Question 1

Consider the reaction Fe2O3(s) + CO(g) → Fe(s) + CO2(g). When this chemical equation is balanced, what will be the correct set of coefficients a, b, c and d respectively in aFe2O3 + bCO → cFe + dCO2?

Accepted Answer

Introduction / Context: This question tests your skill in balancing chemical equations, a fundamental technique in chemistry. The reaction given represents the reduction of iron(III) oxide by carbon monoxide to produce metallic iron and carbon dioxide, a process related to iron extraction in a blast furnace. You are asked to determine the correct set of coefficients a, b, c and d that balance the reaction aFe2O3 + bCO → cFe + dCO2. Balancing ensures that the law of conservation of mass is satisfied, with the same number of each type of atom on both sides of the equation. Given Data / Assumptions: The unbalanced reaction is Fe2O3(s) + CO(g) → Fe(s) + CO2(g). The coefficients a, b, c and d are positive integers to be determined. We must balance the number of Fe, C and O atoms on both sides of the equation. No side reactions are considered, and the reaction is taken as a single step overall process. Concept / Approach: To balance a chemical equation, we equate the total number of atoms of each element on the reactant and product sides. We usually start with the metal or the most complex compound and then balance other elements like oxygen and carbon. In this case, Fe2O3 contains two iron atoms and three oxygen atoms per formula unit. Each CO molecule contains one carbon and one oxygen, while each CO2 molecule contains one carbon and two oxygen atoms. The goal is to find integer coefficients that satisfy these atom counts simultaneously. Step-by-Step Solution: Step 1: Begin with iron. Fe2O3 contains two iron atoms, so we need two Fe atoms on the product side. This suggests setting c = 2 in the balanced equation. Step 2: With a single Fe2O3 on the reactant side, set a = 1. Now we have one Fe2O3 providing two Fe atoms and three O atoms. Step 3: Next, consider carbon. Suppose we use three molecules of CO, so set b = 3. This introduces three carbon atoms and three oxygen atoms from CO on the reactant side. Step 4: The total oxygen atoms on the reactant side are now three from Fe2O3 and three from CO, giving six oxygen atoms in total. On the product side, each CO2 has two oxygen atoms, so to match six oxygens we need three CO2 molecules. This sets d = 3. Step 5: Check carbon balance. With b = 3, we have three carbon atoms from CO on the reactant side, and with d = 3, we have three carbon atoms in three CO2 molecules on the product side. Iron is already balanced with two atoms on each side, and oxygen is balanced with six atoms on each side. Therefore the correct coefficients are a = 1, b = 3, c = 2 and d = 3. Verification / Alternative check: Write the fully balanced equation as Fe2O3 + 3CO → 2Fe + 3CO2. Counting atoms on both sides gives two Fe, three C and six O atoms on each side, confirming that the equation is balanced. This is the standard form of the reaction used in discussions of iron extraction in metallurgy. Comparing this to the answer choices, the correct set of coefficients corresponds to 1, 3, 2, 3 for a, b, c and d respectively. No other set given in the options will produce the same balanced atom counts without introducing fractions or inconsistencies. Why Other Options Are Wrong: The set 3, 2, 3, 1 is wrong because it does not balance the number of oxygen and carbon atoms on both sides; using these coefficients would lead to mismatched totals. The set 2, 3, 3, 1 is incorrect because the number of oxygen atoms and carbon atoms would not be equal on reactant and product sides when these coefficients are applied. The set 3, 3, 2, 1 is also incorrect, as the atoms of oxygen and carbon would again not be balanced simultaneously, violating conservation of mass. Common Pitfalls: A common mistake is to balance one element but forget to recheck others after adjusting coefficients, leading to partially balanced equations. Another pitfall is accepting fractional coefficients as final answers instead of scaling the equation to obtain whole numbers. Students may also confuse this reaction with similar ones involving carbon or carbon monoxide and misremember the coefficient pattern. Systematically balancing one element at a time and verifying all elements at the end helps prevent these errors. Final Answer: The correctly balanced equation is Fe2O3 + 3CO → 2Fe + 3CO2, so the coefficients are 1, 3, 2, 3 for a, b, c and d respectively.

Question 2

In an electric bulb, why is argon gas commonly used along with a tungsten filament instead of leaving the bulb as a vacuum or filling it with ordinary air?

Accepted Answer

Introduction / Context: This question is about the practical application of chemistry and physics in everyday technology, specifically incandescent electric bulbs. It asks why argon gas is used inside a bulb along with a tungsten filament. Understanding the role of inert gases in light bulbs helps explain how manufacturers increase filament life and bulb efficiency. It also reinforces general knowledge about reactive versus inert gases and high temperature behaviour of metals. Given Data / Assumptions: The filament in a traditional incandescent bulb is made of tungsten, which operates at very high temperatures to emit visible light. Argon is a noble gas that is chemically inert under ordinary conditions. Ordinary air contains oxygen, nitrogen and other gases that can react with hot metals. The goal in bulb design is to prevent rapid evaporation and oxidation of the tungsten filament while allowing light to be emitted. Concept / Approach: At the high operating temperatures of a tungsten filament, exposure to oxygen would lead to rapid oxidation and burning of the filament, drastically shortening the life of the bulb. A vacuum would reduce oxidation but can encourage faster evaporation and thinning of the tungsten filament. Filling the bulb with an inert gas like argon reduces the evaporation rate of tungsten by providing a gas environment that does not react chemically with the filament. This slows down filament degradation and significantly increases the bulb lifetime compared with an evacuated or air filled bulb. Step-by-Step Solution: Step 1: Consider a filament in air. At high temperature, tungsten would react with oxygen in the air to form tungsten oxides and would burn out quickly, so air is unsuitable. Step 2: Consider a pure vacuum. Although this prevents oxidation, it allows more rapid evaporation of tungsten atoms from the hot filament surface, causing it to thin and break prematurely. Step 3: Consider filling the bulb with argon. Argon is inert and does not react with tungsten, so it prevents oxidation. It also helps reduce the evaporation rate of tungsten by providing a gas that slows the escape of tungsten atoms from the filament. Step 4: As a result, the filament lasts longer before breaking, which directly increases the useful life of the bulb. Step 5: Therefore, the primary reason for using argon gas in an electric bulb is to increase the life of the bulb by protecting the filament. Verification / Alternative check: Manufacturing descriptions of incandescent bulbs typically state that the bulb is filled with an inert gas such as argon or a mixture of argon and nitrogen. This is done specifically to slow down the evaporation of the tungsten filament and to prevent chemical reactions that would damage it. Experimental and practical experience show that bulbs filled with inert gas last longer than those with a vacuum or air. These facts confirm that the main purpose is to prolong the bulb lifetime, rather than to significantly reduce electricity consumption or to colour the light. Why Other Options Are Wrong: Reducing the consumption of electricity is not the main effect of filling a bulb with argon. The power consumption depends primarily on the electrical characteristics of the filament and supply voltage, not on the filler gas, so option B is incorrect. Making the emitted light coloured is wrong because the gas inside the bulb is essentially transparent; the colour of the light is determined mainly by the filament temperature and not by argon. Reducing the cost of the bulb is not the main reason either, since adding argon gas and sealing the bulb may actually increase manufacturing complexity. The main benefit is longer life, which justifies any added cost. Common Pitfalls: A common misunderstanding is to think that the filler gas directly changes the energy efficiency of the bulb or the colour of light emitted. Another pitfall is assuming that any gas could be used instead of argon. Reactive gases such as oxygen would quickly destroy the filament. Remembering that argon is inert and that the goal is to protect the filament at high temperature helps to see that the main purpose is to increase the bulb life. Final Answer: Argon gas is used in an electric bulb with a tungsten filament primarily to increase the life of the bulb by reducing oxidation and evaporation of the filament.

Question 3

Which one of the following relations correctly connects the Kelvin temperature T with the Celsius temperature tc for the same thermal state?

Accepted Answer

Introduction / Context: This question relates to basic thermometry and the relationship between the Celsius and Kelvin temperature scales. Kelvin is the SI unit of temperature and is an absolute scale starting at absolute zero, while the Celsius scale is commonly used in daily life and in laboratories. The question asks which equation correctly relates a Kelvin temperature T to the corresponding Celsius temperature tc for the same physical condition, such as the temperature of a sample of water. Given Data / Assumptions: The Celsius scale sets the freezing point of water at 0 degrees Celsius and the boiling point at 100 degrees Celsius under standard atmospheric pressure. The Kelvin scale sets absolute zero at 0 Kelvin and the triple point of water at 273.16 Kelvin. The size of one degree Celsius is equal to the size of one Kelvin unit. We assume standard modern definitions of the temperature scales. Concept / Approach: The difference between the Kelvin and Celsius scales is purely an offset. Absolute zero is at 0 K, which corresponds to minus 273.15 degrees Celsius. Since the increments are identical, we can convert from Celsius to Kelvin by adding a constant offset. Specifically, T(K) = tc + 273.15. Any other relationship, such as subtracting 273.15 or treating the scales as independent, would conflict with the standard fixed points like the freezing point of water. Step-by-Step Solution: Step 1: Recognise that at the freezing point of water, the Celsius temperature is 0 degrees Celsius and the Kelvin temperature is approximately 273.15 Kelvin. Step 2: Substitute tc = 0 into the candidate formula T = tc + 273.15, giving T = 0 + 273.15 = 273.15 K, which matches the known value. Step 3: Check the boiling point of water at 100 degrees Celsius. Using T = tc + 273.15 gives T = 100 + 273.15 = 373.15 K, which matches the standard boiling point in Kelvin. Step 4: If we tried T = tc - 273.15, then at 0 degrees Celsius the temperature would be negative in Kelvin, which contradicts the definition of Kelvin as an absolute scale starting at zero. Step 5: Therefore, the correct relation between the Kelvin and Celsius temperature scales is T = tc + 273.15. Verification / Alternative check: The relationship between these scales is often written explicitly in textbooks as T(K) = 273.15 + t(°C). This is derived from the fact that both scales have equal increments and that 0 K corresponds to minus 273.15 degrees Celsius. Rearranging the relationship between absolute zero and water freezing point gives the same conversion formula. Checking multiple known temperature points, such as room temperature or the triple point of water, confirms consistency with the equation T = tc + 273.15, reinforcing that it is the correct relation. Why Other Options Are Wrong: The statement that the Kelvin and Celsius scales are independent is wrong because they are linearly related by a fixed offset and share the same degree size. The relation T = tc is wrong because it would give the same numerical value for both scales, which is not true; for example, 0 degrees Celsius does not equal 0 Kelvin. The relation T = tc - 273.15 is wrong because it would yield negative Kelvin values for temperatures near room temperature and at the freezing point of water, contradicting the concept of an absolute temperature scale where Kelvin values cannot be negative. Common Pitfalls: A frequent mistake is mixing up the direction of the offset and writing T = tc - 273 instead of T = tc + 273. Another pitfall is forgetting the decimal 0.15 and approximating too roughly; while 273 is often used in rough calculations, 273.15 is more accurate. Students also sometimes confuse Kelvin with Rankine or other temperature scales. Remembering that Kelvin is always numerically higher than Celsius by about 273 for the same physical temperature helps to keep the sign correct. Final Answer: The correct relation between Kelvin temperature T and Celsius temperature tc is T = tc + 273.15.

Question 4

In basic chemistry, which one of the following statements is not a characteristic property of a chemical compound as opposed to a mixture?

Accepted Answer

Introduction / Context: This general chemistry question checks whether you can distinguish the properties of a pure chemical compound from those of a mixture. Many learners confuse compounds and mixtures, especially when talking about composition and methods of separation. Knowing the correct characteristic properties of compounds is important for understanding classification of matter in basic chemistry. Given Data / Assumptions: We are comparing the properties of a compound with those of mixtures. Several statements about composition and particle nature are given. Exactly one statement is not a correct characteristic of a compound. Assume simple school level definitions of compounds and mixtures. Concept / Approach: A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass. All particles of a given compound are of the same type of molecule or ion pair, for example only H2O molecules in pure water. The composition of a compound is fixed, and its components can only be separated by chemical methods. In contrast, mixtures have variable composition, and their components can often be separated by physical methods such as filtration or distillation. The wrong statement here will typically describe a mixture rather than a compound. Step-by-Step Solution: Step 1: Check statement about composition being variable. A compound has a fixed composition, so saying that the composition is variable describes a mixture, not a compound. This statement is not a characteristic of a compound. Step 2: Check the statement that all particles of a compound are of only one type. In a pure compound such as sodium chloride, all particles are NaCl units, so this is a correct characteristic. Step 3: Examine the statement that particles of a compound contain two or more elements chemically combined in a fixed ratio. This matches the formal definition of a compound and is correct. Step 4: Consider the statement that the constituents of a compound cannot be separated by simple physical methods. This is correct because you need chemical reactions to break down compounds into elements or simpler compounds. Step 5: The additional statement that a compound is formed by chemical combination of its elements is also a standard textbook fact about compounds. Verification / Alternative check: A good way to verify is to take a specific example such as water. The composition of pure water is always hydrogen and oxygen in the mass ratio 1 to 8, not variable. All particles are H2O molecules. You cannot separate hydrogen and oxygen from water by simple filtration or decantation; you need electrolysis or chemical reactions. Every correct statement about compounds must hold for water. The only statement that clearly fails that test is the idea of variable composition. Therefore it must be the option that is not a characteristic of compounds. Why Other Options Are Wrong: All particles are of one type of molecule: This is true for a pure compound and is therefore not the requested non characteristic statement. Particles contain two or more elements chemically combined: This is the basic definition of a compound, so it is a correct characteristic. Constituents cannot be separated by simple physical methods: Compounds require chemical methods to break down, so this is a correct property. Formed by chemical combination of elements: This again is a correct statement about compounds. Common Pitfalls: A regular source of confusion is mixing up the ideas of compounds and mixtures. Learners sometimes think that any sample whose composition can vary slightly in the real world must be a mixture. However, for a pure compound at the microscopic level, the ratio of atoms in each molecule is fixed. Another common mistake is assuming that any separation process is chemical; in fact many everyday separations such as sieving and filtration are physical and work for mixtures, not for compounds. Always focus on fixed composition and need for chemical change as key identifiers of a compound. Final Answer: The statement that is not a characteristic of a compound is Composition is variable from sample to sample.

Question 5

Which of the following dissolved salts cause temporary hardness in natural water? 1. Mg(HCO3)2 2. Ca(HCO3)2 3. CaCl2 4. MgSO4

Accepted Answer

Introduction / Context: This question tests your understanding of water hardness in environmental chemistry. Hardness of water is a very important practical concept because it affects soap consumption, boiler operation, and household water quality. There are two main types of hardness, temporary and permanent, and they are caused by different dissolved salts. Here you need to identify which of the listed salts produce temporary hardness in water. Given Data / Assumptions: Four salts are listed: Mg(HCO3)2, Ca(HCO3)2, CaCl2 and MgSO4. The question asks specifically about temporary hardness, not total hardness. Assume standard definitions where temporary hardness is removed by boiling. Assume that each salt, when dissolved, provides either bicarbonate ions or non bicarbonate anions. Concept / Approach: Temporary hardness is mainly due to the presence of bicarbonates of calcium and magnesium in water, usually written as Ca(HCO3)2 and Mg(HCO3)2. When hard water containing these bicarbonates is boiled, the bicarbonates decompose, producing insoluble carbonates that precipitate out and so the hardness is removed. Permanent hardness, on the other hand, is due to chlorides and sulfates such as CaCl2 and MgSO4. These salts are not removed simply by boiling and require chemical softening methods. So we must identify which of the listed salts are bicarbonates. Step-by-Step Solution: Step 1: Look at Mg(HCO3)2. This is magnesium bicarbonate, which contributes to temporary hardness because it decomposes on boiling to magnesium carbonate, water and carbon dioxide. Step 2: Look at Ca(HCO3)2. This is calcium bicarbonate, which similarly causes temporary hardness and decomposes on boiling to calcium carbonate, water and carbon dioxide. Step 3: Consider CaCl2. This is calcium chloride. Chlorides do not decompose on boiling in the same simple way, and hardness due to CaCl2 is permanent hardness. Step 4: Consider MgSO4. This is magnesium sulfate. Sulfates behave like chlorides and are associated with permanent hardness. Step 5: Therefore only salts 1 and 2 are responsible for temporary hardness of water. Verification / Alternative check: In textbook explanations of temporary hardness, you will usually see the equations describing the decomposition of bicarbonates on heating. For example, Ca(HCO3)2 when heated gives CaCO3 plus water plus carbon dioxide. The solid CaCO3 precipitates out of solution and can be removed. Similar equations apply for Mg(HCO3)2. In contrast, salts like CaCl2 and MgSO4 remain dissolved even when the water is boiled. This is why boiling removes only temporary hardness and not permanent hardness. Matching this information with the options confirms that 1 and 2 are correct. Why Other Options Are Wrong: 3 and 4 only: These are chlorides and sulfates that cause permanent, not temporary, hardness. 2 and 3 only: This incorrectly includes CaCl2, which does not produce temporary hardness. 1 and 4 only: This incorrectly includes MgSO4, which is a permanent hardness causing salt. 1, 2, 3 and 4 all cause only temporary hardness: This is wrong because only the bicarbonates cause temporary hardness, while chlorides and sulfates cause permanent hardness. Common Pitfalls: A common mistake is to assume that any calcium or magnesium salt must contribute to temporary hardness, but the type of anion is crucial. Bicarbonates are associated with temporary hardness, while chlorides and sulfates are associated with permanent hardness. Another pitfall is to confuse the terms temporary and permanent with the idea of how important the problem is, instead of how easy it is to remove by boiling. Always connect temporary hardness with bicarbonates and boilable removal to answer such questions correctly. Final Answer: The salts that cause temporary hardness in water are 1 and 2 only, that is Mg(HCO3)2 and Ca(HCO3)2.

Question 6

In chemical bonding, which of the following types of bond is generally considered the weakest in nature among those listed?

Accepted Answer

Introduction / Context: This question checks your understanding of the relative strengths of different types of chemical bonds and interactions. In chemistry, we often distinguish between strong intramolecular covalent or ionic bonds that hold atoms together in a molecule or crystal, and weaker intermolecular forces such as hydrogen bonds and van der Waals forces that act between molecules. Recognising which interactions are weaker helps explain properties like boiling points and solubilities. Given Data / Assumptions: The options include single, double and triple covalent bonds, a hydrogen bond, and an ionic bond. We are asked which type is generally the weakest in nature among the listed choices. Assume typical bond strengths in covalent molecules and common ionic solids. Concept / Approach: Single, double and triple covalent bonds are strong intramolecular bonds formed by sharing electrons between atoms. In general, triple bonds are strongest, double bonds are intermediate, and single bonds are somewhat weaker but still much stronger than typical intermolecular interactions. Ionic bonds between oppositely charged ions in a crystal lattice are also strong. Hydrogen bonds, although stronger than many other intermolecular forces, are still weaker than covalent and ionic bonds. Therefore, among the given choices, hydrogen bonds are the weakest. Step-by-Step Solution: Step 1: Consider single, double and triple covalent bonds. These are bonds within molecules, and their bond energies are usually high, especially for double and triple bonds. Step 2: Remember that triple bonds such as in nitrogen molecules are usually the strongest covalent bonds, followed by double bonds and then single bonds. Step 3: Ionic bonds involve strong electrostatic attraction between positively and negatively charged ions in solids like sodium chloride and are also high energy bonds. Step 4: Hydrogen bonds arise when a hydrogen atom bonded to a very electronegative atom such as oxygen, nitrogen or fluorine is attracted to another electronegative atom in a neighbouring molecule. Step 5: Hydrogen bonds are weaker than covalent and ionic bonds, although they are stronger than many other intermolecular forces. This makes hydrogen bond the weakest among the listed options. Verification / Alternative check: Typical bond energy tables show covalent bond energies in the range of hundreds of kilojoules per mole, for example C–C or O–H bonds. Ionic lattice energies are also very high. In contrast, hydrogen bond energies are usually much smaller, often tens of kilojoules per mole. This large difference in bond energy supports the idea that hydrogen bonds are weaker than true covalent or ionic bonds. Many physical properties of water, DNA and proteins show the important but relatively weak nature of hydrogen bonding compared to covalent bonding in their backbones. Why Other Options Are Wrong: Single covalent bond: Although weaker than double and triple covalent bonds, it is still much stronger than hydrogen bonds. Double covalent bond: Stronger than a single bond and far stronger than hydrogen bonds. Triple covalent bond: Typically the strongest among covalent bonds, not the weakest. Ionic bond: Electrostatic attraction holds ions strongly in a lattice, giving high melting points and hardness, so it is not the weakest type listed. Common Pitfalls: Some learners mix up the idea of the number of bonds with their strength and may think that single bonds must be very weak. While single covalent bonds are weaker than double or triple bonds, they are still much stronger than hydrogen bonds. Another common confusion is to treat hydrogen bonds as if they were a kind of full covalent bond. Remember that hydrogen bonds are intermolecular attractions, weaker than the true bonds within molecules, even though they have a big effect on physical properties like boiling point and viscosity. Final Answer: The weakest type of bond among the options given is the Hydrogen bond.

Question 7

Calcium has atomic number 20 and mass number 40. Which one of the following hypothetical nuclides would be an isobar of calcium, that is, would have the same mass number 40 but a different atomic number?

Accepted Answer

Introduction / Context: This nuclear chemistry question examines the concept of isobars. Atomic structure terms such as atomic number, mass number, isotopes and isobars are frequently tested in school exams. Isobars are nuclei that have the same mass number but different atomic numbers. Understanding how to count protons and neutrons and relate them to these definitions is essential to answer such questions correctly. Given Data / Assumptions: Calcium is given with atomic number 20 and mass number 40. Atomic number is the number of protons in the nucleus. Mass number is the total number of protons plus neutrons. We are asked to identify a nuclide that has mass number 40 but a different atomic number from calcium. Concept / Approach: Isobars are defined as atoms or nuclei of different elements that have the same mass number but different atomic numbers. This means the sum of protons and neutrons must be equal to 40 for calcium and for the isobar, but the proton count must be different. For calcium with atomic number 20 and mass number 40, the number of neutrons is 40 minus 20, which is 20. To find a correct isobar, we need any other combination of protons and neutrons that adds up to 40 but has a proton number different from 20. Step-by-Step Solution: Step 1: For calcium, note that atomic number Z is 20 and mass number A is 40. Therefore neutrons N are 40 minus 20 equal to 20. Step 2: For a nuclide to be an isobar of calcium, its mass number must be A equals 40, which means protons plus neutrons equals 40. Step 3: Look at option A: 18 protons and 22 neutrons. The mass number is 18 plus 22 equal to 40, and the atomic number is 18, which is different from 20. This fits the definition of an isobar. Step 4: Look at option B: 18 protons and 19 neutrons. The mass number is 18 plus 19 equal to 37, not 40, so it is not an isobar. Step 5: Look at option C: 20 protons and 19 neutrons. The mass number is 20 plus 19 equal to 39, so it is not an isobar of calcium 40. Step 6: Look at option D: 22 protons and 16 neutrons. The mass number is 22 plus 16 equal to 38, again not 40. Step 7: Look at option E: 21 protons and 18 neutrons. The mass number is 21 plus 18 equal to 39, so it also fails the isobar condition. Verification / Alternative check: A quick way to check is to write A equals Z plus N for each nuclide. Any combination that gives A equals 40 and Z not equal to 20 is a valid isobar of calcium 40. For example, argon 40 has atomic number 18 and mass number 40, and it is a well known isobar of calcium 40. Option A corresponds exactly to Z equals 18 and N equals 22, which matches argon 40. This agreement confirms that option A is the correct isobar, while the other options give mass numbers different from 40 and so cannot be isobars of calcium 40. Why Other Options Are Wrong: 18 protons and 19 neutrons: Mass number is 37, not 40, so it is not an isobar of calcium 40. 20 protons and 19 neutrons: Mass number is 39 and atomic number equals that of calcium, so it would be an isotope of calcium, not an isobar. 22 protons and 16 neutrons: Mass number is 38, so it does not match the mass number of calcium 40. 21 protons and 18 neutrons: Mass number is 39 and so cannot qualify as an isobar of mass 40. Common Pitfalls: Many learners confuse isotopes and isobars. Isotopes have the same atomic number but different mass numbers, while isobars have the same mass number but different atomic numbers. Another frequent mistake is to look only at the number of neutrons rather than the sum of protons and neutrons. Carefully computing A equals Z plus N for each option prevents such errors and helps you apply the correct definition of an isobar in exam questions. Final Answer: The nuclide that is an isobar of calcium 40 is the Nuclide with 18 protons and 22 neutrons.

Question 8

Consider the following minor gases of the Earth atmosphere: 1. Argon 2. Neon 3. Helium 4. Carbon dioxide. Which one of the following shows these gases in the correct ascending order of their volume percentage in dry air?

Accepted Answer

Introduction / Context: This question belongs to environmental chemistry and basic geography. It asks you to compare the relative abundances of some minor gases in the atmosphere of Earth. While nitrogen and oxygen dominate the air, trace gases like argon, carbon dioxide, neon and helium are also present in small but important quantities. Understanding their approximate volume percentages helps in interpreting discussions about climate and air composition. Given Data / Assumptions: The gases listed are argon, neon, helium and carbon dioxide. We are dealing with volume percentage in dry air at ground level. We must arrange the gases in ascending order, that is, from lowest percentage to highest percentage. Nitrogen and oxygen are not part of this comparison; we focus only on the four given gases. Concept / Approach: Standard data for dry air show that argon is the most abundant of the listed gases, with a volume percentage close to 0.93 percent. Carbon dioxide is present at a lower level of roughly a few hundred parts per million, around 0.03 to 0.04 percent. Neon and helium are noble gases present in much smaller amounts, with helium being less abundant than neon. Therefore, from lowest to highest concentration, the order is helium, then neon, then carbon dioxide, and finally argon. We must map this order to the numerical labels given in the question: 3 for helium, 2 for neon, 4 for carbon dioxide, and 1 for argon. Step-by-Step Solution: Step 1: Identify which label corresponds to which gas: 1 is argon, 2 is neon, 3 is helium and 4 is carbon dioxide. Step 2: Recall that helium is present in the smallest amount among the four, so it should appear first in the ascending sequence. This corresponds to label 3. Step 3: Neon is more abundant than helium but still very rare, so it comes next. This corresponds to label 2. Step 4: Carbon dioxide has a higher volume percentage, of the order of a few tenths of a percent, so it comes after neon. This corresponds to label 4. Step 5: Argon has the highest volume percentage among the four gases and therefore appears last. This corresponds to label 1. Step 6: Putting these labels together in order from lowest to highest gives 3 - 2 - 4 - 1, which matches option A. Verification / Alternative check: Standard composition tables of dry air list approximate volume percentages as nitrogen about 78 percent, oxygen about 21 percent, argon about 0.93 percent and carbon dioxide about 0.03 to 0.04 percent. Neon and helium are far lower, in the parts per million or less range, with helium generally the least abundant among the four considered here. If you place these values on a number line, helium is at the lowest end, then neon, then carbon dioxide, and finally argon. Comparing this sequence with the coded labels confirms that 3 - 2 - 4 - 1 is the correct ascending order. Why Other Options Are Wrong: 2 - 3 - 4 - 1: This places neon before helium, but helium is actually less abundant than neon. 4 - 2 - 3 - 1: This places carbon dioxide at the lowest concentration, which is incorrect because noble gases like helium and neon are present in smaller amounts than carbon dioxide. 2 - 4 - 1 - 3: This wrongly places argon before helium, suggesting that helium is more abundant, which is not true. 1 - 4 - 2 - 3: This option starts with argon and ends with helium, which completely reverses the expected order for trace gases. Common Pitfalls: Many learners overestimate the amount of carbon dioxide in air because climate discussions focus heavily on its importance. As a result, they sometimes think that carbon dioxide is more abundant than argon. In reality, argon, although chemically inert, is more abundant by volume. Another mistake is to assume that all noble gases must have very similar abundances. However, helium and neon occur in very small concentrations compared to argon and carbon dioxide. Keeping approximate percentage values in mind will help avoid these misconceptions. Final Answer: The correct ascending order of volume percentage for the given gases is 3 - 2 - 4 - 1, that is helium, neon, carbon dioxide and then argon.

Question 9

In the composition of the Earth atmosphere, which one of the following gases is the most abundant by volume?

Accepted Answer

Introduction / Context: This general knowledge question tests your basic understanding of the composition of air. While we often focus on oxygen because it is essential for respiration, nitrogen actually makes up the largest fraction of the atmosphere by volume. Knowing the major gases and their approximate percentages is important for both chemistry and geography topics. Given Data / Assumptions: The options list argon, nitrogen, oxygen, krypton and carbon dioxide. The question asks for the most abundant gas in the atmosphere by volume. We are referring to the lower atmosphere near the Earth surface and to average dry air. Concept / Approach: Dry air close to the Earth surface is composed mainly of nitrogen and oxygen, with smaller amounts of argon, carbon dioxide and trace gases. Standard composition data state that nitrogen is about 78 percent by volume and oxygen about 21 percent. Argon is less than 1 percent, carbon dioxide is a small fraction of a percent, and krypton is present in trace amounts. Therefore, among the listed gases, nitrogen is clearly the most abundant component of the atmosphere. Step-by-Step Solution: Step 1: Recall approximate percentages of major atmospheric gases in dry air: nitrogen about 78 percent, oxygen about 21 percent, argon about 0.93 percent, and carbon dioxide about 0.03 to 0.04 percent. Step 2: Compare nitrogen with oxygen. Nitrogen has a significantly higher percentage than oxygen, so nitrogen is more abundant than oxygen. Step 3: Compare nitrogen with argon, krypton and carbon dioxide, which are all present in much smaller trace amounts. Step 4: Since no other gas listed reaches the level of 78 percent, nitrogen must be the most abundant gas in Earth atmosphere. Verification / Alternative check: Every basic science textbook and reference chart of air composition begins with nitrogen as the major component. Classroom posters and diagrams often show a large sector representing nitrogen in pie charts of the atmosphere. Meteorology and environmental science resources also highlight the dominance of nitrogen, followed by oxygen. Checking any such reliable source confirms that nitrogen is the correct answer to this question, with oxygen a clear second and other gases far behind. Why Other Options Are Wrong: Argon: It is a noble gas present at less than 1 percent by volume, so it is not the most abundant gas. Oxygen: Oxygen is the second most abundant gas at about 21 percent, but nitrogen still exceeds it. Krypton: This is a trace noble gas present in very tiny amounts and cannot be the major component. Carbon dioxide: Even though it is important for climate and photosynthesis, its volume percentage is small compared with nitrogen and oxygen. Common Pitfalls: Because oxygen is so important for life, some learners mistakenly think it must be the main component of air. Similarly, climate news emphasises carbon dioxide, which can cause people to overestimate its quantity. Remember that importance for life or climate is not the same as abundance. For exam questions, always recall that nitrogen is the most abundant gas in air by volume, followed by oxygen, with other gases present in much smaller proportions. Final Answer: The most abundant gas in the atmosphere of Earth is Nitrogen.

Question 10

In basic building materials chemistry, the typical chemical composition of ordinary Portland cement is best represented by which of the following combinations of raw materials?

Accepted Answer

Introduction / Context: This question covers basic knowledge of building materials, especially cement. Ordinary Portland cement is one of the most widely used construction materials in the world. Understanding its main chemical composition helps in appreciating how it sets and hardens, and why certain raw materials are required in its manufacture. Given Data / Assumptions: The question mentions chemical composition of cement in terms of raw materials. Options include different combinations of limestone, clay, gypsum and sand. We are considering ordinary Portland cement, not special varieties. Concept / Approach: Ordinary Portland cement is manufactured by heating a mixture of limestone and clay in a kiln to form clinker, which is then ground with a small amount of gypsum. Limestone provides calcium oxide, while clay provides silica, alumina and some iron oxide. Gypsum is added mainly to control the setting time of cement. Therefore, the best representation of the composition of cement among the choices is the combination of limestone, clay and gypsum. Step-by-Step Solution: Step 1: Recall that limestone (mainly calcium carbonate) is the primary source of calcium in cement manufacture. Step 2: Clay provides the necessary silica, alumina and iron oxide when heated with limestone in the kiln. Step 3: After the clinker is formed, a small controlled amount of gypsum is added during grinding to regulate the setting time of the cement paste. Step 4: Consider option A, limestone and clay only. This describes the raw mixture before gypsum is added, so it is incomplete as a description of cement composition. Step 5: Option B, limestone, clay and gypsum, correctly includes all three essential ingredients for Portland cement. Verification / Alternative check: Civil engineering and materials science textbooks describe cement manufacture in two basic stages. First, limestone and clay are crushed and heated to form clinker. Second, the clinker is ground with 2 to 5 percent gypsum. Flow diagrams of cement plants always show these three components. Sand is not usually listed as one of the main raw materials in the cement itself, although it is widely used as an aggregate in concrete. This confirms that limestone, clay and gypsum together represent the composition of ordinary Portland cement correctly. Why Other Options Are Wrong: Limestone and clay only: This combination ignores the necessary addition of gypsum for controlling setting time. Limestone and gypsum only: This leaves out clay, which is the major source of silica and alumina. Clay and gypsum only: This does not include limestone, the main source of calcium oxide, so it cannot represent cement composition. Sand, clay and gypsum only: Sand is not a typical raw material inside the cement composition, and limestone is missing. Common Pitfalls: Some learners confuse the composition of cement with the composition of concrete. Concrete is made from cement, sand, aggregates and water, but this question focuses only on the cement itself. Another frequent mistake is to forget the role of gypsum, which is added in smaller amounts but is critical for controlling setting time. Keeping the two stage process in mind, clinker from limestone and clay plus gypsum, makes it easier to remember the correct combination in exams. Final Answer: The chemical composition of ordinary Portland cement is best represented by Limestone, clay and gypsum.

Question 11

A radioactive substance has a half life of six months. After how much time will three fourth of the original substance have decayed, leaving one fourth of the initial amount undecayed?

Accepted Answer

Introduction / Context: This question belongs to nuclear chemistry and radioactive decay. The idea of half life is a standard way of describing how quickly a radioactive substance decays. Many exam questions require you to relate fractions of material remaining to a number of half life periods. In this case, you are asked to find the time when only one fourth of the original sample remains, meaning three fourth of it has decayed. Given Data / Assumptions: The half life of the radioactive substance is six months. Half life is the time in which half of the substance decays. We want the time required for three fourth of the substance to decay, which is equivalent to one fourth remaining. Assume ideal first order radioactive decay where each half life reduces the remaining amount by half. Concept / Approach: Radioactive decay in this context follows an exponential law, but for simple fractions of the original amount, we can think in terms of repeated half life steps. After one half life, half of the substance remains. After two half lives, half of that half remains, meaning one fourth is left. If one fourth is left, then three fourth has decayed. Therefore we simply need to compute the time corresponding to two half life periods and not use the full exponential formula in this simple case. Step-by-Step Solution: Step 1: Note that the half life is six months. This means in six months the amount of radioactive substance becomes one half of the initial amount. Step 2: After the first half life of six months, remaining amount equals one half of the original, and one half has decayed. Step 3: After another half life of six months, the remaining amount is half of the previous half, that is one quarter of the original amount. Step 4: At this point, one quarter remains and three quarters have decayed, which matches the condition in the question. Step 5: The total time that has passed for two half lives is six months plus six months equal to twelve months. Verification / Alternative check: A more formal approach uses the exponential decay relationship N equal to N0 times (1 divided by 2) raised to the power t divided by T where N is the remaining amount, N0 is the initial amount, t is time and T is the half life. We want N equal to N0 divided by 4, so N divided by N0 equals 1 divided by 4. Set 1 divided by 4 equal to (1 divided by 2) raised to the power t divided by T. Recognise that 1 divided by 4 is (1 divided by 2) raised to the power 2. Therefore t divided by T equals 2, so t equals 2T. Substituting T equals six months gives t equals 12 months. This confirms the answer using the general formula. Why Other Options Are Wrong: Six months: After one half life, only half the substance has decayed, not three fourth. Ten months: This time does not correspond to an exact integer number of half lives, so the remaining fraction would not be exactly one quarter. Twenty four months: This equals four half lives, after which only one sixteenth of the original amount remains, not one quarter. Eighteen months: This is three half lives, leaving one eighth of the original substance, not one quarter. Common Pitfalls: One common mistake is to think that three fourth decay must take three times the half life, but half life steps are multiplicative, not additive in terms of fraction remaining. Another error is to subtract three fourth of the half life rather than thinking in terms of repeated halving. The safest method is to track the fraction remaining after each half life step until you reach the required fraction. This approach works well for questions involving simple fractions such as one half, one quarter and one eighth. Final Answer: Three fourth of the substance will have decayed in Twelve months, at which time one fourth of the original amount remains.

Question 12

In basic chemistry, ozone (O3) is related to oxygen (O2) in which way? Ozone is an example of an ______ of oxygen.

Accepted Answer

Introduction / Context: This question is about the relationship between ozone and ordinary oxygen in terms of chemical classification. Both ozone and dioxygen are forms of the same chemical element, oxygen, but they differ in molecular structure and properties. Understanding terms such as allotrope, isotope and isobar is important in basic chemistry when describing different forms of elements and nuclei. Given Data / Assumptions: Ozone has the molecular formula O3 and ordinary oxygen has formula O2. Both substances are composed only of oxygen atoms. We are asked to fill in the blank in the statement that ozone is an example of an something of oxygen. The options include allotrope, isotope, isobar, isotone and coordination complex. Concept / Approach: Allotropes are different structural forms of the same chemical element in the same physical state. For example, diamond and graphite are allotropes of carbon, and O2 and O3 are allotropes of oxygen. Isotopes are atoms of the same element with different numbers of neutrons but the same number of protons, which is a nuclear property rather than a molecular structure property. Isobars and isotones are also nuclear concepts, not used for molecules in this way. Therefore, the correct classification of ozone with respect to oxygen is that it is an allotrope of oxygen. Step-by-Step Solution: Step 1: Identify that both ozone and ordinary oxygen contain only oxygen atoms, with no other elements present. Step 2: Note that O2 and O3 differ only in the number and arrangement of oxygen atoms in the molecule, not in the type of element. Step 3: Recall that the term allotrope is used when one element exists in more than one molecular or crystal form in the same state of matter. Step 4: Recognise that isotope, isobar and isotone refer to nuclear properties of individual atoms, not to whole molecules with different numbers of atoms. Step 5: Conclude that ozone is an allotrope of oxygen and choose option allotrope. Verification / Alternative check: Chemistry textbooks list common allotropes such as diamond and graphite for carbon, white phosphorus and red phosphorus for phosphorus, and dioxygen and ozone for oxygen. Ozone is often described as a triatomic allotrope of oxygen. Isotope examples, such as oxygen 16 and oxygen 18, involve different mass numbers while still forming O2 molecules. This shows that the concept of allotropy, not isotopy, is used to describe the O2 and O3 relationship. These standard references confirm that allotrope is the correct term here. Why Other Options Are Wrong: Isotope: Isotopes refer to atoms of the same element with different numbers of neutrons, not different molecular forms like O2 and O3. Isobar: Isobars are nuclei of different elements that have the same mass number, again not relevant for O2 and O3. Isotone: Isotones have the same number of neutrons but different atomic numbers, a nuclear property unrelated to this question. Coordination complex: Ozone is not a coordination compound; it is a simple molecule composed of three oxygen atoms. Common Pitfalls: Students sometimes confuse allotropes with isotopes because both terms involve different forms of the same element. The key difference is that isotopes relate to nuclear composition of single atoms, while allotropes relate to different structural arrangements of atoms in molecules or crystals. Remember that O2 versus O3 is a structural difference at the molecular level, so allotrope is the appropriate term. Final Answer: Ozone is an Allotrope of oxygen.

Question 13

In redox chemistry, the process of losing electrons from an atom, ion or molecule is known as which of the following?

Accepted Answer

Introduction / Context: This question tests your understanding of basic redox terminology. In oxidation reduction reactions, electrons are transferred between species. Correctly identifying oxidation as loss of electrons and reduction as gain of electrons is a fundamental concept in chemistry that appears in many topics including corrosion, electrochemistry and metabolism. Given Data / Assumptions: The question asks for the name of the process in which electrons are lost. Options include oxidation, reduction, radiation, both oxidation and reduction, and neutralisation. We assume standard modern definitions of oxidation and reduction based on electron transfer. Concept / Approach: The modern view of redox reactions defines oxidation as the loss of electrons by a species and reduction as the gain of electrons by a species. This can be remembered by the common mnemonic OIL RIG, where OIL stands for Oxidation Is Loss and RIG stands for Reduction Is Gain, referring to electrons. Other processes such as radiation and neutralisation involve different phenomena and are not directly about electron loss in this simple sense. Step-by-Step Solution: Step 1: Recall the mnemonic OIL RIG, where Oxidation Is Loss and Reduction Is Gain of electrons. Step 2: The question specifies that the process involves losing electrons, so we focus on the word associated with electron loss. Step 3: From the mnemonic, oxidation corresponds to the loss of electrons. Step 4: Reduction is the gain of electrons and therefore does not match the description of losing electrons. Step 5: Hence, the correct answer is oxidation. Verification / Alternative check: You can check this understanding with simple half reactions. For example, when a sodium atom loses one electron to form Na plus, the half reaction is Na gives Na plus plus e minus. This is described as oxidation of sodium because sodium has lost an electron. Conversely, when chlorine gains an electron to form Cl minus, the half reaction is Cl plus e minus gives Cl minus, which is reduction of chlorine. These textbook examples consistently reinforce that losing electrons is oxidation and gaining electrons is reduction. Why Other Options Are Wrong: Reduction: Defined as the gain of electrons, not their loss. Radiation: Refers to the emission of energy in the form of waves or particles, not specifically the loss of electrons in a redox sense. Both oxidation and reduction: Oxidation and reduction are opposite processes; a given species cannot both gain and lose electrons in the same event. Neutralisation: Describes an acid base reaction that produces salt and water, not an electron transfer process by definition. Common Pitfalls: Many learners confuse the older definition of oxidation as reaction with oxygen with the modern definition based on electron transfer. While they often coincide, electron based definitions are more general and apply to reactions without oxygen. Another mistake is to mix up which process is gain and which is loss of electrons. Using simple mnemonics like OIL RIG or remembering that reduction leads to a reduction of positive charge (more electrons) helps keep the definitions straight. Final Answer: The process of losing electrons is called Oxidation.

Question 14

In basic atomic theory, anions are negatively charged ions. Anions are formed by which of the following processes?

Accepted Answer

Introduction / Context: This question checks your knowledge of how ions are formed, especially anions. In chemistry, ions are atoms or groups of atoms that carry an electric charge due to an imbalance between numbers of protons and electrons. Learning the difference between cations and anions, and how each is formed, is essential for understanding ionic bonding and electrolysis. Given Data / Assumptions: Anions are known to be negatively charged ions. The options describe gaining or losing electrons or neutrons. We assume that the starting point is a neutral atom or group of atoms. Concept / Approach: In a neutral atom, the number of protons equals the number of electrons, so the net charge is zero. If the atom gains extra electrons, it now has more electrons than protons, resulting in a net negative charge. Such a negatively charged ion is called an anion. Conversely, if an atom loses electrons, it has more protons than electrons and becomes a positively charged ion, called a cation. Changes in neutrons do not affect electric charge, only mass and isotopic identity. Step-by-Step Solution: Step 1: Begin with a neutral atom where protons equal electrons, so total charge is zero. Step 2: If this atom gains one or more electrons, the electron count becomes greater than the proton count. Step 3: Because electrons carry negative charge, the net charge of the atom becomes negative. Step 4: A negatively charged ion is called an anion. Step 5: Therefore, anions are formed when neutral atoms gain electrons. Verification / Alternative check: Common examples include chlorine atoms gaining one electron to form chloride ions, Cl minus, and oxygen atoms gaining two electrons to form oxide ions, O two minus. In each case the atom becomes negatively charged by adding electrons. On the other hand, metal atoms such as sodium lose electrons to form Na plus, which are cations. These familiar examples consistently show that gaining electrons leads to anions, while losing electrons leads to cations. This confirms that gaining electrons is the correct mechanism for anion formation. Why Other Options Are Wrong: Losing electrons from neutral atoms: This produces positively charged cations, not anions. Gaining neutrons by neutral atoms: Neutrons are uncharged, so gaining them changes the mass and isotope but not the electric charge. Losing neutrons from neutral atoms: Again, charge remains unchanged because neutrons are neutral, so no anion is formed. Simultaneously gaining protons and electrons: This could keep the charge balanced depending on the numbers gained, and is not the standard simple process for forming anions. Common Pitfalls: Students sometimes rely on the relative size of ions to guess their formation mechanism or confuse anions and cations because both involve electron changes. An easy way to remember the difference is that anion starts with the letter a and can be associated with the word attract, since anions are attracted to the positive anode in electrolysis. They are negative because they have gained electrons. Cations, on the other hand, are positive, and you can think of the letter t in cation as a plus sign. These small memory aids help avoid confusion on exams. Final Answer: Anions are formed by Gaining electrons by neutral atoms.

Question 15

In basic inorganic chemistry, what is the correct chemical formula for common salt, also known as sodium chloride?

Accepted Answer

Introduction / Context: This question tests simple formula writing skills in inorganic chemistry. Common salt used in food and many industrial processes is sodium chloride. Remembering and recognising the correct formula for sodium chloride is a fundamental skill because it is a classic example of an ionic compound formed from a metal and a non metal. Given Data / Assumptions: The substance in question is sodium chloride, also referred to as common salt. We are given several possible formulas containing sodium and chlorine or other elements. We assume standard valencies of sodium and chloride ions. Concept / Approach: Sodium is a group 1 metal and commonly forms a cation with a single positive charge, written as Na plus. Chlorine is a halogen and forms an anion with a single negative charge, written as Cl minus. To form a neutral ionic compound, one Na plus ion combines with one Cl minus ion. The overall formula is therefore NaCl, indicating a one to one ratio of sodium and chloride ions in the crystal lattice of sodium chloride. Step-by-Step Solution: Step 1: Determine the charges of the ions. Sodium forms Na plus and chloride forms Cl minus. Step 2: Combine ions in whole number ratios so that the total positive and negative charges cancel to zero. Step 3: One Na plus ion has plus one charge and one Cl minus ion has minus one charge, so one of each gives a neutral combination. Step 4: Write the empirical formula with one sodium and one chlorine, which is NaCl. Step 5: Check that other combinations such as Na2Cl or NaCl2 would give unbalanced charges and therefore are not correct for simple sodium chloride. Verification / Alternative check: Textbooks and packaging labels for table salt consistently use the formula NaCl for sodium chloride. Structural diagrams of the sodium chloride crystal show a lattice with equal numbers of sodium and chloride ions. Chemical equations for reactions involving salt, such as NaOH plus HCl giving NaCl plus water, also confirm the ratio one to one. All of these sources agree that the correct formula for common salt is NaCl and not any of the other formulas listed. Why Other Options Are Wrong: NaCl2: This would imply one sodium ion and two chloride ions, giving a net negative charge and not representing neutral sodium chloride. Na2Cl: This would imply two sodium ions and one chloride ion, resulting in a net positive charge and not a neutral compound. Na2C: This contains carbon instead of chlorine and does not refer to common salt. NaCl3: This again suggests a charge imbalance and is not the formula for common sodium chloride. Common Pitfalls: Students sometimes try to match subscripts with group numbers or attempt to create formulas by guesswork without checking charge balance. It is always safer to use ionic charges and cross multiply them to get the simplest neutral ratio. In this simple case, because both sodium and chloride carry charges of magnitude one, the formula reduces to NaCl with no subscripts other than one, which are not written explicitly. Final Answer: The correct chemical formula for common salt, sodium chloride, is NaCl.

Question 16

In thermochemistry, an endothermic reaction is best described as which of the following types of process?

Accepted Answer

Introduction / Context: This question belongs to basic thermochemistry and energetics. Reactions can be classified as exothermic or endothermic depending on whether they release or absorb heat. Recognising the correct definition of an endothermic reaction is essential for understanding energy changes in chemical processes such as dissolving, melting and decomposition reactions. Given Data / Assumptions: The term being defined is endothermic reaction. Options describe reactions that either release heat, absorb heat, or do neither. We assume standard chemistry definitions where the system is the reacting substances and surroundings is everything else. Concept / Approach: An endothermic reaction is one in which the reacting system absorbs heat energy from its surroundings. As a result, the surroundings may feel cooler. Typical examples include melting of ice and thermal decomposition reactions. In contrast, exothermic reactions release heat to the surroundings and often cause a temperature increase. Therefore, the correct description of an endothermic reaction must include absorption of heat from the surroundings. Step-by-Step Solution: Step 1: Recall that exothermic means heat flows out of the system and endothermic means heat flows into the system. Step 2: Interpret the phrase endothermic literally as taking in heat, since endo indicates inside and thermo refers to heat. Step 3: Identify the option that explicitly states that heat is absorbed from the surroundings into the reacting system. Step 4: Option B says the reaction absorbs heat from the surroundings, which matches the definition of endothermic. Step 5: Confirm that the other options either describe exothermic processes or do not match standard thermodynamic descriptions. Verification / Alternative check: Consider familiar examples. When ammonium nitrate dissolves in water in an instant cold pack, the pack becomes cold because the dissolution is endothermic and absorbs heat from the surroundings. Similarly, the melting of ice requires heat input, so it is endothermic. In these cases, a thermometer placed in the surroundings registers a temperature drop. These observations are consistent with the definition that endothermic reactions absorb heat from the surroundings, confirming option B as correct. Why Other Options Are Wrong: A reaction in which heat is released to the surroundings: This describes an exothermic reaction, not an endothermic one. A reaction in which neither heat is released nor absorbed: In practical terms most reactions involve some heat effect, and this is not the standard definition of endothermic. A reaction in which heat and work both remain unchanged: This does not relate to the specific concept of endothermic or exothermic processes. A reaction that only produces light but no heat at all: Many light producing reactions still involve heat changes, and this description is not a definition of endothermic behaviour. Common Pitfalls: Students often confuse exothermic and endothermic because both involve energy changes and may think of fire or heat in general terms rather than direction of heat flow. Focusing on the prefixes exo meaning out and endo meaning in helps. Another useful strategy is to draw an energy diagram and note whether the products are at a higher or lower energy level than the reactants. For endothermic reactions, products are at higher energy and the system must absorb heat from the surroundings to climb that energy difference. Final Answer: An endothermic reaction is A reaction in which heat is absorbed from the surroundings.

Question 17

In metallurgy and mineral resources, which of the following minerals is an ore of aluminium used in the extraction of this metal?

Accepted Answer

Introduction / Context: This question relates to basic metallurgy and general knowledge of ores. Different metals are extracted from specific ores, which are naturally occurring minerals with sufficient metal content. Aluminium is an important metal used in many industries. Recognising its ores is a common exam topic in chemistry and general science. Given Data / Assumptions: We are asked to identify an ore of aluminium from the given list. Options include galena, cryolite, cinnabar, Epsom salt and rock salt. We assume conventional school level knowledge of common ores. Concept / Approach: Aluminium is mainly extracted from bauxite, but cryolite (sodium aluminium fluoride) is also associated with aluminium and used in the electrolytic extraction process. Among the given options, cryolite is the only mineral that contains aluminium in its composition. Galena is an ore of lead, cinnabar is an ore of mercury, Epsom salt is a magnesium compound and rock salt is sodium chloride. Therefore, cryolite is the correct choice as an aluminium related ore among the listed minerals. Step-by-Step Solution: Step 1: Recall that cryolite is a mineral with composition close to sodium aluminium fluoride and is historically important in aluminium extraction processes. Step 2: Recognise that galena is lead sulfide and is commonly cited as the main ore of lead. Step 3: Recognise that cinnabar is mercury sulfide and serves as the principal ore of mercury. Step 4: Epsom salt is magnesium sulfate heptahydrate, used medically and not as an aluminium ore. Step 5: Rock salt is sodium chloride and is an ore of sodium, not aluminium. Verification / Alternative check: Metallurgy textbooks list bauxite as the principal ore of aluminium and also mention cryolite as an important mineral in the Hall process, where it serves as a solvent for bauxite in the electrolytic cell. Reference lists of ores match galena with lead, cinnabar with mercury, and cryolite with aluminium electrolytic processes. Because only cryolite has a direct connection to aluminium among the options, it must be the correct answer. Why Other Options Are Wrong: Galena: This is PbS and is the chief ore of lead, not aluminium. Cinnabar: This is HgS and is the chief ore of mercury. Epsom salt: This is MgSO4 dot 7H2O, used as a magnesium source and in medicine, not in aluminium extraction. Rock salt: This is NaCl, an ore of sodium and source of common salt. Common Pitfalls: Students sometimes automatically think of bauxite as the only ore of aluminium and may be confused when it is not listed among the options. In such cases, it is important to recall other aluminium related minerals and choose the one most clearly linked to aluminium, which is cryolite. Another mistake is to guess based on unfamiliar names rather than connecting minerals to their known metal contents. Regular revision of common ores and their metals helps avoid such errors. Final Answer: The ore of aluminium among the options given is Cryolite.

Question 18

In environmental chemistry, which of the following toxic gases was released during the Bhopal gas tragedy of 1984 in India?

Accepted Answer

Introduction / Context: This question refers to one of the worst industrial disasters in history, the Bhopal gas tragedy in India. General science and environmental studies courses often include this incident as an example of the dangers of industrial toxic chemicals. Knowing the name of the main gas involved is part of basic environmental awareness and general knowledge. Given Data / Assumptions: The event is the Bhopal gas tragedy. We are asked which gas was released during this disaster. Options mention different isocyanate related compounds and other gases. Concept / Approach: The Bhopal gas tragedy involved the accidental release of a highly toxic gas called methyl isocyanate. It was stored at a pesticide plant and leaked into the surrounding area, causing thousands of deaths and long term health effects. The other similar sounding names in the options can cause confusion, but methyl isocyanate is the correct chemical identity of the gas involved in this incident. Step-by-Step Solution: Step 1: Recall that the Bhopal gas tragedy happened at a pesticide plant where methyl isocyanate was used and stored. Step 2: The gas that escaped in large quantities from the storage tank was methyl isocyanate. Step 3: Check the options and identify methyl isocyanate explicitly among them. Step 4: The other options mention sodium, nitrogen, and potassium isothiocyanate, which are different substances and were not the principal toxic gas in the Bhopal disaster. Step 5: Therefore, select methyl isocyanate as the correct answer. Verification / Alternative check: Reports, documentaries and textbooks that describe the Bhopal disaster consistently name methyl isocyanate as the toxic gas released. It is often abbreviated as MIC in the literature. The incident is used in case studies of industrial safety and environmental law, and every reliable source repeats this same chemical name for the gas. Other isothiocyanates do not appear in those accounts as the main released gas, confirming that methyl isocyanate is the correct choice. Why Other Options Are Wrong: Sodium isothiocyanate: This is an entirely different chemical compound and was not the primary gas released in Bhopal. Nitrogen isothiocyanate: This is not the gas associated with the tragedy. Potassium isothiocyanate: Again, not the compound linked to the Bhopal disaster. Carbon monoxide: A common toxic gas, but not the one historically connected with the Bhopal gas tragedy. Common Pitfalls: The main pitfall in this question is confusion due to similar sounding chemical names that include cyanate or isothiocyanate. Some learners may also mistakenly recall other famous gas incidents and mix up their details. Focusing on the specific historical association of Bhopal with methyl isocyanate and remembering the abbreviation MIC can help fix the correct answer in memory for exams and general awareness quizzes. Final Answer: The toxic gas released during the Bhopal gas tragedy was Methyl isocyanate.

Question 19

From the following polymer categories, nylon threads used in textiles and ropes are made of which type of synthetic polymer?

Accepted Answer

Introduction / Context: This question belongs to polymer chemistry and materials science. Nylon is a common synthetic fibre used in clothing, ropes, fishing lines and many mechanical parts. Identifying the chemical class of polymers that form nylon helps you understand the link between monomer structure and polymer properties in basic organic chemistry. Given Data / Assumptions: Nylon threads are the materials under discussion. Options include polyester, polyamide, polyvinyl polymer, polysaccharide and natural rubber polymer. We assume standard usage of these terms in polymer chemistry. Concept / Approach: Nylon is the general name for a family of synthetic polymers known as polyamides. These polymers contain repeating units linked by amide bonds, also called peptide bonds. Polyesters, on the other hand, contain ester linkages. Polyvinyl polymers are based on vinyl monomers like vinyl chloride and form PVC or related plastics. Polysaccharides are natural carbohydrate polymers such as starch and cellulose, and natural rubber is a polyisoprene polymer. Because nylon contains amide linkages and is produced from diamines and dicarboxylic acids, it is classified as a polyamide polymer. Step-by-Step Solution: Step 1: Recall that nylon 6,6 is formed from hexamethylene diamine and adipic acid, which join to form amide bonds between monomer units. Step 2: The repeating unit in nylon contains the amide group, which is characteristic of polyamides. Step 3: Polyesters such as Terylene or Dacron have ester linkages and are not called nylon. Step 4: Polyvinyl polymers like PVC have vinyl based backbones without the amide linkages seen in nylon. Step 5: Therefore, nylon threads are made of polyamide polymers. Verification / Alternative check: Textbooks on synthetic fibres state that nylon is a generic name for synthetic linear polyamides. Commercial examples include nylon 6 and nylon 6,6. The polymer chains are formed by condensation reactions that create amide bonds. In contrast, polyester fibres are based on terephthalic acid and ethylene glycol, forming ester linkages. Data sheets and fibre labels in the textile industry also classify nylon under polyamide materials. These facts confirm that nylon threads are polyamide polymers. Why Other Options Are Wrong: Polyester polymer: This refers to a different class of polymers with ester linkages, not the amide linked structure of nylon. Polyvinyl polymer: Examples include PVC, which does not share the chemical structure or name of nylon. Polysaccharide: These are natural polymers like cellulose and starch, not synthetic nylon. Natural rubber polymer: This is mainly polyisoprene and is not the same as synthetic nylon fibres. Common Pitfalls: Some learners confuse polyester and polyamide because both are used as synthetic fibres in clothing. Without careful attention to the names and chemical structures, they may think all synthetic clothing fibres are polyester. Remember that nylon specifically refers to polyamide fibres, while polyester is a different group. Associating the letters am in polyamide with the amide linkage can help you recall this connection for exam questions and practical knowledge. Final Answer: Nylon threads are made of Polyamide polymer.

Question 20

In terms of acidity and basicity, what is the nature of the pH of fresh milk under normal conditions?

Accepted Answer

Introduction / Context: This question concerns the pH of milk, a common household liquid. pH is a measure of acidity or basicity of a solution. Understanding whether milk is acidic, basic or neutral is a simple but useful application of pH concepts in everyday life and is frequently asked in school level science questions. Given Data / Assumptions: The substance being discussed is fresh milk. Options describe it as slightly acidic, slightly basic, highly acidic, highly basic or exactly neutral. We assume normal fresh milk, not spoiled or processed products with added substances. Concept / Approach: Fresh milk contains lactic acid and other components that give it a pH slightly below 7. A pH of 7 is neutral, values below 7 are acidic and values above 7 are basic. Milk typically has a pH in the range of about 6 to 6.8, which makes it slightly acidic rather than strongly acidic or basic. Therefore, the correct description of the nature of pH of milk is that it is slightly acidic. Step-by-Step Solution: Step 1: Recall the pH scale where 7 represents neutrality, values less than 7 are acidic and values greater than 7 are basic. Step 2: Fresh milk contains small amounts of lactic acid and other acidic components. Step 3: Because of these acids, the pH of milk is somewhat less than 7, placing it in the acidic region. Step 4: The acidity is not extreme, so milk is not classified as highly acidic; instead it is only slightly acidic. Step 5: Therefore, the best description of the pH nature of milk is slightly acidic. Verification / Alternative check: Measurements of milk pH in food science studies and quality control laboratories usually show values around 6.5, with small variations depending on source and storage. This value is clearly on the acidic side of the scale but very close to neutral. Sensory experience also suggests that fresh milk does not taste strongly sour, which supports the idea that its acidity is mild. When milk spoils, increased lactic acid production lowers the pH further, making it more acidic and sour. These observations reinforce that fresh milk is slightly acidic in nature. Why Other Options Are Wrong: Slightly basic: This would require a pH above 7, which does not match typical measurements of fresh milk. Highly acidic: Milk is nowhere near the acidity of strong acids like lemon juice or vinegar, so this description is incorrect. Highly basic: Milk does not have high pH values and does not behave like strong bases such as sodium hydroxide. Perfectly neutral with pH exactly 7: In practice, milk pH is usually below 7, so it is not exactly neutral. Common Pitfalls: Some learners assume that because milk is often used to soothe acidity in the stomach, it must be basic. In reality, its pH is only slightly acidic, and its effect in the stomach is more complex, involving buffering rather than simple neutralisation. Others may think of all white liquids as neutral, which is not a reliable rule. Remember instead to link milk with a pH a little less than 7 and describe it as slightly acidic for exam questions. Final Answer: The nature of the pH of fresh milk is Slightly acidic.

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