Question 1

In heat transfer and everyday materials science, which one of the following is a poor (bad) thermal conductor compared with the others?

Accepted Answer

Introduction / Context: Thermal conductivity is a measure of how easily heat flows through a material. Good thermal conductors quickly transfer heat, while bad (poor) conductors act as insulators and slow down heat flow. Understanding which common materials conduct heat well helps explain why certain metals are used for cooking utensils and why some materials are used for insulation or window panes. This question asks you to identify which listed material is a poor conductor of heat. Given Data / Assumptions: Aluminium, copper, and silver are all metals. Glass is a non metallic, brittle solid used in windows and containers. Metals are generally good conductors of heat and electricity. Non metals like glass are usually much poorer conductors of heat. Concept / Approach: Metals conduct heat efficiently due to the presence of free electrons that can move through the lattice and transfer energy. Among metals, silver and copper are especially known for their excellent thermal and electrical conductivity; aluminium is also a good conductor, commonly used in cookware and heat sinks. Glass, however, lacks free electrons and relies on lattice vibrations (phonons) for heat transfer, making it a relatively poor conductor. As a result, glass and similar materials are often used where moderate insulation is acceptable, such as in windows or laboratory apparatus that should not heat up too quickly. Step-by-Step Solution: Step 1: Identify the metals in the list. Aluminium, copper, and silver are all metallic elements and known good conductors of heat. Step 2: Recall their common uses. Copper and silver are used in electrical wiring and high quality cookware due to their excellent conductivity. Aluminium is used in pans, pots, and heat sinks because it transfers heat efficiently. Step 3: Examine glass. Glass is an amorphous or partially crystalline non metal; it does not conduct heat as well as metals and is considered a poor thermal conductor. Step 4: Conclude that glass is the bad conductor in the given list. Verification / Alternative check: Practical experience supports this classification. A metal spoon placed in hot tea becomes hot quickly along its length, showing good thermal conductivity. A glass handle or a glass tumbler heats much more slowly and is often comfortable to hold even when containing a warm drink, indicating poorer heat transfer. Engineering data also show that thermal conductivity values for metals like silver, copper, and aluminium are orders of magnitude higher than those for glass. This confirms that glass is a bad conductor of heat relative to the listed metals. Why Other Options Are Wrong: Option A (Aluminium): Aluminium is widely used where good heat conduction is needed, such as in cookware and radiators. Option B (Copper): Copper is one of the best thermal conductors and is commonly used in high performance heat transfer applications. Option D (Silver): Silver is one of the best conductors of heat and electricity known and cannot be classified as a poor thermal conductor. Common Pitfalls: Students may confuse thermal and electrical conductivity or think that any brittle solid like glass must be a good conductor because it is hard. Another confusion comes from the fact that glass can feel cold to the touch, which is more about surface temperature than conductivity. Remember that metals with free electrons are generally excellent conductors, while non metallic solids like glass, wood, and plastics are typically insulators or poor conductors of heat. This distinction is fundamental in both physics and everyday technology. Final Answer: Among the given materials, the bad thermal conductor is Glass.

Question 2

In environmental chemistry, release of which of the following substances into the atmosphere is considered the primary cause of depletion of the stratospheric ozone layer?

Accepted Answer

Introduction / Context: The stratospheric ozone layer acts as a protective shield, absorbing much of the Sun's harmful ultraviolet radiation and reducing the risk of skin cancer and other UV related damage. In the late 20th century, scientists discovered that certain man made chemicals were thinning this layer, causing the so called ozone hole. Understanding which substances are primarily responsible for ozone depletion is an important part of environmental chemistry and policy awareness. Given Data / Assumptions: The options include nitrous oxide, hydrogen dioxide, chlorofluro carbon (chlorofluorocarbon, CFC), and carbon monoxide. Chlorofluorocarbons are stable compounds used historically in refrigeration, aerosols, and foam blowing. We focus on the primary, historically recognised cause of ozone layer depletion. We assume typical exam spelling of chlorofluorocarbon as given in the option text. Concept / Approach: Chlorofluorocarbons (CFCs) are compounds containing chlorine, fluorine, and carbon. They are chemically inert in the lower atmosphere but eventually diffuse into the stratosphere, where ultraviolet light breaks them down, releasing chlorine atoms. These chlorine atoms participate in catalytic cycles that destroy ozone molecules, converting them into oxygen and thus thinning the ozone layer. Nitrous oxide, carbon monoxide, and water vapour (sometimes loosely called hydrogen dioxide in error) have other environmental impacts but are not considered the primary cause of the ozone hole in standard exam explanations. Step-by-Step Solution: Step 1: Recall which gases are historically associated with the ozone hole. CFCs, halons, and related chlorine and bromine containing compounds are known to damage the ozone layer. Step 2: Identify the CFC like option. The option "Chlorofluro carbon" refers to chlorofluorocarbon (CFC). Step 3: Consider other gases in the list. Nitrous oxide is a greenhouse gas and can affect ozone chemistry but is not the primary historical cause taught in textbooks. Carbon monoxide mainly affects local air quality and is not the key ozone depleting substance. Hydrogen dioxide is another name for water, which plays different roles in the atmosphere and is not the main ozone depleting agent. Verification / Alternative check: International agreements such as the Montreal Protocol specifically target CFCs and related halogenated compounds because they were identified as the major cause of ozone depletion. Scientific models and observations over Antarctica showed a strong correlation between CFC emissions and ozone layer thinning. While other gases may influence atmospheric chemistry, CFCs and their breakdown products are consistently singled out as the primary culprits in both scientific and textbook accounts. This historical and regulatory focus confirms that chlorofluorocarbons are correctly identified as the main cause. Why Other Options Are Wrong: Option A (Nitrous oxide): It is a greenhouse gas and can participate in some ozone related reactions, but it is not the classical primary cause of ozone layer depletion discussed in school level exams. Option B (Hydrogen dioxide): This is essentially water (H2O) and is not responsible for the observed large scale depletion of stratospheric ozone. Option D (Carbon monoxide): CO is a toxic gas affecting local air quality and human health but is not a significant ozone depleting substance in the stratosphere. Common Pitfalls: A common mistake is to confuse greenhouse gases that contribute to global warming (such as CO2, CH4, and N2O) with ozone depleting substances. Another confusion arises from the similar sounding names of various atmospheric pollutants. To avoid this, remember that the ozone hole problem led directly to the ban and phase out of CFCs under the Montreal Protocol, making chlorofluorocarbons the key substances blamed for ozone depletion in exam questions. Final Answer: The primary reason for depletion of the ozone layer is the release of Chlorofluro carbon (chlorofluorocarbon, CFC) compounds into the atmosphere.

Question 3

In basic covalent bonding, which of the following diatomic molecules is held together by a double covalent bond between its two atoms?

Accepted Answer

Introduction / Context: Diatomic molecules consist of two atoms chemically bonded together. Depending on the number of shared electron pairs between the atoms, these molecules may contain single, double, or triple covalent bonds. Knowing the type of bond in common diatomic molecules such as Cl2, O2, and N2 helps in understanding bond strength, bond length, and reactivity. This question asks you to identify which molecule specifically has a double covalent bond. Given Data / Assumptions: The molecules listed are Cl2, O2, N2, and He2. Covalent bonds involve sharing of electron pairs between atoms. Single bonds share one pair, double bonds share two pairs, and triple bonds share three pairs of electrons. We assume ground state, stable molecular forms as commonly taught in school chemistry. Concept / Approach: Chlorine (Cl2) is a diatomic molecule in which each chlorine atom shares one electron with the other, forming a single covalent bond. Oxygen (O2) has a double bond: two pairs of electrons are shared between the oxygen atoms, resulting in a typical O=O bond. Nitrogen (N2) is bound by a triple bond, sharing three electron pairs, making it very stable and relatively inert. Helium is a noble gas that does not readily form stable diatomic molecules like He2 under normal conditions. Hence, among the given options, O2 is the molecule that has a double covalent bond between its atoms. Step-by-Step Solution: Step 1: Consider chlorine, Cl2. Each chlorine atom has 7 valence electrons and shares 1 electron with the other, forming a single bond Cl–Cl. Step 2: Consider oxygen, O2. Each oxygen atom has 6 valence electrons and needs 2 more to complete an octet; they share 2 pairs, forming a double bond O=O. Step 3: Consider nitrogen, N2. Each nitrogen atom has 5 valence electrons and forms a triple bond N≡N by sharing 3 pairs of electrons. Step 4: Consider helium, He2. Helium already has a stable duet of electrons; it does not need to form bonds, and He2 is not a stable covalent molecule under normal conditions. Step 5: Select the molecule with a double bond: O2. Verification / Alternative check: Lewis dot structures and valence bond diagrams consistently show O2 as having a double bond between the two oxygen atoms, with each atom achieving an octet. Bond order tables list O2 with bond order 2, N2 with bond order 3, and Cl2 with bond order 1. Spectroscopic and structural data, such as bond lengths and energies, further support this: the O=O double bond is shorter and stronger than a single bond but weaker than the N≡N triple bond. Helium, being a noble gas with a filled valence shell, does not form such bonds. This evidence clearly confirms that O2 is the double bonded molecule among the choices. Why Other Options Are Wrong: Option A (Cl2): Chlorine forms a single covalent bond between its atoms, not a double bond. Option C (N2): Nitrogen forms a triple covalent bond, one of the strongest covalent bonds known. Option D (He2): Helium does not form a stable diatomic molecule under normal conditions; it remains monatomic due to its full valence shell. Common Pitfalls: A frequent mistake is to assume that all diatomic molecules have single bonds, which is not true for O2 and N2. Another confusion comes from misremembering whether oxygen or nitrogen has the triple bond. A useful memory aid is that N2, with 5 valence electrons, needs to share 3 pairs to reach an octet, leading to a triple bond, while O2, with 6 valence electrons, needs to share 2 pairs, leading to a double bond. Keeping this valence electron counting method in mind helps avoid such errors. Final Answer: The diatomic molecule that is joined by a double covalent bond is O2.

Question 4

In basic nuclear physics, the force that a neutron exerts on a proton inside an atomic nucleus is primarily which fundamental force?

Accepted Answer

Introduction / Context: Inside an atomic nucleus, protons and neutrons are bound together extremely tightly, even though protons are all positively charged and should repel each other strongly. This question tests understanding of the fundamental interactions that operate at nuclear distances and asks specifically which force a neutron exerts on a proton inside the nucleus. Knowing the correct force helps students distinguish between gravity, electrostatic interactions, and the special short range forces that hold nuclei together in nuclear physics. Given Data / Assumptions: A neutron and a proton are considered inside the same atomic nucleus. The distance between nucleons inside the nucleus is extremely small, on the order of 10^-15 m. We consider the four fundamental interactions: gravitational, electrostatic, strong nuclear, and weak nuclear forces. The neutron is electrically neutral, and the proton carries a positive charge. Concept / Approach: The key idea is that the stability of the atomic nucleus is due to the strong nuclear interaction, also called the nuclear force, which acts between nucleons (protons and neutrons). Although gravitational and electrostatic forces are always present in nature, at nuclear scales they are extremely weak compared to the strong nuclear force. Since the neutron is uncharged, it cannot exert an electrostatic force on the proton. Therefore the main attractive interaction that binds the neutron and proton together is the strong nuclear force. Step-by-Step Solution: Step 1: Recall that neutrons and protons together are called nucleons and are held inside the nucleus by a very strong short range force. Step 2: Note that the neutron has no net electric charge, so it cannot cause a direct electrostatic attraction or repulsion with the positively charged proton. Step 3: Consider gravitational force. Gravity acts between all masses, but for subatomic particles its magnitude is extremely tiny compared to other interactions and can be ignored in nuclear binding. Step 4: Understand that the strong nuclear force (nuclear force) acts between nucleons regardless of their electric charge, is highly attractive at typical nuclear separations, and overcomes electrostatic repulsion between protons. Step 5: Conclude that the dominant force between a neutron and a proton inside the nucleus is the nuclear force, not gravitational, electrostatic, tidal, or magnetic forces. Verification / Alternative check: An alternative way to check the answer is to compare magnitudes. The strong nuclear force is roughly 10^38 times stronger than gravity at nuclear distances and far stronger than any magnetic or residual electromagnetic interactions. Since nuclei are stable only because of this extremely strong, short range attraction, any realistic description of neutron proton binding inside the nucleus must involve the nuclear force. This confirms that the correct interaction is the nuclear force. Why Other Options Are Wrong: Gravitational force: It exists between all masses but is far too weak to bind nucleons together inside a tiny nucleus. Electrostatic force: A neutron has no charge, so there is no direct electrostatic attraction or repulsion between a neutron and a proton. Tidal force: This term is used mainly in astrophysics for gravitational effects over distances, not for binding subatomic particles. Magnetic force: Nucleons do have magnetic moments, but magnetic interactions are again much weaker than the strong nuclear force in the nucleus. Common Pitfalls: Many learners mistakenly think that gravity is responsible for all attractions between masses, even at the atomic level. Others may assume that any interaction involving a charged particle must be electrostatic. It is also common to confuse nuclear force with electrostatic force between protons, forgetting that the neutron is neutral. Remember that the strong nuclear force is a distinct fundamental interaction that dominates at nuclear scales, while gravity and magnetic effects are negligible there. Final Answer: The neutron mainly exerts the nuclear force on the proton inside an atomic nucleus.

Question 5

For the molecule ethene (C2H4), which description correctly represents its electron dot and cross (Lewis) structure and the bonding around each carbon atom?

Accepted Answer

Introduction / Context: This question focuses on the electron dot and cross structure, also called the Lewis structure, of the organic molecule ethene with formula C2H4. Ethene is a simple unsaturated hydrocarbon containing a carbon carbon double bond. Understanding its bonding helps students visualise sigma and pi bonds, valence electrons, and the octet rule as applied to carbon atoms in organic chemistry. Given Data / Assumptions: Molecule: ethene, molecular formula C2H4. Each carbon atom has four valence electrons. Each hydrogen atom has one valence electron and forms one covalent bond. The electron dot and cross structure must show how electrons are shared to satisfy the octet rule for carbon. Concept / Approach: In ethene, the two carbon atoms are bonded by a double bond. A double bond consists of one sigma bond and one pi bond. Each carbon atom in ethene is sp2 hybridised, forms three sigma bonds, and uses remaining unhybridised p orbitals to form a shared pi bond. The electron dot and cross representation must show that each carbon is bonded to two hydrogens by single covalent bonds and to the other carbon by a double covalent bond, giving each carbon a total of eight shared electrons in its valence shell. Step-by-Step Solution: Step 1: Count valence electrons. Each carbon contributes 4 electrons, and each hydrogen contributes 1 electron. Total electrons = 2*4 + 4*1 = 12 valence electrons. Step 2: Place the two carbon atoms in the centre, since carbon is more electronegative than hydrogen and usually forms the skeleton of the molecule. Step 3: Connect each carbon atom to two hydrogen atoms using single covalent (sigma) bonds, and connect the two carbon atoms to each other. Step 4: To satisfy the octet rule, add another shared pair of electrons between the two carbon atoms so that there is a double bond. Each carbon now has three sigma bonds (two C H and one C C) and one pi bond (the second bond in the C C double bond). Step 5: Check octets. Around each carbon there are 8 shared electrons (4 pairs) and around each hydrogen there are 2 electrons (1 pair), so all atoms attain stable configurations. Verification / Alternative check: An alternative way to confirm the structure is to use valence bond theory and hybridisation. Each carbon in ethene is sp2 hybridised, which predicts three sigma bonds arranged in a trigonal planar geometry and one unhybridised p orbital forming a pi bond. This exactly matches the description that each carbon forms three sigma bonds and one pi bond. The electron dot and cross diagram that reflects this shows two shared pairs between the carbons and one shared pair between each carbon and each hydrogen. Why Other Options Are Wrong: Option B: Four single bonds to hydrogen and no C C double bond would correspond to ethane (C2H6), not ethene, and would use the wrong number of hydrogen atoms. Option C: A single bond between the carbons leaves each carbon short of an octet and does not match the known double bond in ethene. Option D: A triple bond with only one hydrogen on one carbon describes a different molecule with formula C2H2, similar to ethyne, not C2H4. Option E: Delocalised electrons over all six atoms would suggest an aromatic system like benzene, which is not the case for ethene. Common Pitfalls: Students sometimes confuse ethene with ethane or ethyne and assign the wrong number of bonds between carbons. Another common mistake is to forget that carbon must have four bonds in total and that ethene must contain a carbon carbon double bond, not a single or triple bond. Learners may also misuse the electron dot notation and fail to count total valence electrons correctly. Careful counting and applying the octet rule helps avoid these errors. Final Answer: The correct description is that each carbon atom forms three sigma bonds (two with hydrogen atoms and one with the other carbon atom) and one pi bond between the two carbons, giving each carbon a complete octet.

Question 6

The atoms of the elements germanium, arsenic, selenium and bromine each contain how many electron shells according to their positions in the modern periodic table?

Accepted Answer

Introduction / Context: This question checks understanding of how the periodic table is organised into periods and how the period number relates directly to the number of electron shells in an atom. Germanium, arsenic, selenium and bromine are a sequence of elements located in the same period. Recognising their common period allows us to deduce how many electron shells each atom possesses in its ground state electronic configuration. Given Data / Assumptions: Elements considered: germanium (Ge), arsenic (As), selenium (Se) and bromine (Br). We use the modern long form periodic table. Each period in the periodic table corresponds to a principal quantum number n and to the number of occupied electron shells. All four elements are placed in the same horizontal row (period). Concept / Approach: In the modern periodic table, the period number tells us how many electron shells are occupied in the ground state of an element. Elements in period 1 have 1 shell, elements in period 2 have 2 shells, and so on. Germanium, arsenic, selenium, and bromine are in period 4. Therefore, each of these atoms has electrons occupying four principal energy levels or shells (K, L, M and N shells). Once we identify the period, we can answer the question without detailed electronic configurations. Step-by-Step Solution: Step 1: Locate germanium (Ge), arsenic (As), selenium (Se), and bromine (Br) on the modern periodic table. Step 2: Observe that all four elements lie in the same horizontal row, which is period 4. Step 3: Recall the rule that the period number is equal to the number of electron shells occupied by electrons in the ground state of the element. Step 4: Since the period number is 4, conclude that each of these elements has four electron shells (n = 1, 2, 3, and 4) with electrons. Step 5: Therefore, the number of shells in each of the given atoms is 4. Verification / Alternative check: A more detailed verification can be done by writing approximate electronic configurations. For example, germanium has configuration [Ar] 3d10 4s2 4p2, which clearly uses four shells (levels 1, 2, 3, and 4). Similarly, arsenic has [Ar] 3d10 4s2 4p3, selenium has [Ar] 3d10 4s2 4p4, and bromine has [Ar] 3d10 4s2 4p5. In every case electrons occupy up to the fourth shell. This confirms that each atom has four electron shells. Why Other Options Are Wrong: 3 shells: This would correspond to elements in period 3, such as sodium or chlorine, not to the period 4 elements listed in the question. 5 shells: That would apply to period 5 elements like rubidium or iodine, not to germanium, arsenic, selenium and bromine. 7 shells: This is characteristic of very heavy elements in period 7 and is far above the given elements. 6 shells: This would correspond to period 6 elements, again not matching the positions of the elements in the question. Common Pitfalls: A frequent mistake is to confuse group number with period number or to assume that the number of valence electrons equals the number of shells. Another error is to memorise configurations without linking them back to the periodic table arrangement. To avoid this, always remember that periods run horizontally and indicate the number of shells, while groups run vertically and indicate the pattern of valence electrons. Focusing on the period makes questions like this straightforward. Final Answer: Each of the atoms of germanium, arsenic, selenium, and bromine contains 4 electron shells.

Question 7

Give an example of a salt which, when dissolved in water, produces an aqueous solution with pH less than 7 and therefore behaves as an acidic salt solution.

Accepted Answer

Introduction / Context: This question examines knowledge about acidic and basic salts and how they behave in aqueous solution. Some salts, when dissolved in water, give solutions with pH less than 7, while others yield neutral or basic solutions. The correct example here must be a salt that, due to hydrolysis of its ions, produces an acidic solution. This concept is important in general chemistry and environmental chemistry because many natural waters contain such salts. Given Data / Assumptions: We are looking for a salt that gives an aqueous solution with pH less than 7. The options include common salts of strong and weak acids and bases. Water is assumed to be pure and at standard conditions so that pH 7 is neutral. Hydrolysis of ions in solution determines whether the solution is acidic, basic, or neutral. Concept / Approach: A salt formed from a strong base and a weak acid will generally give an alkaline solution, because the conjugate base of the weak acid hydrolyses and produces hydroxide ions. A salt formed from a weak base and a strong acid will give an acidic solution, because the conjugate acid of the weak base hydrolyses to produce hydronium ions. A salt formed from a strong acid and a strong base will usually give a neutral solution. Ammonium chloride, NH4Cl, is formed from a weak base (ammonia) and a strong acid (hydrochloric acid), so its solution is acidic. Step-by-Step Solution: Step 1: Identify the acid and base that form each salt. Step 2: For ammonium chloride, note that it is formed from ammonia (a weak base) and hydrochloric acid (a strong acid). Step 3: In water, the ammonium ion NH4+ undergoes hydrolysis, releasing H+ (or H3O+) ions and lowering the pH below 7. Step 4: For sodium carbonate and potassium carbonate, recognise that they come from a strong base (NaOH or KOH) and a weak acid (carbonic acid), so their solutions are basic, not acidic. Step 5: For sodium bicarbonate, the hydrolysis is partial and the solution is mildly alkaline, with pH slightly above 7. Step 6: Sodium chloride comes from a strong acid (HCl) and a strong base (NaOH) and gives an almost neutral solution. Verification / Alternative check: The pH of a typical 0.1 molar ammonium chloride solution is around 5.5, clearly below 7, confirming its acidic nature. In contrast, a similar concentration of sodium carbonate gives a pH around 11, and sodium bicarbonate solutions are slightly basic with pH above 7. Sodium chloride solutions are close to neutral with pH near 7. These approximate values confirm that ammonium chloride is the only option among the given salts that produces an acidic solution. Why Other Options Are Wrong: Sodium carbonate: Salt of a strong base and a weak acid, so it hydrolyses to give an alkaline solution with pH greater than 7. Sodium bicarbonate: Also related to carbonic acid and a strong base and generally gives a slightly basic solution. Sodium chloride: Formed from a strong acid and a strong base, giving an essentially neutral solution. Potassium carbonate: Similar to sodium carbonate, this salt yields a basic solution due to hydrolysis of the carbonate ion. Common Pitfalls: Students often think that any salt containing chloride or carbonate must be neutral like common kitchen salt. Others may misinterpret bicarbonate as acidic because it can neutralise acids, forgetting that its solution is actually mildly basic. To avoid confusion, always analyse whether the parent acid and base are strong or weak and then predict the behaviour of the solution based on the hydrolysis of the ions. Final Answer: The salt that gives an aqueous solution of pH less than 7 is Ammonium chloride.

Question 8

The mass number of an isotope of an element is 298. If its nucleus contains 189 neutrons, what is the atomic number of this element, that is, the number of protons in its nucleus?

Accepted Answer

Introduction / Context: This numerical chemistry question tests understanding of basic nuclear terminology, specifically the relationship between mass number, atomic number, and the number of neutrons in an atomic nucleus. Being able to calculate one of these quantities from the other two is essential in nuclear chemistry, radioactivity, and isotopic notation. Here we are given the mass number and the number of neutrons and asked to determine the atomic number. Given Data / Assumptions: Mass number of the isotope, A = 298. Number of neutrons in the nucleus, N = 189. Atomic number Z is defined as the number of protons in the nucleus. Mass number A = Z + N for any nuclide. Concept / Approach: The central concept is that mass number A is the sum of the number of protons and neutrons in the nucleus. In symbols, A = Z + N. The atomic number Z represents the number of protons and defines the identity of the element. If we know A and N, we can rearrange the formula to find Z using Z = A - N. This straightforward relationship allows us to calculate the atomic number directly from the provided data. Step-by-Step Solution: Step 1: Write down the relationship between mass number, atomic number, and number of neutrons: A = Z + N. Step 2: Substitute the known values into the equation. Here A = 298 and N = 189. Step 3: Rearrange to solve for Z: Z = A - N. Step 4: Perform the subtraction: Z = 298 - 189. Step 5: Calculate the result: 298 - 189 = 109. Step 6: Conclude that the atomic number of this isotope is 109, meaning it has 109 protons in its nucleus. Verification / Alternative check: As a check, add the computed atomic number back to the given number of neutrons and confirm that the result equals the mass number. Using Z = 109 and N = 189 gives A = 109 + 189 = 298, which matches the original mass number. This confirms that the calculation is correct. In addition, an atomic number of 109 corresponds to a very heavy, artificially produced element in the transactinide region, which is reasonable for such a high mass number. Why Other Options Are Wrong: 298: This is the mass number, not the atomic number. It represents the total number of protons and neutrons, not just the number of protons. 189: This is the number of neutrons, not the atomic number. 487: This is the result of adding rather than subtracting the numbers (298 + 189), which is not how mass number is defined. 199: This is an arbitrary number with no simple relation to the given data and does not satisfy A = Z + N. Common Pitfalls: Students sometimes confuse mass number with atomic number and may select 298 by mistake. Another common error is to add instead of subtracting when rearranging the formula. It is important to remember that mass number already includes both protons and neutrons, so to obtain the number of protons you must subtract the number of neutrons. Writing the formula A = Z + N clearly and checking units can help avoid such errors. Final Answer: The atomic number of the element is 109, which means its nucleus contains 109 protons.

Question 9

Which of the following common substances is correctly classified as a deliquescent compound that absorbs moisture from the air until it forms an aqueous solution?

Accepted Answer

Introduction / Context: Deliquescent substances are important in both laboratory practice and industry because they can absorb moisture from the atmosphere and eventually dissolve in the absorbed water to form a solution. This question asks you to identify which substance among the options behaves in this characteristic way. Understanding deliquescence helps in storage of chemicals and in designing drying agents or humidity control systems. Given Data / Assumptions: We are asked for a deliquescent compound, not merely a hygroscopic or drying agent. Deliquescent substances absorb so much moisture that they eventually form a liquid solution. The options include several well known laboratory chemicals. Normal atmospheric humidity is assumed. Concept / Approach: Deliquescent compounds are usually ionic solids with strong affinity for water. When exposed to moist air, they absorb water vapour continuously until they dissolve in the absorbed water and form a concentrated solution. Magnesium chloride and certain other salts such as calcium chloride show this behaviour. Some substances, like phosphorous pentoxide, are strongly hygroscopic and can remove water but do not necessarily form a stable solution in the same way. Quick lime reacts chemically with water, while common salt is not significantly deliquescent under ordinary conditions. Step-by-Step Solution: Step 1: Recall the definition of deliquescence: a solid absorbs moisture from air to such an extent that it dissolves in the absorbed water and forms a solution. Step 2: Consider magnesium chloride. It is known to absorb water vapour readily and can turn into a liquid brine when left exposed to humid air. It is therefore a classic example of a deliquescent compound. Step 3: Quick lime (calcium oxide) reacts with water to form calcium hydroxide and does not simply dissolve by absorbing moisture. This is more of a chemical reaction than typical deliquescence. Step 4: Sulphuric acid is a liquid that is highly hygroscopic, but the question is clearly focused on compounds commonly discussed as deliquescent solids. Step 5: Phosphorous pentoxide is a very strong drying agent that forms phosphoric acids when it reacts with water; it is hygroscopic rather than a traditional deliquescent salt that forms a simple aqueous solution. Step 6: Sodium chloride is relatively stable in air and does not normally absorb enough moisture at ordinary humidity to dissolve by itself. Verification / Alternative check: An alternative check is to consult typical lists of deliquescent substances from school level chemistry, which commonly mention magnesium chloride, calcium chloride, and zinc chloride as examples. These salts are troublesome to store because they liquefy in moist air. Quick lime and phosphorous pentoxide are usually listed as hygroscopic or drying agents rather than deliquescent salts. This supports the choice of magnesium chloride as the correct answer. Why Other Options Are Wrong: Quick lime: It reacts with water to form calcium hydroxide; this is slaking, not simple deliquescence. Sulphuric acid: It is a liquid, strongly hygroscopic, but the question focuses on solid compounds that become solutions. Phosphorous pentoxide: Powerful drying agent, reacts with water to form acids, generally classified as hygroscopic rather than deliquescent in school chemistry. Sodium chloride: Ordinary table salt does not significantly absorb moisture to dissolve under normal conditions. Common Pitfalls: Learners often confuse hygroscopic substances, drying agents, and deliquescent compounds. Not every substance that absorbs water is deliquescent. The key feature of deliquescence is the formation of a liquid solution from a solid because of absorbed moisture. Another pitfall is to assume that all chlorides or all drying agents behave the same way. It is important to remember specific standard examples like magnesium chloride and calcium chloride. Final Answer: The correctly identified deliquescent compound is Magnesium chloride.

Question 10

Which of the following is not considered a subatomic particle of an atom in basic chemistry, that is, it is not one of the fundamental particles found inside atoms?

Accepted Answer

Introduction / Context: The structure of the atom in basic chemistry is built from subatomic particles such as protons, neutrons, and electrons. This question tests the ability to distinguish genuine subatomic particles from whole atoms or other entities. It asks which of the listed options is not a subatomic particle of an atom and therefore should not be considered part of the basic internal structure of a single atom. Given Data / Assumptions: We use the simple model of the atom consisting of protons and neutrons in the nucleus and electrons in surrounding shells. Subatomic particles are smaller than atoms and are constituents of atoms. Some options may refer to elementary particles, while others may refer to entire atoms or chemical elements. We are working at the level of introductory general chemistry. Concept / Approach: Subatomic particles are the building blocks of atoms. In basic chemistry, the three main ones are the proton (positively charged, in the nucleus), neutron (neutral, in the nucleus), and electron (negatively charged, around the nucleus). Other subatomic or elementary particles exist in advanced physics, such as positrons. However, xenon is not a subatomic particle but a complete atom and a noble gas element in the periodic table. Recognising xenon as a chemical element rather than a constituent particle of an atom is the key to answering this question. Step-by-Step Solution: Step 1: Identify known subatomic particles. Protons, neutrons, and electrons are standard subatomic particles present in atoms. Step 2: Recall that positron is an elementary particle, essentially the antiparticle of the electron, and is considered subatomic even though it is not a normal constituent of stable atoms. Step 3: Recognise that xenon is a noble gas element with symbol Xe and atomic number 54. It represents whole atoms, not a component inside atoms. Step 4: Conclude that xenon is not a subatomic particle but an atom of an element itself. Step 5: Therefore, xenon is the correct choice as the entity that is not a subatomic particle of an atom. Verification / Alternative check: To verify, think about where each entity appears in atomic structure diagrams. Protons, neutrons, and electrons are shown inside or around a nucleus. Positrons may appear in nuclear reactions or particle physics diagrams but are at the same scale as electrons. Xenon, however, is drawn as an entire atom with its own nucleus and electron cloud. Additionally, xenon appears as an entry in the periodic table, whereas subatomic particles do not. This confirms that xenon is not a subatomic particle. Why Other Options Are Wrong: Neutron: A neutral subatomic particle found in the nucleus of most atoms and crucial for isotope differences. Electron: A negatively charged subatomic particle that occupies regions around the nucleus and determines chemical bonding. Proton: A positively charged subatomic particle located in the nucleus that defines the atomic number and element identity. Positron: The antiparticle of the electron, present in some nuclear and particle reactions, and considered a subatomic particle even though it is not part of ordinary stable atoms. Common Pitfalls: Learners sometimes confuse the names of elements and particles, especially when options mix both. Another common error is to assume that anything mentioned in physics contexts is a subatomic particle. Remember that chemical elements like xenon, oxygen, or iron refer to atoms or collections of atoms, while subatomic particles are smaller entities that make up the atoms. Keeping this distinction clear helps in answering similar questions correctly. Final Answer: The entity that is not a subatomic particle of an atom is Xenon.

Question 11

In the modern periodic table, how many elements are present in Group 18, the noble gas group commonly discussed in school level chemistry?

Accepted Answer

Introduction / Context: The periodic table is organised into groups (vertical columns) and periods (horizontal rows). Group 18 is the noble gas group, which includes chemically inert gases used in many applications. This question checks knowledge of how many elements belong to Group 18 in the typical school level modern periodic table. Knowing the members of Group 18 helps in understanding trends in reactivity, valence electrons, and uses of noble gases. Given Data / Assumptions: We are using the modern long form periodic table. Group 18 is the noble gas group. At school level, the commonly discussed noble gases are helium, neon, argon, krypton, xenon, and radon. Superheavy elements such as oganesson may not be emphasised in basic curricula. Concept / Approach: Group 18 elements are characterised by having complete valence electron shells, which makes them very stable and mostly chemically inert. The standard noble gases traditionally listed are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). These six elements occupy the Group 18 positions from period 1 through period 6. While a seventh member, oganesson, has been discovered, in many school level treatments the focus remains on the six classic noble gases. Step-by-Step Solution: Step 1: List the known noble gases: helium, neon, argon, krypton, xenon and radon. Step 2: Count these elements. There are six such gases that are commonly studied. Step 3: Recognise that all these gases appear in the same vertical column, Group 18, of the periodic table. Step 4: Therefore conclude that there are six elements in Group 18 at the standard school level of study. Step 5: Select the option that corresponds to the number 6. Verification / Alternative check: To verify, one can draw or consult a modern periodic table and visually inspect Group 18. Starting from the top of the column: helium in period 1, neon in period 2, argon in period 3, krypton in period 4, xenon in period 5 and radon in period 6. Counting them confirms that there are six noble gas elements traditionally included. Although a superheavy element called oganesson with atomic number 118 is also placed in Group 18, its chemistry is highly specialised and often not included in introductory discussions, so the answer remains six in the context of school examinations. Why Other Options Are Wrong: 5: This would omit one of the recognised noble gases and does not match the periodic table. 7: This could be argued only if oganesson is counted explicitly along with the six traditional noble gases, but most basic curricula focus on six. 2: This would only account for helium and neon and ignores the others. 8: There is no basis for eight noble gases in Group 18 in the modern table. Common Pitfalls: Students may be confused by recent discoveries of superheavy elements and think that every group must always have seven or eight entries. Others may mix up the group number with the number of elements in the group. A simple way to avoid mistakes is to memorise the list of noble gases and count them, rather than guessing based on group numbering. Final Answer: There are 6 commonly studied elements in Group 18 of the modern periodic table.

Question 12

Which of the following is an example of a solid sol, that is, a colloidal system in which a solid is dispersed in a solid medium?

Accepted Answer

Introduction / Context: Colloids are mixtures in which one substance is dispersed in another in the form of very small particles. Depending on the physical states of dispersed phase and dispersion medium, colloids are classified into types such as sols, gels, foams and emulsions. This question asks specifically for a solid sol, which is a colloid where both the dispersed phase and the dispersion medium are solids. Identifying correct examples helps strengthen understanding of colloid classification in general chemistry. Given Data / Assumptions: A solid sol is defined as a colloidal system of solid particles dispersed in a solid medium. Each option represents a familiar colloidal or related system. We assume standard textbook definitions of sols, foams, and other colloids. The task is to match the example that fits the solid in solid category. Concept / Approach: To identify a solid sol, recall that in such a system both phases are solid, for example, coloured glass or gemstones where tiny solid particles impart colour to a solid matrix. Milk of magnesia is a sol where solid is dispersed in a liquid. Foam is a gas dispersed in a liquid or solid. Rubber can be treated as a type of solid polymer, often classified as a gel or elastic solid, not a classic solid sol in elementary classification. Fog is a liquid in gas system. Therefore coloured gemstones provide a typical example of solid solid colloids. Step-by-Step Solution: Step 1: Recall the definition of a solid sol: solid dispersed phase in a solid dispersion medium. Step 2: Examine coloured gemstones. Their colour often arises from finely dispersed transition metal oxides or other solid particles embedded in a solid silica or crystal matrix, which fits the definition of a solid sol. Step 3: Consider milk of magnesia. It consists of solid magnesium hydroxide in water, which is a sol but with liquid medium, so it is a liquid sol, not a solid sol. Step 4: Foam refers to gas bubbles dispersed in a liquid or solid, for example shaving foam or foam rubber, which is gas in liquid or gas in solid, not solid in solid. Step 5: Rubber is typically a solid polymer with some internal structure, often treated as a gel, where a liquid or flexible network exists, but it is not usually cited as a textbook example of a solid sol. Step 6: Fog consists of liquid water droplets dispersed in air, a liquid in gas colloid, so it is not a solid sol. Verification / Alternative check: Standard tables of colloid types in school chemistry list several examples. For solid sols, common examples include coloured glass, ruby glass, and certain coloured gemstones where the pigment is solid and the medium is also solid. For sols (solid in liquid), examples include paints and muddy water. For foams, examples include froth and whipped cream. For aerosols, fog is often listed. This comparison confirms that coloured gemstones belong to the solid sol category. Why Other Options Are Wrong: Milk of magnesia: A suspension or sol with solid in liquid, not solid in solid. Foam: Gas in liquid or gas in solid, not solid in solid. Rubber: Usually regarded as a polymeric solid or gel, not a typical solid sol example in introductory texts. Fog: Liquid droplets dispersed in a gas, classified as an aerosol, not a solid sol. Common Pitfalls: Students sometimes try to memorise lists without understanding the underlying definitions, which leads to confusion when examples are slightly unfamiliar. Another common mistake is to focus only on the common name and not on the physical states of the dispersed phase and medium. A better approach is to identify what phase is dispersed and what phase acts as the medium, then classify the colloid accordingly. Doing this makes it clear why coloured gemstones are solid colloids of solid in solid. Final Answer: The correct example of a solid sol is Coloured gemstones.

Question 13

Which metal is mainly responsible for causing the painful Itai-Itai disease in humans as a result of environmental pollution and contaminated water or rice?

Accepted Answer

Introduction / Context: Itai Itai disease is one of the classic examples of a pollution related health disaster described in environmental chemistry and public health studies. The disease name comes from the severe pain suffered by affected people. This question asks which specific metal is responsible for Itai Itai disease, highlighting the importance of understanding heavy metal toxicity and its impact on human health. Given Data / Assumptions: Itai Itai disease is associated with long term exposure to a particular heavy metal. Exposure occurred mainly through contaminated rice and water in certain regions. The options include several heavy metals known for various health effects. We assume standard textbook descriptions of major pollution related diseases. Concept / Approach: Itai Itai disease was first reported in Japan and is primarily linked to chronic cadmium poisoning. Cadmium from mining and industrial discharge contaminated rivers and rice fields. Over many years, people consuming contaminated rice suffered severe damage to bones and kidneys, leading to extreme pain in the joints and spine. Other metals like mercury, lead, and chromium cause different classical syndromes, but the name Itai Itai is specifically associated with cadmium. Step-by-Step Solution: Step 1: Recall the classic environmental disease associations: Minamata disease with mercury, Itai Itai disease with cadmium, and lead poisoning with neurological effects. Step 2: Focus on Itai Itai disease, which is characterised by bone pain, fractures, and kidney problems due to long term cadmium exposure. Step 3: Note that cadmium contamination often occurred through industrial waste released into rivers, which then entered agricultural lands and rice crops. Step 4: Recognise that other metals listed do cause health problems but are linked to different named diseases, not Itai Itai. Step 5: Conclude that cadmium is the metal responsible for Itai Itai disease. Verification / Alternative check: Basic environmental chemistry references describe several famous pollution incidents. Minamata disease in Japan is due to methyl mercury poisoning from industrial discharge into Minamata Bay. In contrast, Itai Itai disease occurred mainly in the Jinzu River basin due to cadmium pollution. The name Itai Itai, meaning "ouch ouch" in Japanese, reflects the intense bone pain caused by cadmium induced osteomalacia and osteoporosis. These associations are consistently reported across textbooks, which confirms that cadmium is the correct answer. Why Other Options Are Wrong: Nickel: Can cause dermatitis and allergic reactions but is not linked to the named Itai Itai disease. Chromium: Especially hexavalent chromium is toxic and carcinogenic but associated with different health issues. Mercury: Responsible for Minamata disease, not Itai Itai, and mainly affects the nervous system. Lead: Causes lead poisoning, particularly in children, leading to neurological and developmental problems, but not Itai Itai disease. Common Pitfalls: Students may mix up the names of pollution related diseases and the metals involved, especially when revising quickly. Confusing Minamata disease with Itai Itai disease is common. A helpful way to remember is to link Minamata with mercury and fish consumption, and Itai Itai with cadmium and severe bone pain from contaminated rice. Organising these associations in a small table during revision can reduce confusion. Final Answer: The metal mainly responsible for Itai Itai disease is Cadmium.

Question 14

Acid rain is primarily caused by atmospheric pollution due to which combination of gaseous oxides released from industrial, vehicular and power plant emissions?

Accepted Answer

Introduction / Context: Acid rain is an important environmental issue discussed in atmospheric chemistry and environmental science. It involves rainfall with a pH significantly lower than 7 due to the presence of strong acids dissolved in raindrops. This question focuses on identifying which gaseous pollutants in the atmosphere are mainly responsible for the formation of acid rain, particularly in industrial and urban regions. Given Data / Assumptions: Acid rain results from acids formed in the atmosphere and then dissolved in rainwater. Common air pollutants include oxides of sulphur, oxides of nitrogen, carbon dioxide, and others. We assume typical industrial and vehicular emission scenarios. The main acids in acid rain are sulphuric acid and nitric acid. Concept / Approach: The key concept is that sulphur dioxide (SO2), sulphur trioxide (SO3), and various nitrogen oxides (NO and NO2, collectively called NOx) are emitted from fossil fuel burning, power stations and vehicle exhausts. In the atmosphere, these oxides react with oxygen and water to form sulphuric acid (H2SO4) and nitric acid (HNO3). These strong acids dissolve in raindrops and lower the pH, producing acid rain. Oxides of carbon such as CO2 mainly cause mild acidity and contribute to acidification of oceans but are not the primary cause of strong acid rain in the classical sense. Step-by-Step Solution: Step 1: Identify the main acidic components of acid rain, which are sulphuric acid and nitric acid. Step 2: Recall that sulphuric acid forms from sulphur oxides, mainly SO2 and SO3, released by burning coal and oil containing sulphur. Step 3: Recall that nitric acid forms from nitrogen oxides produced at high combustion temperatures in engines and power plants. Step 4: Connect the presence of these acids in rainwater to the original gaseous pollutants, namely oxides of nitrogen and sulphur. Step 5: Match this understanding with the options and select the combination that mentions both nitrogen and sulphur oxides. Verification / Alternative check: An alternative way to verify the answer is to remember specific chemical reactions. For example, in the atmosphere, SO2 can be oxidised to SO3, which then reacts with water to form H2SO4. Nitrogen dioxide can react with water to form HNO3 and nitrous acid. These reactions explain why regions with high SO2 and NOx emissions experience more severe acid rain. Environmental reports also list sulphur and nitrogen oxides as the main precursors of acid rain, confirming the choice of oxides of nitrogen and sulphur. Why Other Options Are Wrong: Oxides of carbon and nitrogen: Carbon dioxide contributes some acidity but is not the major cause of strong acid rain. The key combination must explicitly include sulphur oxides too. Oxides of nitrogen and phosphorous: Phosphorus oxides are not generally major atmospheric pollutants causing acid rain. Oxides of nitrogen and methane: Methane is a greenhouse gas and contributes to climate change but is not a direct major source of strong mineral acids in rain. Oxides of carbon and sulphur: This combination leaves out nitrogen oxides, which are essential contributors to nitric acid in acid rain. Common Pitfalls: Students sometimes think that carbon dioxide alone is responsible for acid rain because it can form carbonic acid in water. However, typical acid rain associated with industrial pollution is due mainly to strong acids from sulphur and nitrogen oxides, not weak carbonic acid. Another pitfall is to focus only on one type of oxide, forgetting that both sulphur and nitrogen compounds are important. Remembering the formation of sulphuric and nitric acids helps avoid this error. Final Answer: Acid rain is mainly caused by atmospheric pollution due to oxides of nitrogen and sulphur.

Question 15

After fermentation, the most extensive industrial use of molasses is for the production of which of the following substances?

Accepted Answer

Introduction / Context: Molasses is a thick, dark syrup that remains after sugar crystallisation from sugarcane or sugar beet juice. It is rich in fermentable sugars and is an important raw material in industrial chemistry. This question focuses on what product is most extensively obtained from molasses after fermentation, highlighting the link between agricultural by products and useful chemicals. Given Data / Assumptions: Molasses contains residual sugars suitable for fermentation by microorganisms. Fermentation typically uses yeast to convert sugars into other compounds. The question asks about the main industrial product obtained after fermentation of molasses. We assume standard industrial practice in the sugar and distillery industries. Concept / Approach: Fermentation of molasses by yeast (usually Saccharomyces species) converts the sugars present into ethanol and carbon dioxide. Ethanol produced from molasses is widely used as industrial alcohol, fuel ethanol, or for making beverages after proper processing. Methanol, on the other hand, is not obtained by this simple carbohydrate fermentation process but largely from synthesis gas in chemical industry. Sugar itself is extracted from sugarcane juice before molasses remains. Sugarcane is the original plant, not a product. Acetic acid can be produced by further oxidation of ethanol but is not the primary direct product of molasses fermentation. Step-by-Step Solution: Step 1: Recall that molasses is a by product from sugar manufacture, containing uncrystallised sugars. Step 2: Understand that in distilleries, molasses is diluted and fermented with yeast, which converts sugars mainly into ethanol and carbon dioxide. Step 3: Note that the main economic purpose of fermenting molasses is to produce ethanol, which can be used as industrial spirit, fuel additive, or raw material for many chemicals. Step 4: Recognise that methanol is usually produced from synthesis gas and not from sugar fermentation. Step 5: Recognise that sugar and sugarcane are not products of fermentation; instead sugarcane is the starting crop and sugar is the main crystallised product before molasses is formed. Step 6: Although acetic acid can be produced from ethanol by oxidation, the direct major product of molasses fermentation is ethanol. Verification / Alternative check: If we look at a simple fermentation reaction, glucose (as a representative sugar from molasses) is converted by yeast according to the approximate equation: C6H12O6 gives 2 C2H5OH plus 2 CO2. This shows that ethanol and carbon dioxide are the main products. Industrially, the ethanol is distilled and used in various applications. Textbooks on industrial chemistry explicitly mention molasses as a major raw material for ethanol production, confirming that ethanol is the main product. Why Other Options Are Wrong: Methanol: Typically produced from synthesis gas (carbon monoxide and hydrogen), not by fermentation of molasses. Sugar: Already crystallised from the original juice; molasses is what remains after sugar is removed and is not fermented back into sugar. Sugarcane: This is the original plant and source of juice, not a product of molasses processing. Acetic acid: Can result from the oxidation of ethanol by bacteria, but it is not the primary direct product of molasses fermentation in distilleries. Common Pitfalls: Students may see terms like sugar and sugarcane and think that molasses is somehow converted back to these, but that is not correct. Another confusion is to mix methanol and ethanol because both are alcohols. However, their sources and manufacturing processes are different. Remember that molasses fermentation by yeast is a classic route to ethanol, sometimes called fermentation alcohol. Final Answer: After fermentation, the most extensive industrial use of molasses is for producing Ethanol.

Question 16

In water softening and ion exchange processes, zeolite is best described chemically as which of the following types of compound?

Accepted Answer

Introduction / Context: Zeolites are widely used in water softening, detergents, catalysis, and gas separation. They are important inorganic materials with a specific three dimensional framework structure. This question tests understanding of the basic chemical nature of zeolites by asking which description correctly identifies their composition. Given Data / Assumptions: Zeolite is used as an ion exchange material in water treatment. It has a crystalline framework structure with channels and cavities. The options provide several possible chemical types, including silicates and phosphates. We are considering the general composition, not a specific trade name. Concept / Approach: Natural and synthetic zeolites are aluminosilicate minerals. They are crystalline hydrated sodium aluminium silicates or related compounds in which aluminium and silicon atoms occupy tetrahedral positions, linked through oxygen atoms. The structure contains channels where cations such as sodium, calcium, or potassium and water molecules reside and can move. Because of this structure, zeolites can exchange their sodium ions with calcium or magnesium ions in hard water. Therefore, describing zeolite as a hydrated sodium aluminium silicate is correct at the school level. Step-by-Step Solution: Step 1: Recall that zeolites are aluminosilicate minerals with a framework of AlO4 and SiO4 tetrahedra. Step 2: Recognise that cations such as sodium and water molecules are present in the cavities and channels of this framework. Step 3: Understand that water softening by zeolites involves exchange of sodium ions in the zeolite with calcium and magnesium ions in hard water. Step 4: Connect this behaviour with a chemical description of zeolite as a hydrated sodium aluminium silicate. Step 5: Compare this description with the options and select the one that mentions hydrated sodium aluminium silicate. Verification / Alternative check: Typical textbook formulas for zeolites can be written approximately as Na2O.Al2O3.xSiO2.yH2O or similar, which shows sodium, aluminium, silicon and water in a crystalline structure. The presence of water (hydration), aluminium, and silicate framework matches the idea of a hydrated sodium aluminium silicate. Other compounds listed in the options, such as sodium hexameta phosphate or sodium tetraborate, have very different compositions and uses. Why Other Options Are Wrong: Hydrated ferric oxide: This describes rust like materials, not zeolites, and has no aluminosilicate framework. Sodium hexameta phosphate: A water softening agent sometimes used in detergents but is not structurally a zeolite. Sodium tetraborate: Known as borax, a borate compound, not an aluminosilicate mineral. Calcium carbonate: A common mineral (limestone) whose deposits often cause hardness but does not represent zeolite. Common Pitfalls: Students may confuse zeolites with other water softening agents like polyphosphates or borax, because they may all appear together in detergent discussions. The key for zeolites is the aluminosilicate framework and ion exchange capacity. Remembering that zeolites are hydrated sodium aluminium silicates helps distinguish them from other compounds. Final Answer: Zeolite is best described chemically as hydrated sodium aluminium silicate.

Question 17

Which of the following materials is an electrical insulator, that is, a substance that does not allow electric current to pass through it easily?

Accepted Answer

Introduction / Context: Electrical conductors and insulators play a central role in circuits, wiring, and safety. Conductors allow electric current to flow easily, while insulators resist current flow and are used to protect people and equipment. This question asks which of the listed substances is an insulator, meaning it is a poor conductor of electricity under normal conditions. Given Data / Assumptions: We consider electrical behaviour at room temperature and normal conditions. Metals are generally good conductors of electricity. Non metallic materials like dry wood are usually insulators. The options include several metals and one non metal material. Concept / Approach: In metals such as copper, aluminium, mercury and iron, there are free electrons that can move easily through the lattice, making these substances good conductors of electricity. In contrast, in materials like dry wood, electrons are not free to move, so they do not conduct electric current well. Such materials are used as insulators in electrical equipment. Therefore, identifying wood as the insulator among mostly metallic options demonstrates understanding of conductor versus insulator behaviour. Step-by-Step Solution: Step 1: Identify which options are metals: copper, mercury, aluminium and iron are all metals. Step 2: Recall that metals are generally good conductors of electricity because of their sea of delocalised electrons. Step 3: Recognise that wood is an organic, non metallic material that in dry condition does not have free electrons for conduction. Step 4: In practice, wooden handles are often used on tools and equipment to provide insulation from electric shock. Step 5: Conclude that wood is the electrical insulator among the listed materials. Verification / Alternative check: Everyday examples support this conclusion. Electrical wires are usually made of copper or aluminium because they conduct electricity very well, but they are covered with insulating materials such as plastic or rubber. Wooden poles have historically been used to support power lines because wood does not conduct electricity effectively when dry. Mercury is used in some switches because it conducts, not because it insulates. These practical uses confirm which materials behave as conductors or insulators. Why Other Options Are Wrong: Copper: A highly efficient electrical conductor widely used in wiring and electrical components. Mercury: A metal that is liquid at room temperature but still conducts electricity well. Aluminium: A light metal used in power lines due to its good conductivity. Iron: A metal used in electrical machines and structures and is also a conductor, although not as good as copper or aluminium. Common Pitfalls: Students may occasionally be misled by the physical state of mercury and think that because it is a liquid, it behaves differently from other metals. However, it remains a conductor. Another mistake is to think that slightly damp wood can conduct electricity and then generalise that wood is always a conductor. In standard theory, dry wood is treated as an insulator, which is the context of the question. Final Answer: The material that acts as an electrical insulator is Wood.

Question 18

Which class of amines reacts with nitrous acid (HNO2) to form diazonium salts in aromatic systems and alcohols in aliphatic systems, a characteristic test in organic chemistry?

Accepted Answer

Introduction / Context: Reactions of amines with nitrous acid are very important in organic chemistry, especially for distinguishing primary, secondary and tertiary amines. Aromatic primary amines can form stable diazonium salts, while aliphatic primary amines give alcohols and nitrogen gas. This question asks which class of amines shows the characteristic reaction with nitrous acid that leads to diazonium salt formation in aromatic systems and is widely used as a diagnostic test. Given Data / Assumptions: We are considering the reaction of amines with nitrous acid (HNO2). Aromatic primary amines can form diazonium salts in cold conditions. Aliphatic primary amines give alcohols with evolution of nitrogen gas. Secondary and tertiary amines behave differently with nitrous acid. Concept / Approach: The classic distinction is that primary amines (especially primary aromatic amines) react with nitrous acid at low temperature to form diazonium salts, such as benzene diazonium chloride. Aliphatic primary amines form unstable diazonium salts that decompose to alcohols and nitrogen gas. Secondary amines generally form N nitrosoamines with nitrous acid. Tertiary amines can form salts but do not give diazonium salts in the same characteristic way. Quaternary ammonium salts do not show the diazotisation reaction. Therefore, primary amines are the correct class that shows this well known reaction. Step-by-Step Solution: Step 1: Recall the diazotisation reaction: primary aromatic amine plus nitrous acid in cold conditions produces an aromatic diazonium salt. Step 2: Remember that aliphatic primary amines also react with nitrous acid, producing unstable diazonium salts that decompose to alcohols with evolution of nitrogen gas. Step 3: Note that secondary amines do not form diazonium salts; instead they form N nitrosoamines. Step 4: Recognise that tertiary amines may form soluble salts with nitrous acid but do not produce diazonium salts in the typical sense used for aromatic substitution reactions. Step 5: Conclude that only primary amines are associated with the classical diazotisation reaction and therefore answer the question. Verification / Alternative check: Standard organic chemistry texts emphasise that the diazotisation reaction is a characteristic test for primary aromatic amines and is carried out with sodium nitrite and hydrochloric acid at 0 to 5 degrees Celsius to generate nitrous acid in situ. They also state that secondary amines form N nitrosoamines and tertiary amines do not give diazonium salts. Laboratory tests for primary amines often involve the evolution of nitrogen gas and formation of stable diazonium salts in aromatic cases. These points confirm that primary amines are the required answer. Why Other Options Are Wrong: Secondary amines: React with nitrous acid to form N nitrosoamines, not diazonium salts. Tertiary amines: May form addition products or salts, but they do not undergo classical diazotisation to give diazonium salts. All of these: Incorrect because only primary amines show the characteristic diazonium salt formation reaction. Quaternary ammonium salts: Already fully substituted and do not show typical amine reactions with nitrous acid. Common Pitfalls: Students often memorise that amines react with nitrous acid without distinguishing between the classes. This can lead to choosing options such as all of these. It is important to remember the specific products: primary amines give diazonium salts or alcohols plus nitrogen, secondary amines give N nitrosoamines, and tertiary amines essentially do not give diazotisation products. Keeping a small comparison table for these reactions helps avoid confusion. Final Answer: The class of amines that reacts with nitrous acid to form diazonium salts in aromatic systems is Primary amines.

Question 19

Which one of the following substances is not regarded as a true allotrope of carbon in basic chemistry, even though it contains a high proportion of carbon?

Accepted Answer

Introduction / Context: Carbon exhibits several different structural forms in which the same element is arranged differently in space. These forms are called allotropes and include diamond, graphite, and newer forms such as graphene and fullerenes. This question tests which of the listed materials is not considered a true allotrope of carbon, even though it is carbon rich and often associated with carbon in everyday life. Given Data / Assumptions: Allotropes of carbon are pure forms with specific crystal structures or arrangements. Diamond, graphite, graphene and fullerenes are widely accepted as allotropes. Coal is a common carbon containing substance but has a complex composition. The question is asked at the level of school chemistry definitions. Concept / Approach: Allotropy refers to the existence of an element in more than one form in the same physical state, with different structures and properties. For carbon, classic allotropes include diamond (three dimensional tetrahedral network), graphite (layered hexagonal sheets), graphene (single layer of graphite), and fullerenes (closed cage structures). Coal, although rich in carbon, is not a pure substance but a mixture of various forms of carbon, hydrocarbons, mineral matter, and impurities. Because it does not have a definite, uniform crystal structure or composition, coal is not classified as a true allotrope of carbon. Step-by-Step Solution: Step 1: Recall the definition of allotropes and list known carbon allotropes such as diamond, graphite, graphene, and fullerenes. Step 2: Examine the structure of diamond, which consists of a three dimensional network of sp3 hybridised carbon atoms with strong covalent bonds. Step 3: Examine graphite, which has layers of sp2 hybridised carbon atoms arranged in hexagonal rings, with weak forces between layers. Step 4: Graphene is a single layer of graphite, and fullerenes are molecular cages of carbon; both arise from pure carbon arrangements. Step 5: Recognise that coal is a naturally occurring material that contains carbon along with hydrogen, oxygen, nitrogen, sulphur and mineral impurities, with no single uniform structure. Step 6: Conclude that coal is not regarded as a true allotrope of pure carbon. Verification / Alternative check: Textbooks on carbon chemistry typically list diamond, graphite and fullerenes as allotropes and may mention graphene as a modern addition. Coal, on the other hand, is discussed in the context of fossil fuels and is described as a complex mixture of carbonaceous material and other substances. It is not given a definite chemical formula or crystal structure. This clear difference in treatment confirms that coal is not an allotrope. Why Other Options Are Wrong: Diamond: A classic allotrope of carbon with a tetrahedral network structure and high hardness. Graphite: Another classic allotrope with layered structure and good electrical conductivity along the planes. Graphene: A single layer of graphite and accepted as a distinct carbon allotrope in modern discussions. Fullerene: A molecular allotrope of carbon with cage like structures such as C60, called buckminsterfullerene. Common Pitfalls: Students may incorrectly think that any carbon rich material, including coal or charcoal, is an allotrope. However, only pure forms of carbon with distinct structures qualify. Coal varies widely in composition from one deposit to another and includes many non carbon components. To avoid confusion, memorise the standard list of pure carbon allotropes discussed in your syllabus and treat substances like coal and charcoal as mixtures or impure forms. Final Answer: The substance that is not regarded as a true allotrope of carbon is Coal.

Question 20

Bee sting venom mainly contains which type of liquid, leading to a characteristic burning sensation that can be relieved by applying a mild alkali?

Accepted Answer

Introduction / Context: Bee stings are common in everyday life and are often discussed in basic chemistry when learning about acids, bases, and neutralisation. The nature of the bee sting venom determines the type of household treatment that can provide relief. This question asks what kind of liquid is present in a bee sting, connecting chemistry concepts to a real life situation. Given Data / Assumptions: Bee sting venom causes pain and a burning sensation at the site of the sting. Simple home remedies often involve applying substances like baking soda. We consider general school level knowledge of bee venom composition. The options describe the liquid as acidic, alkaline, neutral, salty or corrosive. Concept / Approach: Bee sting venom is known to be acidic due to the presence of formic acid and other components. An acidic substance causes burning and inflammation when injected into the skin. Relief can be obtained by applying a mild base, such as baking soda paste, which neutralises the acid. This reasoning aligns with the concept of acid base neutralisation. Therefore, identifying the venom as an acidic liquid is correct at the level of school chemistry. Step-by-Step Solution: Step 1: Recall that bee sting venom contains formic acid and related acidic compounds. Step 2: Understand that acids cause irritation and a stinging sensation when they contact skin or tissues. Step 3: Remember that first aid advice for bee stings often includes applying a mild alkaline substance such as baking soda. Step 4: Neutralisation occurs when an acid and a base react, reducing the acidity and reducing pain. Step 5: Conclude that because a base is used to counteract the sting, the venom itself must be acidic. Verification / Alternative check: Biology and chemistry references confirm that bee venom contains several components, including formic acid, histamine and enzymes, many of which contribute to its acidity and irritant properties. The fact that alkaline treatments can relieve symptoms supports the conclusion that the sting is acidic. Additionally, a contrast can be made with wasp stings, which are more alkaline in nature and can be soothed by mild acids like vinegar, further illustrating acid base relationships. Why Other Options Are Wrong: An alkaline liquid: This would be treated with a mild acid such as vinegar, which is more typical for wasp stings, not bee stings. A salt solution: Although body fluids contain salts, the key painful effect of the bee sting is due to acidity, not mere salt content. A corrosive liquid: Bee venom is irritating but not classed as a strongly corrosive industrial chemical; the term does not match school level descriptions. A neutral liquid: A neutral liquid would not cause strong acid base reactions and would not be relieved by applying a base. Common Pitfalls: Students sometimes confuse bee and wasp stings and misremember which is acidic and which is more alkaline. Another confusion arises from the general use of the term venom, leading some to think of it as simply poisonous without considering its acid base character. To avoid this, associate bee stings with formic acid and baking soda treatment, and wasp stings with alkaline venom and vinegar treatment. Final Answer: Bee sting venom mainly contains an acidic liquid.

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