Question 1

The polar ice caps at the Earth poles are mainly composed of which type of water when considered by salt content?

Accepted Answer

Introduction / Context: This question belongs to environmental science and basic geography. It asks about the type of water that makes up the polar ice caps. These huge masses of frozen water are important for global climate regulation and sea level. Knowing whether this ice is salty or fresh helps in understanding the water cycle and the behavior of oceans when ice melts. Given Data / Assumptions: The location considered is the polar ice caps around the Earth poles. We are asked about the salt content type of this water. Options include saline water, salt water, sour water, fresh water, and only distilled water. Concept / Approach: When sea water freezes, most of the salts are excluded from the ice crystals and remain in the surrounding liquid. As a result, the ice that forms is relatively low in dissolved salts and is close to fresh water. Similarly, ice formed from snowfall is naturally fresh. The polar ice caps are therefore made of fresh water rather than salt water. Sour water is not a standard classification in this context, and distilled water is a laboratory term rather than a natural description. Step-by-Step Solution: Step 1: Recall that salt in sea water does not easily fit into the crystal structure of ice as it forms. Step 2: During freezing, salts are mostly left behind in the remaining liquid water. Step 3: The resulting ice is low in salt content and behaves like fresh water when melted. Step 4: Polar ice caps consist of such frozen fresh water from both sea ice and accumulated snow. Step 5: Therefore, among the given options, fresh water best describes the composition of polar ice caps. Verification / Alternative check: Scientific studies and textbooks on oceanography state that sea ice is significantly less salty than the seawater from which it forms. Ice cores drilled from polar ice caps show that the trapped water has very low salt content. This is why melting polar ice directly adds mainly fresh water to the ocean system. These findings confirm that the polar ice caps are made of fresh water rather than saline water. Why Other Options Are Wrong: Saline water: Refers to water with dissolved salts, typical of oceans, not frozen ice caps. Salt water: Same as saline, which does not describe the actual ice composition. Sour water: Not a standard scientific term for natural water types in this context. Only chemically treated distilled water: Ice caps are not formed from laboratory distilled water; they are natural accumulations of fresh water. Common Pitfalls: Some learners assume that because ice at the poles often forms on the oceans, it must be salty. However, the process of freezing excludes most salts from the ice structure. Recognizing that the salinity of water and the salinity of ice are different allows for a more accurate understanding of climate change discussions and predictions about sea level. Practising questions like this helps fix the correct concept that polar ice caps contain mostly fresh water. Final Answer: The polar ice caps are mainly composed of Fresh water.

Question 2

Which of the following chemical elements is essential for the synthesis of the thyroid hormone thyroxine in the human body?

Accepted Answer

Introduction / Context: This question relates to basic human physiology and nutrition. The hormone thyroxine, produced by the thyroid gland in the neck, plays an important role in controlling metabolism, growth, and development. A particular chemical element is needed for the synthesis of thyroxine. Deficiency of this element can lead to thyroid disorders such as goitre and hypothyroidism. Given Data / Assumptions: The hormone considered is thyroxine, produced by the thyroid gland. We are asked which element is essential for its synthesis. Options include potassium, calcium, iodine, sodium, and magnesium. Concept / Approach: Thyroxine is often written as T4, indicating that it contains four atoms of iodine in its structure. Another thyroid hormone, triiodothyronine (T3), contains three iodine atoms. The thyroid gland uses iodine from the diet to build these hormones. Without adequate iodine, the gland cannot produce enough hormone and may enlarge, forming a goitre. Therefore, iodine is the essential element for thyroxine synthesis. Step-by-Step Solution: Step 1: Recall that thyroid hormones are iodine containing compounds produced in the thyroid gland. Step 2: The most common hormone, thyroxine (T4), has four iodine atoms in each molecule. Step 3: The body must obtain iodine from food sources such as iodised salt, seafood, and some vegetables. Step 4: If dietary iodine is insufficient, thyroxine production drops and the thyroid gland may enlarge to try to trap more iodine. Step 5: Therefore, among the listed options, iodine is the essential element for thyroxine synthesis. Verification / Alternative check: Biology textbooks and nutrition guides identify iodine deficiency as a major cause of endemic goitre in regions with low natural iodine levels in soil and water. Public health measures such as iodised salt programs are designed to ensure adequate iodine intake for thyroid hormone production. The chemical names and formulas of thyroid hormones explicitly include iodine as part of their structures. All of this confirms that iodine is the correct answer. Why Other Options Are Wrong: Potassium: Important for nerve and muscle function but not specifically required to build the thyroxine molecule. Calcium: Essential for bones, teeth, and muscle contraction, but not the key element in thyroxine structure. Sodium: Important for fluid balance and nerve function, but not part of thyroxine. Magnesium: Involved in enzyme activity and energy production, but not the defining element of thyroid hormones. Common Pitfalls: Some students focus on general importance of elements like calcium or sodium and may incorrectly choose them simply because they are well known. It is important to remember the specific connection between iodine and thyroid hormones. Associating the letter I in words like iodine and thyroid helps create a memory link that reduces confusion in multiple choice questions about endocrine and nutritional health. Final Answer: The element essential for the synthesis of thyroxine is Iodine.

Question 3

In food chemistry, the main cause of rancidity in fatty foods is the gradual ______ of fats and oils when exposed to air, light, or heat. Which process correctly completes the sentence?

Accepted Answer

Introduction / Context: This question concerns the concept of rancidity in food chemistry. Rancidity is the unpleasant smell and taste that develop in fats and oils when they spoil. Understanding the main chemical process behind rancidity helps in learning about food preservation and storage conditions as well as the role of antioxidants in packaging and formulation. Given Data / Assumptions: The focus is on rancidity of foods containing fats and oils. The sentence states that rancidity is due to some chemical change of fats and oils. Options include hydrolysis, clarification, reduction, oxidation, and crystallisation. Concept / Approach: Rancidity, especially oxidative rancidity, occurs when unsaturated fats react with oxygen in the air. This oxidation produces aldehydes, ketones, and other compounds with strong off odors and flavors. Although hydrolysis can also contribute to certain types of rancidity, the main cause commonly discussed at basic level is oxidation. Thus, the correct word to complete the sentence is oxidation. Step-by-Step Solution: Step 1: Recall that fats and oils are mostly triglycerides containing fatty acid chains. Step 2: When exposed to air, light, and heat, unsaturated fatty acid chains can react with oxygen. Step 3: This process is oxidation, which leads to breakdown products like aldehydes and peroxides. Step 4: These breakdown products are responsible for the stale, unpleasant smell and taste known as rancidity. Step 5: Therefore, the blank should be filled with oxidation rather than hydrolysis or other processes. Verification / Alternative check: Food science references define oxidative rancidity as the main mechanism by which edible oils spoil. Antioxidants are added to packaged foods to slow this oxidative process, confirming that oxidation is at the center of the problem. Sensory changes such as off odors and off flavors correlate with increased levels of oxidation products. While hydrolytic rancidity can occur when fats react with water and enzymes, the general exam level statement usually points to oxidation as the main cause. Why Other Options Are Wrong: Hydrolysis: This process breaks fats into glycerol and fatty acids and can play a role in some spoilage, but the typical textbook definition of rancidity focuses on oxidation. Clarification: Refers to making a liquid clear by removing impurities, not a spoilage process. Reduction: The opposite of oxidation and does not describe the main cause of rancidity. Crystallisation: Refers to formation of solid crystals from a solution or melt, not the chemical spoilage of fats. Common Pitfalls: Students sometimes choose hydrolysis because they associate it with splitting fats, or they may simply guess from unfamiliar terms. Remembering that rancidity is closely linked with oxygen and that antioxidants are used to prevent it is a helpful clue that oxidation is involved. In exam questions, words like air, oxygen, and exposure often point directly to oxidation as the underlying process. Final Answer: The main cause of rancidity in foods is the oxidation of fats and oils.

Question 4

If the atomic number of krypton is 36, what is the correct electronic configuration of this noble gas element?

Accepted Answer

Introduction / Context: This general chemistry question tests your understanding of electronic configuration and how electrons are arranged in shells (energy levels) for a neutral atom. Krypton is a noble gas with atomic number 36, and exam questions often use such elements to check whether students really know how to distribute electrons across different shells in the correct order. A solid grasp of electronic configuration helps you understand chemical reactivity, valency, and periodic trends such as atomic radius and ionisation energy. Given Data / Assumptions: Element considered: krypton (Kr). Atomic number of krypton = 36. The configuration is to be written in terms of shells: K, L, M and N. We assume a neutral atom, so number of electrons = atomic number = 36. Maximum electrons in a shell are given by the formula 2 * n^2, where n is the shell number. Concept / Approach: The main concept is that the atomic number of an element gives the number of electrons in a neutral atom. These electrons are arranged in shells in order of increasing principal quantum number, subject to the capacity rule for each shell. For school level shell notation the K shell can hold up to 2 electrons, L up to 8, M up to 18, and N up to 32. For krypton we need to place 36 electrons into these shells, filling each shell to its allowed maximum before moving to the next, while respecting the total of 36 electrons. Step-by-Step Solution: Step 1: Total electrons in neutral krypton atom = 36. Step 2: Fill the K shell (n = 1). Maximum capacity is 2 * 1^2 = 2 electrons. So K shell gets 2 electrons. Step 3: Remaining electrons = 36 - 2 = 34. Step 4: Fill the L shell (n = 2). Maximum capacity is 2 * 2^2 = 8 electrons. So L shell gets 8 electrons. Step 5: Remaining electrons = 34 - 8 = 26. Step 6: Fill the M shell (n = 3). Maximum capacity is 2 * 3^2 = 18 electrons. So M shell gets 18 electrons. Step 7: Remaining electrons = 26 - 18 = 8. Step 8: Put the remaining 8 electrons into the N shell (n = 4). So N shell has 8 electrons. Step 9: Collecting these values we get K = 2, L = 8, M = 18, N = 8, written as 2,8,18,8. Verification / Alternative check: Add up the electrons in all shells and verify that the total equals the atomic number. Calculation: 2 + 8 + 18 + 8 = 36, which matches the given atomic number of krypton. Also, krypton is a noble gas in period 4 and group 18, so it should have a completely filled outer shell. An outer shell with 8 electrons (octet) is consistent with a stable noble gas configuration. Why Other Options Are Wrong: Option B, 2,8,8,18, gives a total of 36 electrons but gives an unrealistic distribution because the N shell is shown as containing 18 electrons while the M shell has only 8, which does not follow the usual filling rule for shells in this notation. Option C, 2,18,8,8, places 18 electrons in the L shell, which exceeds its maximum capacity of 8 in the simple shell model. Option D, 2,8,16,10, has the wrong distribution and also gives a total of 36, but does not match the allowed shell capacities. Option E, 2,10,18,6, violates the usual capacities and also does not correspond to the standard shell filling for krypton. Common Pitfalls: A common mistake is to mix up subshell notation (like 3d and 4s) with simple shell notation. At this level, the question only expects the K, L, M, N shell distribution. Another frequent error is to forget the maximum capacity of the L shell and incorrectly put more than 8 electrons there. Students may also add correctly to 36 but ignore the rule that lower shells must be filled before higher ones in this notation scheme. Final Answer: The correct electronic configuration of krypton in shell notation is 2,8,18,8, which uses all 36 electrons and respects the maximum capacity of each shell.

Question 5

Chlorine gas is widely used in industry for the manufacture of which common chemical product?

Accepted Answer

Introduction / Context: This question checks your knowledge of important industrial uses of chlorine gas, which is a highly reactive and widely used chemical in inorganic chemistry and everyday life. Knowing which common products are made using chlorine helps you connect textbook chemistry to real industrial applications such as water treatment, disinfection, and manufacturing of cleaning agents. Given Data / Assumptions: Reactant of interest: chlorine gas Cl2. We are asked about manufacture of a common compound used in households or industry. Options include baking powder, bleaching powder, baking soda, washing soda and caustic soda. Assume standard large scale industrial processes. Concept / Approach: Chlorine gas is a strong oxidising agent. It is used in many processes, including water chlorination and production of various chlorine containing compounds. Bleaching powder is chemically calcium oxychloride, often written as CaOCl2 or more precisely Ca(OCl)Cl. It is produced industrially by passing chlorine gas over dry slaked lime Ca(OH)2. On the other hand, compounds like baking soda and washing soda are usually produced by processes such as the Solvay process, which do not use chlorine gas as the main reactant. Therefore, we identify which product in the options clearly uses chlorine gas in its manufacturing route. Step-by-Step Solution: Step 1: Recall the chemical formula and preparation of bleaching powder. Step 2: Bleaching powder is essentially calcium oxychloride, produced by the reaction of chlorine gas with dry slaked lime. Step 3: The simplified reaction is: Ca(OH)2 + Cl2 → CaOCl2 + H2O. Step 4: In this process chlorine gas is a key reactant, so bleaching powder is directly linked to the industrial use of chlorine. Step 5: Baking soda (sodium hydrogen carbonate, NaHCO3) and washing soda (sodium carbonate, Na2CO3) are prepared mainly via the Solvay process from sodium chloride, ammonia and carbon dioxide, not from chlorine gas itself. Step 6: Baking powder is a mixture containing baking soda and an acid salt, and is not manufactured by direct reaction with chlorine gas. Step 7: Caustic soda (sodium hydroxide) is produced mainly by electrolysis of brine rather than by direct reaction of chlorine gas with a base in this context. Verification / Alternative check: A simple memory check is that bleaching powder is associated with chlorine both in name and in use. It is widely used for bleaching cloth, paper and for disinfecting water, all processes that rely on the oxidising power of chlorine. In textbooks the preparation of bleaching powder from slaked lime and chlorine is a standard example in the chapter dealing with chlorine and its compounds, which confirms that the correct product is bleaching powder. Why Other Options Are Wrong: Baking powder is a mixture of baking soda and acid salts and is not directly manufactured using chlorine gas. Baking soda and washing soda are sodium carbonate based compounds produced mainly by the Solvay process, where chlorine does not act as a main reactant. Caustic soda is produced industrially by electrolysis of brine, which also produces chlorine and hydrogen as by products, but the question clearly asks for a compound manufactured using chlorine gas, and bleaching powder is the direct example given in textbooks. Common Pitfalls: Students sometimes confuse the role of sodium chloride and chlorine gas. While sodium chloride contains the chloride ion, it is not the same as free chlorine gas. Another confusion arises between bleaching powder and bleaching agents in general. Some may mistakenly think of washing soda or baking powder as bleaching agents, but in school chemistry bleaching powder is the classic chlorine based compound. Carefully reading the phrase used in the manufacture helps avoid these mix ups. Final Answer: Chlorine gas is widely used in industry to manufacture bleaching powder, a common disinfectant and bleaching agent.

Question 6

Which scientist laid the foundation of modern chemical science by establishing two important laws of chemical combination?

Accepted Answer

Introduction / Context: This question belongs to the history of chemistry and focuses on the early development of laws of chemical combination. Understanding which scientist first formulated key quantitative laws, such as the law of conservation of mass, helps you see how modern chemistry moved from qualitative observations to quantitative science. Antoine L. Lavoisier is often called the father of modern chemistry because of his pioneering work in this area. Given Data / Assumptions: The question asks about a scientist who laid the foundation of chemical sciences. Clue: he established two important laws of chemical combination. Options include Proust, Lavoisier, Rutherford, Democritus and Dalton. We assume standard school syllabus coverage of these scientists. Concept / Approach: The central idea is to recall which chemist is associated with quantitative laws regarding how substances react and combine. Lavoisier carried out careful experiments on combustion and chemical reactions. He formulated the law of conservation of mass and contributed significantly to the understanding of elements and compounds. Proust is known mainly for the law of definite proportions. Dalton is known for atomic theory and the law of multiple proportions. Rutherford is associated with the nuclear model of the atom. Democritus proposed a philosophical idea of indivisible atoms but not quantitative laws. Step-by-Step Solution: Step 1: Recall that Antoine L. Lavoisier performed accurate mass measurements during chemical reactions. Step 2: From these experiments he formulated the law of conservation of mass, which states that mass is neither created nor destroyed in a chemical reaction. Step 3: Lavoisier also helped clarify the concept of elements and compounds, and laid groundwork for later laws of combination, providing a quantitative basis for chemistry. Step 4: Joseph L. Proust proposed the law of definite proportions, which states that a given compound always contains the same elements in the same proportion by mass, but he is generally associated with one major law rather than two. Step 5: John Dalton later proposed atomic theory and the law of multiple proportions, building on earlier work including that of Lavoisier and Proust. Step 6: Rutherford worked in nuclear physics and experimental atomic structure, and Democritus was a philosopher with no experimental chemical laws. Step 7: Therefore the scientist who laid the foundation of chemical science by establishing two important laws of chemical combination is Antoine L. Lavoisier. Verification / Alternative check: In most school textbooks Lavoisier is given the title father of modern chemistry and is credited with establishing the law of conservation of mass and clarifying the concept of elements. He also helped move chemistry away from phlogiston theory towards an oxygen based understanding of combustion. When a question mentions foundation of chemical sciences and two important laws, it almost always points to Lavoisier, while Proust and Dalton are presented as later contributors who extended this foundation. Why Other Options Are Wrong: Joseph L. Proust contributed one key law, the law of definite proportions, but not two major laws. Ernest Rutherford is famous for his alpha scattering experiment and the nuclear model of the atom, not for laws of chemical combination. Democritus proposed the idea that matter is composed of indivisible atoms, but his ideas were philosophical and did not involve quantitative chemical laws. John Dalton developed atomic theory and the law of multiple proportions, but his work came after the fundamental mass based work of Lavoisier. Common Pitfalls: Students sometimes confuse Dalton and Lavoisier because both names appear frequently in early chapters. A simple way to remember is that Lavoisier focused on careful mass measurements and laws of conservation, while Dalton focused on a model of atoms and multiple proportions. Another pitfall is to choose Proust because the question mentions laws of chemical combination, but Proust is mainly linked with one law rather than a broader foundation of modern chemical science. Final Answer: The scientist who laid the foundation of modern chemical science by establishing two important laws of chemical combination is Antoine L. Lavoisier.

Question 7

In a Dobereiner triad containing elements A, B and C, if the atomic mass of A is 7 and that of C is 39, what is the atomic mass of the middle element B?

Accepted Answer

Introduction / Context: This question deals with Dobereiner triads, an early attempt to classify elements before the modern periodic table. The idea was that certain groups of three elements had similar properties and a simple numerical relationship between their atomic masses. Understanding how to use the Dobereiner triad rule helps you appreciate the development of periodic classification and shows how chemists first noticed patterns among the elements. Given Data / Assumptions: Elements A, B and C form a Dobereiner triad. Atomic mass of element A = 7. Atomic mass of element C = 39. We are asked to find the atomic mass of the middle element B. We assume standard definition of Dobereiner triads from school level chemistry. Concept / Approach: In a Dobereiner triad, three elements with similar chemical properties are arranged in order of increasing atomic mass. The characteristic property is that the atomic mass of the middle element is approximately the arithmetic mean of the atomic masses of the first and third elements. In simple terms, mass of B = (mass of A + mass of C) / 2. This relation is only approximate for real elements, but in exam questions the numbers are chosen to work out exactly. Step-by-Step Solution: Step 1: Write down the given masses: A has atomic mass 7, C has atomic mass 39. Step 2: Apply the Dobereiner triad rule, which states that the mass of B is the average of the masses of A and C. Step 3: Use the formula: atomic mass of B = (atomic mass of A + atomic mass of C) / 2. Step 4: Substitute the given values: atomic mass of B = (7 + 39) / 2. Step 5: Compute the numerator: 7 + 39 = 46. Step 6: Divide by 2: 46 / 2 = 23. Step 7: Therefore the atomic mass of the middle element B is 23. Verification / Alternative check: You can verify by checking that the three numbers 7, 23 and 39 form a sequence where the middle value is exactly halfway between the first and the last. The difference between 7 and 23 is 16, and the difference between 23 and 39 is also 16. This symmetric spacing is another way to see the arithmetic mean relation. This property is exactly what Dobereiner observed in some sets of elements, such as lithium, sodium and potassium, where the atomic mass of sodium is roughly the average of lithium and potassium masses. Why Other Options Are Wrong: Option A, 20, does not satisfy the arithmetic mean relation because (7 + 39) / 2 is not 20. Option B, 40, is even larger than the mass of C, which contradicts the requirement that B is the middle element. Option D, 12, is too close to A and far from the average of 7 and 39. Option E, 28, also does not equal the computed mean of 23. Only 23 fits the rule that the mass of B equals the average of the masses of A and C. Common Pitfalls: A frequent mistake is to confuse Dobereiner triads with other periodic trends and simply choose the numerical middle of the options rather than calculate the arithmetic mean. Another pitfall is adding the masses and forgetting to divide by 2, which would give 46 instead of 23. Always remember that triads are based on an average, not on any geometric or harmonic mean in this syllabus. Writing out the simple formula before substituting values can help avoid errors. Final Answer: By applying the Dobereiner triad rule, the atomic mass of the middle element B comes out to be 23.

Question 8

When a metal reacts with a dilute acid, which products are typically formed in the chemical reaction?

Accepted Answer

Introduction / Context: This question checks your understanding of basic inorganic reaction patterns, specifically the reaction between a metal and a dilute acid. Such reactions are standard in school chemistry and appear frequently in examinations and laboratory work. Knowing the general products of this reaction helps you predict what happens when metals are added to hydrochloric acid or sulphuric acid and explains observable effects such as effervescence due to gas release. Given Data / Assumptions: Reactants: a metal and a dilute acid. Acids considered are typically dilute hydrochloric acid or dilute sulphuric acid. We assume that the metal is above hydrogen in the reactivity series so that the reaction actually proceeds. We are asked to identify the products formed. Concept / Approach: The general reaction of a reactive metal with a dilute acid is: metal + dilute acid → salt of the metal + hydrogen gas. For example, zinc reacts with dilute sulphuric acid to give zinc sulphate and hydrogen gas. The metal displaces hydrogen from the acid because the metal is more reactive than hydrogen. No chlorine is produced unless chlorine containing oxidising agents are used in a different type of reaction. Water is not the principal product in this simple displacement reaction. Step-by-Step Solution: Step 1: Recall the general form of the reaction: Metal + Dilute acid → Salt + Hydrogen gas. Step 2: Take a typical example. Zinc reacting with dilute sulphuric acid: Zn + H2SO4 (dilute) → ZnSO4 + H2. Step 3: In this reaction, ZnSO4 is the salt of zinc and sulphuric acid, and H2 is hydrogen gas. Step 4: Another example is magnesium reacting with dilute hydrochloric acid: Mg + 2HCl → MgCl2 + H2. Step 5: In all such reactions, the pattern remains the same. You always obtain a salt and hydrogen gas as the main products. Step 6: Therefore the correct description of the products is a salt and hydrogen. Verification / Alternative check: You can verify by remembering the reactivity series of metals. Metals above hydrogen in the reactivity series can displace hydrogen from dilute acids. The displaced hydrogen appears as hydrogen gas, which can be tested by the pop sound when a burning splint is brought near the mouth of the test tube. The remaining ionic part of the acid combines with the metal cation to form a salt. There is no mechanism in this simple displacement process to form chlorine gas, water, or a base as the main product. Why Other Options Are Wrong: Option A, a salt and chlorine, is incorrect because chlorine gas is produced from reactions involving oxidising chlorine compounds or electrolysis, not from simple metal acid reactions in school level examples. Option C, a salt and water, describes neutralisation reactions between acids and bases, not the reaction of acids with metals. Option D, a salt and a base, does not match any standard reaction type for metal with acid. Option E, only hydrogen, ignores the formation of the ionic salt, which is always produced in aqueous solution. Common Pitfalls: Students often confuse metal acid reactions with acid base neutralisation, where salt and water are formed. It is important to notice that in this question the second reactant is a metal, not a base. Another pitfall is to assume that any reaction involving hydrochloric acid and metals might produce chlorine gas, but at this level the standard pattern for such questions always emphasises hydrogen gas. Always check which species is being displaced from the acid, which is hydrogen in this case. Final Answer: When a metal reacts with a dilute acid, the typical products are a salt and hydrogen gas.

Question 9

What is the approximate percentage by mass of oxygen in the compound aluminium sulphate, Al2(SO4)3?

Accepted Answer

Introduction / Context: This numerical question tests your ability to calculate percentage composition by mass of an element in a compound, a fundamental skill in stoichiometry. Understanding how to work out mass percentages helps in preparing solutions, verifying empirical formulae and handling many types of chemical calculations in both school and competitive exams. Given Data / Assumptions: Compound: aluminium sulphate, formula Al2(SO4)3. We need the percentage by mass of oxygen. Use approximate atomic masses: Al ≈ 27, S ≈ 32, O ≈ 16, unless otherwise specified. Assume one mole of Al2(SO4)3 for calculation. Concept / Approach: Percentage by mass of an element in a compound is given by the formula: (mass of that element in one mole of the compound / molar mass of the compound) * 100. So we first calculate total molar mass of Al2(SO4)3, then calculate total mass of oxygen atoms present in that formula unit. Finally we divide the mass of oxygen by the total molar mass and multiply by 100 to get the percentage. Step-by-Step Solution: Step 1: Write the formula Al2(SO4)3 clearly. This means there are 2 aluminium atoms, 3 sulphur atoms and 12 oxygen atoms in one formula unit. Step 2: Compute the total molar mass of the compound. Step 3: Use approximate atomic masses: Al = 27, S = 32, O = 16. Step 4: Mass contribution from aluminium: 2 * 27 = 54. Step 5: Mass contribution from sulphur: 3 * 32 = 96. Step 6: Mass contribution from oxygen: there are 12 oxygen atoms (3 sulphate groups, each having 4 oxygen atoms). So mass of oxygen = 12 * 16 = 192. Step 7: Total molar mass M of Al2(SO4)3 = 54 + 96 + 192 = 342. Step 8: Now calculate percentage of oxygen by mass: percentage of oxygen = (mass of oxygen / total molar mass) * 100. Step 9: Substitute values: percentage of oxygen = (192 / 342) * 100. Step 10: Evaluate the fraction 192 / 342 ≈ 0.561. Step 11: Multiply by 100 to obtain percentage: 0.561 * 100 ≈ 56.1%. Step 12: Therefore, the percentage by mass of oxygen in aluminium sulphate is approximately 56.1%. Verification / Alternative check: You can quickly check the reasonableness of this answer by noting that oxygen has a large share of the molar mass because there are 12 oxygen atoms. The mass from oxygen alone is 192, which is more than half of the total molar mass 342. Thus a value slightly above 50% makes sense. An answer around 56% is consistent with this logic, while values near 50% or 60% would need careful checking. Performing the division with a calculator or more precise atomic masses will still give a value close to 56.1%, confirming the option. Why Other Options Are Wrong: Option A, 57.7%, is a bit higher than the calculated value and would require either a different formula or different atomic masses. Option C, 53.1%, is too low relative to the large oxygen contribution. Option D, 52.6%, is also too low and not supported by the ratio 192 to 342. Option E, 60.0%, overestimates the oxygen fraction and would suggest oxygen mass dominates much more than it actually does. Only 56.1% matches the correct calculation using standard atomic masses and the correct formula. Common Pitfalls: Common mistakes include miscounting the number of oxygen atoms in the formula, sometimes taking only 4 instead of 12, or forgetting that the entire SO4 group is repeated three times. Another error is to use incorrect atomic masses or to round too aggressively early in the calculation. To avoid such issues, always expand the formula term by term, write the count of each element clearly, and only round the final percentage at the end. Final Answer: The percentage by mass of oxygen in Al2(SO4)3 is approximately 56.1%.

Question 10

Which of the following gaseous hydrocarbons is commonly used as an illuminating gas in lamps and welding torches?

Accepted Answer

Introduction / Context: This question relates to the practical uses of hydrocarbons, specifically which gas is used as an illuminating gas. Illuminating gases were historically used in lamps and continue to be used for welding and cutting metals. Knowing which hydrocarbon is employed in such applications deepens your understanding of fuel properties, such as flame temperature and brightness. Given Data / Assumptions: The options are methane, propane, ethyne, ethane and butane. The question asks which gaseous hydrocarbon is used as an illuminating gas. We assume standard school level knowledge of hydrocarbon uses. Concept / Approach: Ethyne, also called acetylene, is a hydrocarbon with formula C2H2 that burns in oxygen with a very hot and luminous flame. This property makes it useful as an illuminating gas and in oxy acetylene welding. Methane, propane, ethane and butane are also fuels but are more commonly used for domestic cooking, heating and as LPG components, not specifically referred to as illuminating gas in basic chemistry textbooks. Step-by-Step Solution: Step 1: Recall that ethyne (acetylene) has a triple bond and burns with a very bright flame in oxygen. Step 2: Historically, ethyne was used in lamps because its flame is luminous and can provide bright light, which is why it is called an illuminating gas. Step 3: In modern applications, ethyne is mainly used in oxy acetylene welding, where its high flame temperature is important. Step 4: Methane is the main component of natural gas and is used for heating and cooking, but it is not commonly referred to as illuminating gas in school chemistry. Step 5: Propane and butane are components of LPG used in household gas cylinders, again not described as illuminating gas in this context. Step 6: Ethane is a simple alkane used in some industrial processes, but again not the typical illuminating gas mentioned in textbooks. Step 7: Therefore, ethyne is the correct answer. Verification / Alternative check: A quick memory check is that many textbooks specifically state that acetylene is used as an illuminating gas and in welding. The brightness of its flame and its ability to burn at a high temperature in oxygen make it suitable for these tasks. If you have seen diagrams of oxy acetylene torches in your chemistry book, that is a strong hint that ethyne is the special case among the listed hydrocarbons. Why Other Options Are Wrong: Methane, propane, ethane and butane are fuels, but they are not usually labelled as illuminating gas in school level terminology. Methane is associated with natural gas, propane and butane with LPG cylinders, and ethane with petrochemical feedstock. The question expects the specific textbook phrase illuminating gas, which is tied to ethyne. Therefore the other options, while combustible, are not the intended answer here. Common Pitfalls: Students may think of methane because it is the simplest hydrocarbon gas, or propane and butane because they are common in domestic gas cylinders. However, the key phrase illuminating gas points to older historical uses where acetylene lamps were common. Another pitfall is confusing ethane and ethyne due to similar names. Remember that the triple bond in ethyne gives it distinctive burning characteristics compared to ethane. Final Answer: The hydrocarbon commonly used as an illuminating gas is ethyne (acetylene).

Question 11

Among the elements F, O, H and Li, which one has the largest atomic radius?

Accepted Answer

Introduction / Context: This question focuses on periodic trends, specifically the variation of atomic radius across periods and down groups in the periodic table. Understanding atomic radius is important because it influences many other properties such as ionisation energy, electronegativity and chemical reactivity. Here, you are asked to compare the sizes of four simple elements: fluorine, oxygen, hydrogen and lithium. Given Data / Assumptions: Elements to compare: fluorine (F), oxygen (O), hydrogen (H) and lithium (Li). We need to determine which has the largest atomic radius. We assume neutral isolated atoms in their ground state. Periodic positions: H and Li are in period 1 and 2 respectively; O and F are in period 2. Concept / Approach: Atomic radius increases down a group and decreases from left to right across a period. This is because additional shells are added when moving down a group, making atoms larger, while increasing nuclear charge across a period pulls electrons closer, making atoms smaller. In period 2, lithium is on the far left, oxygen and fluorine are on the right side and are nonmetals. Hydrogen is in period 1 with only one shell. For elements in the same period, the leftmost element has the largest atomic radius. When comparing period 1 and period 2, period 2 elements usually have a larger radius because they have two shells instead of one. Step-by-Step Solution: Step 1: Note that hydrogen is in period 1 with electron configuration 1s1, so it has only one shell. Step 2: Lithium is in group 1 and period 2 with configuration 2,1, so it has two shells. Step 3: Oxygen is in period 2, group 16 and fluorine is in period 2, group 17, with higher nuclear charge than lithium. Step 4: Across a period from left to right, atomic radius decreases due to increasing nuclear charge pulling electrons closer. Step 5: Therefore, in period 2, lithium has the largest radius, followed by beryllium, while oxygen and fluorine are much smaller. Step 6: When comparing hydrogen and lithium, lithium has an extra shell, so its atomic radius is larger than that of hydrogen. Step 7: Thus, among F, O, H and Li, lithium has the largest atomic radius. Verification / Alternative check: Standard periodic tables often provide approximate atomic radii. If you check these values you will see that lithium has a radius larger than that of hydrogen and much larger than that of oxygen and fluorine. Conceptually, the key point is that additional shells always increase radius significantly, while moving right across a period gradually decreases radius. Lithium being a period 2 alkali metal combines both effects to give a relatively large atomic size among the given options. Why Other Options Are Wrong: Fluorine and oxygen are both to the right of lithium in period 2, so they are smaller because their larger nuclear charge pulls the same shell of electrons closer. Hydrogen, although it is in the first period, has only one shell and therefore a smaller radius than lithium, which has two shells. The idea that all have the same radius is incorrect, since periodic trends clearly show variation in atomic sizes across periods and down groups. Common Pitfalls: Students sometimes assume that hydrogen, being very light, must also be very small in atomic radius, leading them to ignore the role of additional shells in lithium. Others incorrectly think that elements with higher atomic number are always larger, without considering where they lie in the periodic table. The correct approach is always to use trends: radius increases down a group and decreases from left to right across a period. Final Answer: Among F, O, H and Li, the element with the largest atomic radius is lithium (Li).

Question 12

When zinc metal is added to an aqueous solution of copper sulphate, which substance is displaced from the solution?

Accepted Answer

Introduction / Context: This question examines your understanding of displacement reactions in inorganic chemistry, specifically the reaction between zinc metal and copper sulphate solution. Such reactions illustrate the reactivity series of metals and show how a more reactive metal can displace a less reactive metal from its compound in solution. Given Data / Assumptions: Reactants: zinc metal (Zn) and aqueous copper sulphate solution (CuSO4). We are asked which substance is displaced from the solution when zinc is added. We assume standard conditions and that the reaction occurs. We use the usual reactivity series where zinc is more reactive than copper. Concept / Approach: The reactivity series shows that a more reactive metal can displace a less reactive metal from its salt solution. Zinc is placed above copper in the reactivity series. When zinc is added to a solution of copper sulphate, zinc will displace copper from the solution, forming zinc sulphate and depositing copper metal. The sulphate ion remains in solution and is not displaced. Hydrogen and oxygen are not involved as displaced elements in this reaction. Step-by-Step Solution: Step 1: Write the word equation for the reaction: Zinc + Copper sulphate → Zinc sulphate + Copper. Step 2: Write the balanced chemical equation: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s). Step 3: In copper sulphate, copper is present as Cu2+ ions and sulphate as SO4 2− ions. Step 4: Zinc atoms lose electrons and enter the solution as Zn2+ ions, while Cu2+ ions gain electrons and are deposited as copper metal. Step 5: As a result, copper is removed from the solution and zinc takes its place in the sulphate salt. Step 6: Therefore, copper is the substance displaced from the solution. Verification / Alternative check: Observation in the laboratory supports this explanation. When zinc is placed in blue copper sulphate solution, the blue colour gradually fades because Cu2+ ions are being removed, and a red brown deposit of copper metal appears on the zinc surface or at the bottom of the vessel. The solution gradually becomes colourless as it becomes zinc sulphate solution. These visual changes confirm that copper is the species displaced. Why Other Options Are Wrong: Hydrogen is not part of copper sulphate and therefore cannot be displaced in this reaction. Zinc is not displaced; it is the metal that enters the solution and replaces copper, so it is the displacing metal, not the displaced substance. The sulphate ion remains in solution associated with zinc and is not displaced as a separate substance. Oxygen is not an individual reactant or product here, so it is also incorrect. Common Pitfalls: Students sometimes confuse which metal is more reactive and may think copper displaces zinc. Remember that in the reactivity series zinc lies above copper, so zinc is more reactive. Another mistake is to misinterpret the word displaced and think of any species that changes form in the reaction, rather than focusing on the less reactive metal ion being removed from its compound. Final Answer: When zinc is added to copper sulphate solution, the substance displaced from the solution is copper.

Question 13

In which mode of heat transfer is heat transmitted from a region of higher temperature to a region of lower temperature by the actual bulk motion of the molecules of the medium?

Accepted Answer

Introduction / Context: Although listed under chemistry, this question connects to basic physics and thermal science. It asks about the mechanism of heat transfer that involves the actual movement of molecules, or bulk motion of fluid, from a hotter region to a cooler region. Understanding the three main modes of heat transfer, conduction, convection and radiation, is important in both physics and applied chemistry, especially in topics like heat exchangers and everyday heat flow. Given Data / Assumptions: We are looking for the mode where heat moves through actual motion of molecules. Options include conduction, convection, radiation, both conduction and convection, and insulation. Assume standard textbook definitions of conduction, convection and radiation. Concept / Approach: There are three basic modes of heat transfer. Conduction is transfer of heat through a material without bulk movement of the material itself, mainly via collisions and vibrations of particles. Convection is transfer of heat in fluids (liquids and gases) by the actual movement of heated fluid from one place to another. Radiation is transfer of energy by electromagnetic waves that does not require any medium. The phrase actual motion of the molecules clearly points to convection. Step-by-Step Solution: Step 1: Recall the definition of conduction: heat transfer through a solid or stationary medium without net movement of matter, only energy is passed on from particle to particle. Step 2: Recall the definition of convection: heat transfer in a fluid where warmer, less dense regions rise and cooler, denser regions sink, causing bulk motion of the fluid and carrying heat with it. Step 3: Recall the definition of radiation: transfer of energy by electromagnetic waves, which can occur even through vacuum. Step 4: The question explicitly mentions actual motion of the molecules from a high temperature region to a lower temperature region. Step 5: This description matches convection, because conduction does not involve bulk motion and radiation does not involve any material particles. Step 6: Therefore, the correct mode of heat transfer described is convection. Verification / Alternative check: Everyday examples help verify this. When water is heated in a pot, the water at the bottom becomes hot, becomes less dense and rises, while cooler water sinks. This circulation pattern is convection. Similarly, warm air rising in the atmosphere and cool air replacing it is also convection. In contrast, heating a metal rod at one end and feeling the other end become hot over time is conduction. The warmth felt from the Sun is due to radiation. Only convection fits the description that the medium itself moves as a whole while carrying heat. Why Other Options Are Wrong: Conduction does not involve bulk motion of the medium, only microscopic energy transfer between neighbouring particles. Radiation does not require any medium at all and hence does not involve molecular motion. The combination both conduction and convection is too broad and does not answer the specific phrase used in the question. Insulation is actually meant to reduce heat transfer and is not a mode of heat transfer itself. Common Pitfalls: A common confusion is between conduction and convection, especially in fluids, because in real systems both can occur together. To avoid this, focus on the phrase bulk motion or actual movement of the fluid, which is the hallmark of convection. Another pitfall is to forget that radiation can occur through vacuum and therefore does not involve molecular motion at all. Final Answer: Heat transmitted by actual bulk motion of the molecules of the medium from a hotter region to a cooler region is transferred by convection.

Question 14

In the modern long form of the periodic table, how many groups and how many periods are present?

Accepted Answer

Introduction / Context: This question checks your basic factual knowledge of the structure of the modern periodic table. The number of groups and periods is a standard fact that appears in almost every chemistry textbook. Knowing this structure is essential because later concepts such as periodic trends, block classification, and element families are all defined in terms of groups and periods. Given Data / Assumptions: The periodic table referred to is the modern long form periodic table. Groups are vertical columns, periods are horizontal rows. We need to recall the total number of each. Options present various combinations such as 18 and 7, 18 and 8, and so on. Concept / Approach: In the long form periodic table, elements are arranged according to increasing atomic number. There are 18 vertical columns called groups and 7 horizontal rows called periods. Groups represent families of elements with similar valence electron configurations, while periods represent elements with the same number of electron shells. This arrangement allows clear display of s, p, d and f block elements. Step-by-Step Solution: Step 1: Recall that the modern periodic table consists of 7 horizontal rows. Step 2: These rows are called periods and are numbered from 1 to 7. Step 3: Recall that there are 18 vertical columns called groups. Step 4: Groups are numbered from 1 to 18 in the modern IUPAC system, replacing older A and B group notations. Step 5: Therefore the correct pair is 18 groups and 7 periods. Step 6: Match this with the options provided; it corresponds to option 18 groups and 7 periods. Verification / Alternative check: You can quickly verify by looking at any standard periodic table chart. The top row shows elements from hydrogen to helium, which is period 1. Subsequent periods go down to period 7, which includes the actinides. For groups, you can count from the alkali metals as group 1 through to the noble gases as group 18. This universal structure confirms the answer 18 groups and 7 periods. Why Other Options Are Wrong: Options suggesting 8 periods are incorrect because there are currently only 7 recognised periods in the modern table. Options suggesting 16 groups do not match the IUPAC numbering, which clearly defines 18 groups. The combination 7 groups and 18 periods reverses the reality and is also incorrect. Thus, only the combination 18 groups and 7 periods matches the established structure of the modern periodic table. Common Pitfalls: Some students confuse older notations in which main groups were labelled using Roman numerals and letters A and B. They may also confuse the number of periods with the highest principal quantum number of known elements, but the correct count in basic chemistry is fixed at 7 periods. Remember that the periodic table you see on classroom walls or in textbooks uses 18 numbered groups and 7 numbered periods. Final Answer: The modern periodic table has 18 groups and 7 periods.

Question 15

Which of the following everyday materials is an example of a mixture rather than a pure substance?

Accepted Answer

Introduction / Context: This question tests your understanding of the difference between pure substances and mixtures. Many everyday materials look uniform but are actually mixtures of several components. Recognising examples of mixtures helps you apply theoretical definitions of elements, compounds and mixtures to real world substances like milk, fuels and gases used in homes. Given Data / Assumptions: Options: milk, gasoline, liquefied petroleum gas (LPG) and combinations of these. We must identify which of these are mixtures. Assume normal commercial forms of these materials, not chemically purified versions. Concept / Approach: A pure substance has a fixed composition and uniform properties throughout, and is either an element or a compound. A mixture is a physical combination of two or more substances in variable proportions. Milk is an emulsion of fats, proteins, lactose and minerals in water. Gasoline is a mixture of many different hydrocarbons. Liquefied petroleum gas is a mixture of propane, butane and sometimes other gases. Therefore, each of the listed substances is a mixture. Step-by-Step Solution: Step 1: Consider milk. Milk contains water, fats, proteins, lactose, minerals and vitamins. These components are not present in fixed chemical proportions and can be separated by physical methods such as centrifugation. Thus, milk is a mixture. Step 2: Consider gasoline. Gasoline is not a single compound but a carefully blended mixture of many hydrocarbons, including alkanes and aromatic compounds. The exact composition can vary between refineries and fuel grades, which is characteristic of a mixture. Step 3: Consider liquefied petroleum gas. LPG is typically a mixture of propane and butane, sometimes with small amounts of other gases. The proportions can vary depending on climate or supplier. This again is a mixture, not a single compound. Step 4: Since all three substances are mixtures, the correct answer is that all options are correct. Verification / Alternative check: Another way to test whether a material is a mixture is to ask if its composition can vary and whether it can be separated into components by physical methods. Different samples of milk from different animals or brands will have slightly different fat and protein contents. Gasoline composition is adjusted seasonally to improve engine performance. LPG composition varies between countries. These variations are typical of mixtures, whereas pure substances like distilled water always have the same composition H2O. Why Other Options Are Wrong: Choosing any one of milk, gasoline or LPG alone as the only mixture would ignore the fact that the others also meet the definition. The option None of these is incorrect because all three are well known mixtures. Therefore, the only fully correct statement among the options is that all options are correct. Common Pitfalls: Students may mistakenly think that because gasoline and LPG are sold under standard names they must be pure substances. In reality, fuels are carefully formulated mixtures tailored for specific performance requirements. Similarly, the uniform appearance of milk might suggest a pure substance, but it is actually a colloidal mixture. Always relate back to the definition of a pure substance and ask whether composition is fixed and unique or variable and separable. Final Answer: Milk, gasoline and liquefied petroleum gas are all mixtures, so the correct choice is all options are correct.

Question 16

Researchers at the University of Illinois reported a new form of matter called excitonium. The term excitonium was first coined in the 1960s by which physicist?

Accepted Answer

Introduction / Context: This question links modern physics and chemistry by asking about excitonium, a newly confirmed state of matter related to excitons in condensed matter systems. While the discovery is recent, competitive exams sometimes ask about the historical origin of such terms. Here, the focus is on which theoretical physicist first coined the term excitonium in the 1960s, long before its experimental confirmation. Given Data / Assumptions: Researchers at the University of Illinois reported evidence for excitonium. The term excitonium was coined in the 1960s. Options list several well known theoretical physicists. We assume standard general knowledge sources regarding this discovery. Concept / Approach: Excitonium is a predicted condensate state formed by excitons, which are bound states of an electron and an electron hole. The concept and the term were introduced theoretically decades before experimental confirmation. The question asks about the person who coined the term, not who discovered excitons or who did the later experiments. According to widely cited reports, the term excitonium was coined by Harvard theoretical physicist Bertrand Halperin in the 1960s, sometimes mentioned along with Rice in the theoretical work. Step-by-Step Solution: Step 1: Recall that excitons are quasiparticles formed when an electron in a solid is bound to a positively charged hole. Step 2: The idea that a macroscopic condensate of excitons might form a new state of matter led to the theoretical proposal of excitonium. Step 3: Historical accounts state that the term excitonium was coined in the 1960s. Step 4: Among the options, Bertrand Halperin is the condensed matter theorist associated with coining this term. Step 5: Other names in the options, such as Wigner, Dirac and Bloch, are famous for other fundamental contributions but not for naming excitonium. Step 6: Therefore, the correct answer is Bertrand Halperin. Verification / Alternative check: If you consult reliable science news sources or notes for general science exams, they typically mention that the existence of excitonium was theorised after the term was coined by Bertrand Halperin, a Harvard theoretical physicist, in the 1960s. The University of Illinois team published experimental evidence in 2017 that supported the existence of this state of matter. This historical sequence confirms that the question refers to Halperin rather than the experimental team or other famous physicists. Why Other Options Are Wrong: Eugene Wigner made significant contributions to quantum mechanics and nuclear physics but is not associated with the name excitonium. Paul Dirac is famous for the Dirac equation and prediction of antimatter. Felix Bloch is known for Bloch waves and work in solid state physics but not for naming excitonium. Niels Bohr developed the Bohr model of the atom and contributed to quantum theory. None of these are credited with coining the term excitonium. Common Pitfalls: Students may be tempted to choose a more familiar name such as Dirac or Bohr simply because those names are well known, even if they do not recall a specific connection to excitonium. General knowledge questions often test whether you can remember lesser known names linked to very specific discoveries or terms. Reading the science and technology sections of exam preparation material carefully helps avoid such guess based errors. Final Answer: The term excitonium was first coined in the 1960s by theoretical physicist Bertrand Halperin.

Question 17

Magnesium is an essential constituent metal of which important biological molecule?

Accepted Answer

Introduction / Context: This question connects basic inorganic chemistry with biology by asking about the role of magnesium in living systems. Many bioinorganic questions focus on which metal ion is central in a particular biomolecule, such as iron in haemoglobin or magnesium in chlorophyll. Knowing these associations is important for both chemistry and biology examinations. Given Data / Assumptions: We are asked which important biological molecule contains magnesium as a constituent metal. Options include chlorophyll, DNA, mitochondria, ribosomes and haemoglobin. We assume standard school level biology and chemistry content. Concept / Approach: Chlorophyll is the green pigment in plants responsible for capturing light energy for photosynthesis. Its core structure is a porphyrin like ring with a central magnesium ion. This is similar in shape to haemoglobin, which has a porphyrin ring with a central iron ion. DNA, mitochondria and ribosomes are important structures or molecules but do not contain magnesium as a central metal ion in the same characteristic way that chlorophyll does. Step-by-Step Solution: Step 1: Recall that chlorophyll is a green pigment located in the chloroplasts of plant cells. Step 2: The structural formula of chlorophyll shows a large ring system called a chlorin ring, which is related to porphyrin. Step 3: At the centre of this ring is a magnesium ion that coordinates with nitrogen atoms of the ring. Step 4: This central magnesium ion is crucial for the light absorption properties of chlorophyll, allowing efficient capture of solar energy. Step 5: DNA is a nucleic acid polymer composed of sugar, phosphate and nitrogenous bases; while magnesium ions may stabilise structures in solution, they are not permanent constituent metals in the molecule in the same way. Step 6: Mitochondria and ribosomes are cell organelles, not single molecules with a central metal ion. Step 7: Haemoglobin contains iron at the centre of a heme group, not magnesium. Step 8: Therefore, the biological molecule with magnesium as a constituent metal is chlorophyll. Verification / Alternative check: You can verify this by recalling the common exam pairings: iron in haemoglobin, magnesium in chlorophyll, calcium in bones and teeth, and iodine in thyroid hormones. These associations are often listed together in science textbooks. When you see a question pairing a metal and a biomolecule, mentally link magnesium with chlorophyll and iron with haemoglobin to avoid confusion. Why Other Options Are Wrong: DNA does not have a central metal ion as part of its basic covalent structure. Mitochondria and ribosomes contain many different proteins and molecules, plus various metal ions, but they are not defined by a single central metal the way chlorophyll is defined by magnesium. Haemoglobin is the oxygen carrying protein in blood and contains iron in the heme group, not magnesium. Common Pitfalls: A common mistake is to confuse the metal in chlorophyll with that in haemoglobin, since both structures involve a large ring and a central metal ion. Remember the simple rule: green plants, green pigment, magnesium; red blood, red pigment, iron. Another pitfall is to overthink the role of magnesium in biochemical reactions in general. While magnesium ions can activate enzymes and stabilise nucleic acids, the distinctive structural example in exams is magnesium in chlorophyll. Final Answer: Magnesium is a constituent metal of the chlorophyll molecule in plants.

Question 18

Which gas is passed over dry slaked lime in the industrial preparation of bleaching powder?

Accepted Answer

Introduction / Context: This question tests your knowledge of the industrial preparation of bleaching powder, an important chlorine based compound. Bleaching powder is used for disinfection and bleaching in households and industry. Knowing which gas is used in its production helps reinforce your understanding of the chemistry of chlorine and its compounds. Given Data / Assumptions: Substance prepared: bleaching powder. The process involves passing a gas over dry slaked lime Ca(OH)2. Options include oxygen, hydrogen, nitrogen, chlorine and ammonia. Assume the standard process described in school chemistry textbooks. Concept / Approach: Bleaching powder is essentially calcium oxychloride, written as Ca(OCl)Cl or CaOCl2. It is produced industrially by passing chlorine gas over dry slaked lime. The chlorine reacts with calcium hydroxide to form bleaching powder and water. Therefore, the correct gas is chlorine, not oxygen or other gases. Step-by-Step Solution: Step 1: Recall the formula of slaked lime, which is calcium hydroxide, Ca(OH)2. Step 2: In the manufacturing process, dry Ca(OH)2 is spread on trays in a chamber. Step 3: Chlorine gas is passed over the dry slaked lime. Step 4: The simplified reaction is often written as: Ca(OH)2 + Cl2 → CaOCl2 + H2O. Step 5: CaOCl2 is bleaching powder, which contains both CaCl2 and Ca(OCl)2 like species. Step 6: Other gases such as oxygen, hydrogen, nitrogen and ammonia are not used in this preparation step. Step 7: Hence, the gas used is chlorine. Verification / Alternative check: Most introductory chemistry texts describe chlorine and its compounds in one chapter. In that chapter, the manufacture of bleaching powder is highlighted as a key industrial use of chlorine gas. The fact that bleaching powder is a chlorine containing bleaching agent should remind you of the direct role of chlorine gas in its production. Oxygen is associated more with combustion and oxidation processes, while ammonia is associated with fertilisers, not bleaching powder. Why Other Options Are Wrong: Oxygen does support combustion and is involved in many oxidation reactions but is not the gas used to make bleaching powder. Hydrogen and nitrogen do not form bleaching powder with slaked lime under these conditions. Ammonia reacts with certain substances to form fertilisers and other compounds, but not bleaching powder. Only chlorine reacts with Ca(OH)2 in the described industrial process to yield bleaching powder. Common Pitfalls: Because bleaching powder and bleaching agents in general are associated with oxidation, some students might guess oxygen. However, the key to avoiding this mistake is remembering that bleaching powder is a chlorine based compound. Another pitfall is to misremember the manufacturing process and confuse chlorine gas with other gases used in different industrial reactions. Keeping a separate mental list of key industrial processes like Haber process for ammonia, Contact process for sulphuric acid and bleaching powder preparation helps clarify such questions. Final Answer: In the industrial preparation of bleaching powder, the gas passed over dry slaked lime is chlorine.

Question 19

In arc welding, why is argon gas commonly used around the metal being welded?

Accepted Answer

Introduction / Context: This question connects chemistry with engineering practice by asking about the role of argon gas in arc welding. Arc welding involves very high temperatures and molten metal, and shielding gases like argon play an important role in controlling the welding environment. Understanding why argon is chosen relies on knowledge of inert gases and their chemical properties. Given Data / Assumptions: Process: arc welding of metals. Shielding gas used: argon. We must identify the main reason for using argon. Options mention reactivity, melting point, flammability, calorific value and oxidising nature. Concept / Approach: Argon is a noble gas and is chemically inert under most conditions. In arc welding, a shielding gas is required to prevent the hot metal and molten weld pool from reacting with air, which contains oxygen and nitrogen. If these reactive gases reach the weld, they can cause oxidation, porosity and weak joints. An inert gas like argon provides a protective atmosphere that does not react with the metal. Therefore, low reactivity with the metal is the key reason for using argon. Step-by-Step Solution: Step 1: Recall that argon is a group 18 element, so it has a full valence shell and is chemically inert. Step 2: In arc welding, an electric arc melts the metal at the joint, creating a molten pool that is very hot and reactive. Step 3: If this hot metal is exposed to air, oxygen and nitrogen can react with it, forming oxides or nitrides that weaken the weld. Step 4: To prevent this, a shielding gas is used to surround the arc and the molten pool, driving away air. Step 5: Argon is chosen because it does not react with the molten metal and does not introduce any unwanted chemical species. Step 6: Therefore the best description is that argon has low reactivity with the metal. Verification / Alternative check: In welding technology, arc processes are often named after their shielding gas, such as gas tungsten arc welding, where argon is commonly used, or mixtures of argon and other gases. Technical manuals emphasise that the shielding gas must be inert or nearly inert to protect the weld from atmospheric contamination. This confirms that the main requirement for argon is its inert character, not its energy content or flammability. Why Other Options Are Wrong: Argon does not lower the melting point of the metal; the melting point is determined mainly by the metal itself and any alloying elements. Argon is not flammable and does not have a high calorific value because it is not a fuel. It is also not a strong oxidising agent, which would be undesirable, since oxidation would damage the weld. These properties make the other options incorrect for the question asked. Common Pitfalls: Some students might assume that any gas used in welding must be combustible, but in reality the purpose of the shielding gas is to protect, not to fuel the arc. Another confusion is to think that argon somehow cools or alters melting points; in fact, its role is mainly chemical protection. Remember that noble gases like argon and helium are used in situations where an inert protective atmosphere is needed. Final Answer: Argon is used in arc welding mainly because of its low reactivity with metal, providing an inert shielding atmosphere around the weld.

Question 20

Which statement correctly describes the chemical behaviour of inert gases (noble gases)?

Accepted Answer

Introduction / Context: This question tests your understanding of the basic properties of inert gases, also known as noble gases, such as helium, neon, argon, krypton, xenon and radon. These gases occupy group 18 of the periodic table and are known for their very low chemical reactivity. Recognising this property is fundamental in both general chemistry and periodic table questions. Given Data / Assumptions: We are asked about the chemical behaviour of inert gases. Options include statements about miscibility with water, stability, reactivity and reducing ability. We assume standard school level understanding that noble gases have complete valence shells. Concept / Approach: Inert gases have a completely filled valence shell of electrons. This stable electronic configuration makes them extremely reluctant to gain, lose or share electrons, so they do not normally form chemical compounds. As a result, they are described as chemically unreactive or inert. While some heavier noble gases can form a few compounds under extreme conditions, at school level they are treated as unreactive. Therefore, the statement that they are chemically unreactive is the best description. Step-by-Step Solution: Step 1: Recall the group of inert gases: helium, neon, argon, krypton, xenon and radon. Step 2: Each noble gas has a full valence shell, such as 2 for helium and 8 for neon and argon. Step 3: Because their valence shell is full, there is no strong tendency to gain or lose electrons. Step 4: This electronic stability makes them very unreactive under normal conditions. Step 5: Therefore, the phrase chemically unreactive is the correct description of their behaviour. Verification / Alternative check: You can verify this by recalling that noble gases were long thought to be completely inert and were called inert gases in older textbooks. They do not normally form compounds with other elements and are used in applications where a non reactive atmosphere is required, such as in lighting, welding and preserving reactive materials. Only advanced courses discuss rare compounds of xenon and krypton, but these do not change the general statement that noble gases are chemically unreactive compared to other elements. Why Other Options Are Wrong: The phrase miscible with water refers to physical mixing, not chemical reactivity, and noble gases are only slightly soluble in water. Saying they are not stable is incorrect because their filled valence shells make them very stable. Chemically very active is the opposite of their actual behaviour. Strong reducing agents are substances that readily donate electrons, while noble gases do not easily donate or accept electrons, so this description is also wrong. Common Pitfalls: Students sometimes equate inert with physically inactive in every sense, forgetting that noble gases can dissolve slightly in fluids or move freely as gases. However, the term inert in this context relates specifically to chemical reactivity. Another pitfall is to overemphasise exceptional xenon compounds and think that inert gases are generally reactive, which is not true within the scope of school level chemistry. Final Answer: Inert gases are best described as chemically unreactive under normal conditions.

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