and has density around 7.9 grams per cubic centimetre. Option D, copper, has density slightly less than iron and is used for electrical wiring and coins. Option E, aluminium, is the least dense of all and is considered a light metal used in aircraft and packaging. None of these match the very high density of gold, so only option A is correct. Common Pitfalls: Students sometimes assume that metals used in heavy machinery, such as iron, must always be the heaviest metal. Others may think of silver as very heavy because of its name and appearance. To avoid confusion, remember that gold is famous not only for its value and colour but also for its very high density. A simple way to remember this is to think of how small a gold bar can be for a given mass compared with bars of other metals. Recognising gold as exceptionally dense will help you answer similar questions quickly. Final Answer: Among the listed metals, the heaviest by density is Gold, which has a much higher density than silver, iron, copper or aluminium.

Question 17

In the nitrogen cycle, nitrification is the biological process that primarily converts which nitrogen form into which other form in its first step?

Accepted Answer

Introduction / Context: The nitrogen cycle describes how nitrogen moves through the atmosphere, soil, water and living organisms. Several key biological processes are involved, including nitrogen fixation, nitrification, assimilation and denitrification. Nitrification is especially important in soil chemistry and wastewater treatment because it transforms ammonia into more oxidised forms that plants can use and that affect water quality. This question asks you to identify the main conversion that happens in the first step of nitrification, focusing on which nitrogen form is converted into which other form by nitrifying bacteria. Given Data / Assumptions: Nitrification is carried out by specific aerobic bacteria in soil and water. It involves oxidation reactions that increase the oxidation state of nitrogen. Ammonia is often produced from the decomposition of organic nitrogen compounds or from fertilizers. In many texts, nitrification is described as a two step process: ammonia to nitrite and nitrite to nitrate. The question emphasises the first main biological conversion. Concept / Approach: In nitrification, ammonia (NH3) or ammonium ions (NH4 plus) are first oxidised to nitrite ions (NO2 minus) by bacteria such as Nitrosomonas. This is the first step. In the second step, nitrite is further oxidised to nitrate ions (NO3 minus) by bacteria such as Nitrobacter. Nitrification does not involve direct conversion of nitrogen gas to nitrate and is different from nitrogen fixation. It also does not convert ammonia back to nitrogen gas; that is denitrification. The correct approach is to select the option that describes ammonia being converted into nitrite, which represents the initial nitrification step. Step-by-Step Solution: Step 1: Recall that nitrification is an oxidation process that converts reduced nitrogen species into more oxidised ones in the soil. Step 2: The starting nitrogen form for nitrification is usually ammonia or ammonium, produced by decay of organic matter or added as fertilizer. Step 3: In the first step, bacteria such as Nitrosomonas oxidise ammonia (NH3 or NH4 plus) to nitrite ions (NO2 minus). Step 4: In a subsequent second step, other bacteria oxidise nitrite to nitrate (NO3 minus), but that second step is beyond the scope of this particular question. Step 5: Nitrogen gas (N2) is not directly involved in nitrification; processes involving N2 are nitrogen fixation and denitrification. Step 6: Therefore, the correct description of nitrification in its first step is the conversion of ammonia into nitrite ions. Verification / Alternative check: Standard environmental science and microbiology textbooks describe nitrification as a two stage process: Stage 1: NH3 plus 1.5 O2 gives NO2 minus plus H plus plus H2O. Stage 2: NO2 minus plus 0.5 O2 gives NO3 minus. These reactions show clearly that ammonia is converted first to nitrite, then to nitrate. Field studies of soil nitrogen confirm that when ammonia based fertilizers are applied to soil, they are gradually converted into nitrite and then nitrate by nitrifying bacteria. Nitrogen gas appears in other parts of the cycle, such as nitrogen fixation (N2 to ammonia) and denitrification (nitrate to N2), not in nitrification. This evidence supports option C as correct. Why Other Options Are Wrong: Option A describes nitrogen gas being turned directly into nitrate, which would require nitrogen fixation plus additional oxidation, not nitrification alone. Option B suggests N2 to nitrite, which again is not the standard nitrification step and does not match the biological processes carried out by nitrifying bacteria. Option D claims ammonia is converted directly into nitrogen gas, which would be denitrification in reverse and is not observed in nitrification. Option E describes nitrate being converted into ammonia, which is the opposite of oxidation and is sometimes called nitrate reduction, not nitrification. Only option C correctly states that the first step of nitrification is the conversion of ammonia into nitrite ions. Common Pitfalls: Learners often confuse the different nitrogen transformations and mix up terms like nitrogen fixation, nitrification and denitrification. Another pitfall is to forget that nitrification is specifically an oxidation process and therefore must move nitrogen from a more reduced state (ammonia) to more oxidised states (nitrite and nitrate). To keep them straight, remember the sequence: fixation brings nitrogen into the system by converting N2 to ammonia, nitrification oxidises ammonia to nitrate via nitrite, and denitrification returns nitrogen to the atmosphere by reducing nitrate back to N2. This mental map helps to answer nitrogen cycle questions accurately. Final Answer: In the first step of nitrification, bacteria convert Ammonia (NH3 or NH4 plus) into nitrite ions (NO2 minus), which is then further oxidised to nitrate in the second step.

Question 18

In everyday chemistry, vinegar used in kitchens is essentially an aqueous solution of which acid?

Accepted Answer

Introduction / Context: Vinegar is a very common household substance used in cooking, pickling and cleaning. It is known for its sour taste and characteristic smell. Chemically, vinegar is not a pure substance but a dilute solution of a specific carboxylic acid in water. Recognising which acid this is helps connect organic chemistry with everyday life and also appears frequently in school level chemistry questions. This question asks you to identify the correct acid that makes up vinegar. Given Data / Assumptions: Vinegar is prepared by fermentation processes where ethanol is oxidised. The main acid in vinegar is a simple carboxylic acid with two carbon atoms. Glacial acetic acid refers to nearly pure, concentrated ethanoic acid. Formic acid, also called methanoic acid, is a one carbon carboxylic acid found in ant stings. Vinegar used in food typically contains about 4 to 8 percent acid by volume in water. Concept / Approach: Vinegar is essentially a dilute aqueous solution of acetic acid, also known as ethanoic acid. During production, bacteria oxidise ethanol in wine or other alcoholic solutions to ethanoic acid, which is then diluted to the appropriate strength. Glacial acetic acid is the pure, concentrated form and is not used directly as table vinegar because it is too strong and corrosive. Formic acid and citric acid are different acids associated with ant stings and citrus fruits respectively, not with standard vinegar. The approach is to recall that the acid in vinegar is acetic acid and that in table vinegar it is present in dilute form. Step-by-Step Solution: Step 1: Remember that vinegar is produced by the bacterial oxidation of ethanol, giving ethanoic acid as the main product. Step 2: Ethanoic acid is commonly called acetic acid and has the formula CH3COOH. Step 3: Commercial vinegar for cooking is not pure acetic acid; it is a dilute solution of acetic acid in water, usually around 4 to 8 percent. Step 4: Glacial acetic acid refers to nearly pure acetic acid, which is too strong to be used directly as food vinegar. Step 5: Formic acid (methanoic acid) is a one carbon acid associated with ant and bee stings and is not the main acid in vinegar. Step 6: Citric acid is present in lemons and oranges, contributing to their sour taste, but vinegar is not defined as a citric acid solution. Step 7: Therefore, the correct description of vinegar is that it is diluted acetic acid in water. Verification / Alternative check: Food labelling regulations in many countries require vinegar products to list acetic acid as the acidic component. For example, the label may state that the vinegar contains 5 percent acetic acid. Chemistry laboratory tests, such as neutralisation titrations, can be used to determine the concentration of acetic acid in vinegar samples. The characteristic smell of vinegar is also associated specifically with acetic acid. No standard reference describes vinegar as a formic acid or citric acid solution, confirming that dilute acetic acid is the correct answer. Why Other Options Are Wrong: Option B, glacial acetic acid, is concentrated acetic acid with minimal water and is used in industry and laboratories but not directly as kitchen vinegar. Option C, glacial formic acid, is a different acid and is not used for ordinary vinegar. Option D, diluted formic acid, would have a different smell and taste and is not the traditional culinary vinegar. Option E, diluted citric acid, describes citrus based sour solutions, not standard vinegar. Only option A correctly identifies vinegar as diluted acetic acid in water. Common Pitfalls: A common mistake is to confuse acetic acid with other carboxylic acids because they share similar sour tastes and can all be present in foods. Another pitfall is misunderstanding the term glacial and thinking that glacial acetic acid must be the same as ordinary vinegar. To avoid confusion, remember that vinegar is always associated with acetic acid, and that glacial simply refers to the concentrated, almost pure form of ethanoic acid. Table vinegar is always a diluted solution, not the glacial form. Final Answer: Vinegar used in kitchens is essentially Diluted acetic acid (ethanoic acid) in water, not concentrated acid or other types of organic acids.

Question 19

Which of the following statements about oxides of metals and non metals is correct according to general periodic trends?

Accepted Answer

Introduction / Context: Oxides are compounds formed when elements combine with oxygen. Their behaviour in water and with acids or bases depends strongly on whether the parent element is a metal or a non metal. Recognising the general patterns for metal oxides and non metal oxides is essential for predicting acid base reactions, understanding environmental issues like acid rain and answering basic periodic table questions. This question asks which statement correctly describes the typical behaviour of non metal oxides with respect to acidity. Given Data / Assumptions: We are considering simple oxides of typical metals and non metals. Metal oxides such as Na2O, CaO and MgO are often basic oxides. Non metal oxides such as CO2, SO2 and P2O5 are often acidic oxides. We are focusing on general trends, not rare exceptions. Acidic oxides form acids when dissolved in water, while basic oxides form bases. Concept / Approach: Most metal oxides are basic; they react with water to form metal hydroxides, which are bases, and they neutralise acids. In contrast, many non metal oxides are acidic; they react with water to form oxyacids and neutralise bases. For example, CO2 reacts with water to form carbonic acid, and SO3 reacts with water to form sulfuric acid. A few oxides are amphoteric or neutral, but the general rule in school chemistry is that non metal oxides are acidic. The correct approach is to identify the statement consistent with this rule, which is that most non metal oxides are acidic in water. Step-by-Step Solution: Step 1: Consider examples of non metal oxides such as CO2, SO2, SO3 and P2O5. These all form acids when dissolved in water. Step 2: Carbon dioxide and water give carbonic acid, H2CO3. Sulfur dioxide and water give sulfurous acid, H2SO3, while sulfur trioxide and water give sulfuric acid, H2SO4. Step 3: These reactions show that non metal oxides are generally acidic in nature. Step 4: Metal oxides such as Na2O and CaO react with water to produce bases like NaOH and Ca(OH)2, which turn red litmus blue and neutralise acids, showing basic character. Step 5: Therefore, it is incorrect to say that metal oxides are acidic in general, and it is also wrong to claim that all oxides are neutral. Step 6: Among the options, the statement that most oxides of non metals are acidic in water is the correct general rule. Verification / Alternative check: Laboratory tests confirm these trends. Passing CO2 through water and testing with litmus shows that the resulting solution is acidic. Similarly, solutions prepared from sulfur dioxide show acidic behaviour. Metal oxides such as sodium oxide, when dissolved or reacted with water, produce strongly basic solutions. Environmental chemistry also supports this pattern, since many non metal oxide emissions, such as SO2 and NO2, contribute to acid rain by forming acids in the atmosphere. Textbooks summarise these observations by stating that metal oxides are usually basic and non metal oxides are usually acidic, with a few amphoteric exceptions like aluminium oxide. Why Other Options Are Wrong: Option A claims that most oxides of non metals are basic, which is the opposite of the observed trend. Option C states that most oxides of metals are acidic, which contradicts the fact that metal oxides usually form bases and neutralise acids. Option D claims that all oxides are neutral, which is clearly false because many oxides form acids or bases in water. Option E says that all the statements are correct, which cannot be true since options A, C and D are incorrect. Only option B accurately states that most oxides of non metals are acidic in water. Common Pitfalls: Students sometimes memorise that metal equals basic and non metal equals acidic but then mix up which oxides correspond to which behaviour. Another issue is forgetting that there are exceptions such as amphoteric oxides, but examination questions like this one usually ask for the general rule. To avoid confusion, associate metal oxides with basic solutions and non metal oxides with acidic solutions, and remember that these patterns are especially strong among s block metals and p block non metals. Final Answer: The correct statement is that Most oxides of non metals are acidic in water, forming acids when dissolved and neutralising bases.

Question 20

Which of the following is a well known natural source of oxalic acid found in foods and plants?

Accepted Answer

Introduction / Context: Many organic acids occur naturally in foods and plants, giving them characteristic sour tastes and influencing their nutritional properties. Oxalic acid is one such acid, known for forming insoluble salts with calcium and contributing to kidney stone formation when consumed in large amounts. Recognising natural sources of oxalic acid helps in food science and health related discussions. This question asks you to select a natural source that is correctly associated with oxalic acid, distinguishing it from sources of other common organic acids. Given Data / Assumptions: Oxalic acid occurs naturally in certain vegetables and fruits. Sour milk contains lactic acid produced by bacterial fermentation. Ant stings contain formic acid, which causes the burning sensation. Tamarind is rich in tartaric acid, used in some culinary and industrial applications. Lemon and citrus fruits contain citric acid as the main organic acid. Concept / Approach: Oxalic acid is commonly found in vegetables such as spinach, rhubarb and tomatoes, as well as in some other plant sources. It can combine with calcium to form sparingly soluble calcium oxalate. Other listed food items are better known for different acids: sour milk for lactic acid, ant sting for formic acid, tamarind for tartaric acid and lemons for citric acid. The approach is to match oxalic acid with its characteristic food and plant sources, especially tomatoes and certain leafy vegetables, and not confuse it with the acids present in other familiar substances. Step-by-Step Solution: Step 1: Recall that oxalic acid is present in vegetables like spinach, tomatoes and rhubarb and that high intake can contribute to kidney stone risk in susceptible individuals. Step 2: Tomatoes are often mentioned in school chemistry as a natural source of oxalic acid, along with some leafy vegetables. Step 3: Sour milk is produced when lactose is fermented by bacteria into lactic acid, not oxalic acid. Step 4: Ant stings contain formic acid (methanoic acid), which causes pain and irritation at the sting site. Step 5: Tamarind pulp contains a high proportion of tartaric acid, which provides its sharp sour taste. Step 6: Lemon juice is rich in citric acid, the main acid in citrus fruits. Step 7: Therefore, the correct natural source of oxalic acid among the options is tomato and some leafy vegetables. Verification / Alternative check: Nutrition and food chemistry references list oxalic acid content for various vegetables. Spinach, beet greens, Swiss chard, rhubarb and tomatoes are consistently identified as high oxalate foods. In contrast, lactic acid is associated with fermented milk products, formic acid with insects such as ants and bees, tartaric acid with grapes and tamarind and citric acid with citrus fruits. These associations are used in both health advice and food processing. This independent information confirms that tomatoes and certain vegetables are indeed recognised natural sources of oxalic acid. Why Other Options Are Wrong: Option B, sour milk, is dominated by lactic acid, not oxalic acid. Option C, ant stings, involve formic acid as the main active acid. Option D, tamarind, is well known for its tartaric acid content rather than oxalic acid. Option E, lemon juice, is characterised by citric acid and is used as a standard example in acidity discussions. None of these are identified primarily as oxalic acid sources in basic chemistry teaching. Only option A correctly points to tomato and some leafy vegetables as natural sources of oxalic acid. Common Pitfalls: Students may confuse different organic acids because many foods taste sour and because names like tartaric, citric and oxalic sound similar. A useful memory aid is to pair each acid with a key natural source: citric with citrus fruits, tartaric with tamarind and grapes, lactic with milk products, formic with ant stings and oxalic with certain vegetables including tomatoes and spinach. Organising information this way helps avoid mixing them up in exam questions. Final Answer: A well known natural source of oxalic acid is Tomato and some leafy vegetables, which contain measurable amounts of this dicarboxylic acid.

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