Question 1

Which one of the following fundamental properties of an element is always an exact whole number by definition?

Accepted Answer

Introduction / Context: This question checks understanding of basic atomic properties and their numerical nature. In particular, it contrasts atomic number, atomic mass, and macroscopic properties such as atomic radius and atomic volume. Recognising that atomic number is defined as a whole number representing the count of protons is fundamental for reading the periodic table and for distinguishing it from atomic mass, which is usually a decimal value due to isotopes. Given Data / Assumptions: - The options are atomic mass, atomic number, atomic radius, and atomic volume. - The task is to identify which of these is always an exact whole number for any element. - We assume standard definitions of these terms in school level chemistry. Concept / Approach: Atomic number is defined as the number of protons in the nucleus of an atom and is therefore always a positive integer. Atomic mass listed on a periodic table is typically a weighted average of the masses of all naturally occurring isotopes, resulting in a decimal value. Atomic radius and atomic volume depend on how atoms pack and interact and are measured quantities that can vary and be expressed with decimal values. Therefore, atomic number is unique in being always a whole number. Step-by-Step Solution: Step 1: Recall that atomic number is the count of protons in the nucleus of an atom, which must be an integer since you cannot have a fraction of a proton. Step 2: Look at atomic mass values on a periodic table. They are often non integer, for example 35.5 for chlorine, because they represent average masses of isotopes weighted by natural abundance. Step 3: Atomic radius is measured experimentally and depends on many factors such as bonding and coordination number. The values are not restricted to whole numbers and are expressed with decimal precision. Step 4: Atomic volume is derived from molar volume and atomic packing and again is a measured property that can take any positive real value, not just integers. Step 5: Since only atomic number is inherently a count of discrete particles, it is always an exact whole number and is the correct answer. Verification / Alternative check: If you examine a standard periodic table, the small whole number often printed above the element symbol is the atomic number. This number always increases by one from element to element, such as 1 for hydrogen, 2 for helium, and 3 for lithium. In contrast, the larger decimal number printed below the symbol is the relative atomic mass and is very often not an integer. Additionally, reference books always talk about atomic number as an integer quantity, confirming that it must be a whole number by definition. Why Other Options Are Wrong: - Atomic mass: It is usually a decimal because it represents the weighted average of different isotopes, so it is not always a whole number. - Atomic radius: It is a measured length that can take any positive real value and is typically written with decimal places, not restricted to integers. - Atomic volume: It is calculated from bulk measurements and varies with conditions, so it is also not constrained to integer values. Common Pitfalls: Students sometimes confuse atomic number and atomic mass because both appear near the element symbol in the periodic table. Another mistake is to think that because some elements have approximate whole number masses, atomic mass is always an integer, which is incorrect. To avoid these issues, remember that atomic number is the fundamental count of protons and defines the identity of the element, while atomic mass is a measured average that can be fractional. Final Answer: Atomic number is always an exact whole number, since it counts the number of protons in the nucleus of an atom.

Question 2

Which pair of scientists proposed the theory of radioactive disintegration to explain the spontaneous decay of unstable atomic nuclei?

Accepted Answer

Introduction / Context: This question belongs to the history of atomic physics and nuclear chemistry. It asks about the scientists who developed the theory of radioactive disintegration, which explains how unstable nuclei spontaneously decay into more stable forms. Recognising the contribution of Rutherford and Soddy is important for understanding how early observations of radioactivity were converted into a coherent theoretical model. Given Data / Assumptions: - Four pairs of scientists are listed: Rutherford and Soddy, Soddy and Fajan, Thomson and Rutherford, and Hahn and Strassmann. - The question asks which pair presented the theory of radioactive disintegration. - We assume standard historical accounts used in chemistry and physics curricula. Concept / Approach: Radioactive disintegration theory explains that radioactive elements undergo spontaneous and random decay, transforming into other elements while emitting radiation such as alpha or beta particles. Ernest Rutherford and Frederick Soddy worked together at the start of the twentieth century to interpret the experimental observations of radioactivity and proposed that radioactive substances decay at characteristic rates. This work led to the idea of half life and the understanding that radioactivity involves nuclear transformations. Therefore, the correct pair is Rutherford and Soddy. Step-by-Step Solution: Step 1: Recall that Ernest Rutherford is widely known for his work on radioactivity, alpha scattering, and the nuclear model of the atom. Step 2: Frederick Soddy collaborated with Rutherford on early studies of radioactive substances. Together they observed that radioactive elements transform into other elements over time. Step 3: From their work, they proposed the theory of radioactive disintegration, which states that a radioactive atom has a certain probability of decaying per unit time and that such decays lead to transmutation of elements. Step 4: Soddy and Fajan worked later on the radioactive displacement law, which describes how the position of an element in the periodic table changes during radioactive decay, but that is a separate concept from the original theory of disintegration. Step 5: Thomson and Rutherford are both important figures in atomic physics, but J J Thomson is best known for discovering the electron, not for creating the theory of radioactive disintegration. Step 6: Hahn and Strassmann discovered nuclear fission in uranium, a later development in nuclear physics, and did not propose the original theory of radioactive disintegration. Verification / Alternative check: Standard nuclear chemistry texts attribute the theory of radioactive decay and the concept of half life to the collaborative work of Rutherford and Soddy. Their papers describe how a radioactive element decays according to an exponential law and how daughter elements form. Historical timelines place their work before later contributions by Fajan and Hahn, which confirms that Rutherford and Soddy are the correct pair associated with the theory of radioactive disintegration. Why Other Options Are Wrong: - Soddy and Fajan: This pair is associated with the radioactive displacement law, not with the original disintegration theory itself. - Thomson and Rutherford: Thomson discovered the electron and proposed the plum pudding model, while Rutherford later proposed the nuclear model, but the radioactivity theory was developed specifically by Rutherford and Soddy. Why Other Options Are Wrong (continued): - Hahn and Strassmann: They are famous for the discovery of nuclear fission, which is a different phenomenon from the general exponential decay described by the theory of radioactive disintegration. Common Pitfalls: Students may confuse different milestones in nuclear science and attribute the wrong discovery to a particular scientist. Because Rutherford appears in several important discoveries, learners might pair him with Thomson by habit. Others may recall Hahn and Strassmann from discussions of atomic energy and assume they were responsible for all radioactivity theories. To avoid such confusion, it is helpful to remember that Rutherford and Soddy dealt with natural radioactive decay and half life, whereas later scientists worked on fission and reactor physics. Final Answer: Rutherford and Soddy proposed the theory of radioactive disintegration to explain the spontaneous decay of unstable nuclei.

Question 3

For which of the following aqueous solutions at the same temperature will the pH be highest, that is, the solution will be the most strongly basic, assuming complete dissociation of strong electrolytes?

Accepted Answer

Introduction / Context: This chemistry question tests understanding of pH, strong and weak acids and bases, and the effect of polybasic bases on hydroxide ion concentration. Learners must recognise that pH is determined by the concentration of hydrogen ions or hydroxide ions in solution, not just by the molarity of the compound. The comparison includes strong bases, a strong acid, and a weak acid, all at the same molarity, so the key is to determine which produces the greatest hydroxide ion concentration. Given Data / Assumptions: - All solutions have the same volume, 50.0 mL, and the same molarity, 0.0500 M. - The solutes are Ca(OH)2, NaOH, HNO3, and HNO2. - Ca(OH)2 and NaOH are strong bases, HNO3 is a strong acid, and HNO2 is a weak acid. - We assume complete dissociation for strong electrolytes in dilute solution and standard conditions for pH calculations. Concept / Approach: pH is related to hydrogen ion concentration by the relation pH = minus log10[H plus]. For bases, it is often easier to think in terms of hydroxide ion concentration and then relate pOH and pH. A strong base such as NaOH dissociates completely to give one hydroxide ion per formula unit. A strong base like Ca(OH)2 dissociates to give two hydroxide ions per formula unit, making its effect on pH stronger at the same molarity. Strong acids give high hydrogen ion concentration, which lowers pH, and weak acids only partly dissociate. The solution with the highest pH will be the one with the highest hydroxide ion concentration, which is the Ca(OH)2 solution. Step-by-Step Solution: Step 1: For 0.0500 M NaOH, which is a strong base, the dissociation NaOH gives Na plus and OH minus. Therefore, [OH minus] is 0.0500 M. Step 2: For 0.0500 M Ca(OH)2, which is also treated as a strong base in such questions, the dissociation Ca(OH)2 gives Ca2 plus and 2 OH minus. Therefore, [OH minus] is 2 multiplied by 0.0500 M, which is 0.1000 M. Step 3: A higher hydroxide ion concentration corresponds to a higher pOH but a lower [H plus], and therefore a higher pH, since pH plus pOH equals 14 at 25 degree celsius. Step 4: For the strong acid HNO3 at 0.0500 M, complete dissociation gives [H plus] equal to 0.0500 M, which produces a low pH, not a high pH. Step 5: For the weak acid HNO2 at 0.0500 M, [H plus] will be less than 0.0500 M because the acid does not fully dissociate, but the solution will still be acidic and have a pH lower than neutral, not the highest pH among the options. Step 6: Comparing all cases, 0.0500 M Ca(OH)2 gives the highest hydroxide ion concentration, 0.1000 M, and hence the highest pH value. Verification / Alternative check: An alternative way is to compare the basic strength qualitatively. Strong polybasic bases such as Ca(OH)2 supply more than one hydroxide ion per formula unit, effectively doubling the basic effect compared with a monobasic base of the same molarity. Strong acids like HNO3 clearly lower pH, and weak acids like HNO2 lower pH less but still produce acidic solutions. Therefore, the Ca(OH)2 solution must be the most basic among the four, confirming that its pH is highest. Why Other Options Are Wrong: - 50.0 mL of 0.0500 M NaOH: This is a strong base but provides only 0.0500 M hydroxide ions, which is lower than the 0.1000 M provided by Ca(OH)2 at the same molarity. - 50.0 mL of 0.0500 M HNO3: This is a strong acid that gives a high hydrogen ion concentration, so its pH is low, not high. - 50.0 mL of 0.0500 M HNO2: This weak acid provides some hydrogen ions but still yields an acidic solution, so it cannot have the highest pH among the options. Common Pitfalls: Students often compare only the molarity of the compounds and forget to account for the number of hydroxide ions per formula unit in polybasic bases. Another common mistake is to think that any strong electrolyte at the same molarity will have the same impact on pH, which is not correct because the stoichiometry of ion production matters. Misclassifying HNO2 as a strong acid can also lead to errors. Keeping track of how many ions of each type are produced per mole of solute helps avoid these mistakes. Final Answer: 50.0 mL of 0.0500 M calcium hydroxide Ca(OH)2 produces the highest hydroxide ion concentration and therefore has the highest pH among the given solutions.

Question 4

Which of the following statements together gives the best overall description of glycolysis in cellular respiration?

Accepted Answer

Introduction / Context: This biochemistry related question asks about glycolysis, a central metabolic pathway in almost all living cells. Glycolysis is the first major stage in the breakdown of glucose to extract usable energy in the form of ATP. The options provide different, but compatible, descriptions of glycolysis, and the learner must recognise that they all describe accurate aspects of the same process. The question therefore checks both factual knowledge and the ability to integrate multiple statements. Given Data / Assumptions: - Three specific statements are given about glycolysis and one combined option All the above. - The statements cover role, function, and outcome of glycolysis. - We assume standard textbook description of glycolysis as part of cellular respiration in aerobic and anaerobic conditions. Concept / Approach: Glycolysis is a ten step enzymatic pathway that occurs in the cytoplasm. It begins with one molecule of glucose and ends with the formation of two molecules of pyruvate, with a net yield of ATP and NADH. It is indeed the first stage in the overall chemical oxidation of glucose, whether the cell ultimately uses aerobic respiration or fermentation. It can correctly be described as the breakdown of glucose and as the conversion of glucose to pyruvate. Therefore, each of the first three options is correct, which means that All the above is the most accurate choice. Step-by-Step Solution: Step 1: Consider statement 1: glycolysis represents the first stage in the chemical oxidation of glucose by a cell. This is correct, because glycolysis begins the catabolic pathway that leads to energy extraction from glucose. Step 2: Consider statement 2: glycolysis is the breakdown of glucose. This is also correct, because the pathway splits a six carbon glucose molecule into two three carbon molecules. Step 3: Consider statement 3: glycolysis converts glucose to pyruvate. This is again correct, because the end product of glycolysis in most cells is pyruvate, which can then enter the citric acid cycle or undergo fermentation. Step 4: Note that none of these statements contradicts the others. Instead, they emphasise different aspects of the same biochemical process. Step 5: Since all three statements are individually correct and complementary, option All the above is the best answer. Verification / Alternative check: Biology and biochemistry textbooks describe glycolysis as a pathway that converts one molecule of glucose into two molecules of pyruvate, with a net gain of two ATP molecules and two NADH molecules under standard conditions. They also make it clear that glycolysis is the first step of both aerobic respiration and many anaerobic pathways. This confirms that glycolysis is both the initial stage of glucose oxidation and a process that breaks down glucose into pyruvate, supporting all three statements in the question. Why Other Options Are Wrong: - It represents the first stage in the chemical oxidation of glucose by a cell: This is true but incomplete alone, because it does not mention the specific product pyruvate. - It is the biochemical breakdown of glucose: This is also true, yet on its own it does not highlight the formation of pyruvate as the key end product. Why Other Options Are Wrong (continued): - It converts one molecule of glucose to molecules of pyruvate: This is again correct but omits the context that glycolysis is the first stage in a larger energy yielding process. The option All the above brings all these aspects together and is therefore superior. Common Pitfalls: Some learners hesitate to choose All the above because they assume the exam setter wants only one detailed statement to be correct. Others may incorrectly think glycolysis is only part of anaerobic respiration and forget its role in aerobic pathways. Another mistake is to confuse glycolysis with the citric acid cycle or oxidative phosphorylation. Remember that glycolysis is the universal initial pathway that breaks glucose into pyruvate in the cytoplasm, regardless of what subsequent pathway the cell uses. Final Answer: All the above correctly describe glycolysis as the first stage in the chemical oxidation of glucose, as the breakdown of glucose, and as the conversion of glucose into pyruvate.

Question 5

What is the spin only magnetic moment of the tetrahedral complex ion [NiCl4]2 minus, assuming that Ni2 plus is a high spin d8 metal ion?

Accepted Answer

Introduction / Context: This coordination chemistry question tests understanding of crystal field theory, electron configuration of transition metal ions, and the calculation of spin only magnetic moments. The complex [NiCl4]2 minus contains Ni2 plus in a tetrahedral environment. Learners must determine the number of unpaired electrons in the d orbitals and then apply the spin only formula for magnetic moment. Such problems are common in inorganic chemistry examinations and require careful reasoning through several steps. Given Data / Assumptions: - The complex ion is [NiCl4]2 minus, where nickel is in the plus two oxidation state. - Chloride ligands are weak field ligands and usually form high spin complexes. - The geometry is tetrahedral, which leads to a particular pattern of d orbital splitting. - The magnetic moment is to be calculated using the spin only formula mu equals square root of n multiplied by n plus 2 Bohr magneton, where n is the number of unpaired electrons. Concept / Approach: First, we determine the electron configuration of Ni2 plus. Neutral nickel has atomic number 28, which corresponds to [Ar]3d8 4s2. The Ni2 plus ion loses two 4s electrons, giving [Ar]3d8. In a tetrahedral crystal field, the d orbitals split into two sets, but the splitting is relatively small and the complex is almost always high spin with maximum number of unpaired electrons. For a d8 high spin tetrahedral complex, there are two unpaired electrons. Substituting n equals 2 into the spin only formula mu equals square root of n multiplied by n plus 2 gives the magnetic moment, which evaluates to approximately 2.83 Bohr magneton, rounded in the options to 2.82 Bohr magneton. Step-by-Step Solution: Step 1: Write the electron configuration of neutral nickel. Atomic number 28 gives [Ar]3d8 4s2. Step 2: For Ni2 plus, remove two electrons from the 4s orbital, giving [Ar]3d8 as the electronic configuration. Step 3: Recognise that chloride ligands are weak field and that tetrahedral complexes generally experience smaller splitting than octahedral ones, so pairing of electrons is not favoured and the complex is high spin. Step 4: For a d8 configuration in a high spin tetrahedral field, the electrons occupy the split d orbitals in such a way that there are two unpaired electrons. Step 5: Use the spin only formula for magnetic moment: mu equals square root of n multiplied by n plus 2 Bohr magneton. Here n equals 2. Step 6: Substitute n equals 2. Then mu equals square root of 2 multiplied by 4, which is square root of 8, approximately 2.828 Bohr magneton. Step 7: Match this value to the closest option, which is 2.82 Bohr magneton. Verification / Alternative check: You can cross check by recalling common patterns. For d8 octahedral complexes that are low spin, such as some strong field ligands, there can be zero unpaired electrons. However, high spin d8 complexes, especially in tetrahedral fields with weak ligands like chloride, typically have two unpaired electrons. Reference tables for magnetic moments list mu values around 2.8 Bohr magneton for complexes with two unpaired electrons. This independent check confirms that 2.82 Bohr magneton is the correct approximate spin only magnetic moment. Why Other Options Are Wrong: - 1.41 Bohr magneton: This would correspond to a system with one unpaired electron using the spin only formula, which does not match the high spin d8 tetrahedral arrangement of Ni2 plus. Why Other Options Are Wrong (continued): - 1.82 Bohr magneton: This does not correspond neatly to a standard integer number of unpaired electrons under the spin only approximation for this complex. - 5.46 Bohr magneton: This value is roughly twice 2.73 and would correspond to a system with four or five unpaired electrons, which is not possible for a d8 high spin tetrahedral Ni2 plus complex. Common Pitfalls: A common error is to assume octahedral geometry for every complex and forget that [NiCl4]2 minus is tetrahedral. Another mistake is to miscount the electrons when forming Ni2 plus, sometimes incorrectly writing d6 or d7 instead of d8. Students can also confuse low spin and high spin situations and think that chloride might cause pairing, which it does not. Finally, some learners misuse the spin only formula by substituting the wrong value of n or by performing the calculation incorrectly. Careful attention to geometry, ligand field strength, and correct electron counting avoids these mistakes. Final Answer: 2.82 BM is the spin only magnetic moment of the tetrahedral complex [NiCl4]2 minus, corresponding to two unpaired electrons in high spin Ni2 plus.

Question 6

Ethylene glycol is used as an antifreeze in cold climates. What mass of ethylene glycol should be added to 4 kg of water so that the solution freezes at minus 6 degree celsius? (Kf for water = 1.86 K kg mol minus1, molar mass of ethylene glycol = 62…

Accepted Answer

Introduction / Context: This numerical chemistry question involves colligative properties, specifically freezing point depression. Antifreeze solutions are classic real world applications of this concept. The learner must apply the formula that relates the lowering of freezing point to molality of the solute and then convert from moles of ethylene glycol to mass. Working through such problems strengthens understanding of solution behaviour and the practical use of colligative property constants. Given Data / Assumptions: - The solvent is water and its freezing point depression constant Kf is given as 1.86 K kg mol minus1. - Desired freezing point of the solution is minus 6 degree celsius, so the freezing point depression is 6 K from zero degree for pure water. - Mass of water used is 4 kg. - Molar mass of ethylene glycol is 62 g mol minus1. - We assume ethylene glycol is a non electrolyte and does not dissociate, so van hoff factor is 1. Concept / Approach: The key relation for freezing point depression is delta Tf equals Kf multiplied by m, where delta Tf is the decrease in freezing point and m is the molality of the solution. Molality is defined as moles of solute per kilogram of solvent. First we calculate the required molality using delta Tf and Kf. Then we compute the total moles of ethylene glycol required for 4 kg of water. Finally we convert this amount into grams using the molar mass. The nearest suitable value among the options is selected as the answer. Step-by-Step Solution: Step 1: Calculate the freezing point depression. The solution freezes at minus 6 degree celsius and pure water freezes at 0 degree celsius. Therefore, delta Tf = 6 K. Step 2: Use the relation delta Tf = Kf multiplied by m to find molality m. Rearranging gives m = delta Tf divided by Kf. Step 3: Substitute the values: m = 6 divided by 1.86, which gives approximately 3.2258 mol kg minus1. For simple calculation we can round this to about 3.23 mol kg minus1. Step 4: Molality is moles of solute per kilogram of solvent. For 4 kg of water, moles of ethylene glycol required = m multiplied by 4, which is about 3.2258 multiplied by 4, giving around 12.9032 moles. Step 5: Convert moles of ethylene glycol to mass using its molar mass 62 g mol minus1. Mass = 12.9032 multiplied by 62, which is very close to 800 g. Step 6: Rounding to a suitable number of significant figures, we obtain a required mass of about 800.00 g of ethylene glycol. Verification / Alternative check: As a quick check, note that m equals 6 divided by 1.86 is about 3.2 mol kg minus1. For 4 kg solvent, that gives roughly 3.2 multiplied by 4 equals 12.8 moles. Multiplying 12.8 by 62 g mol minus1 gives 793.6 g, which is very close to 800 g. The small differences are due to rounding at intermediate steps. Therefore, a value of about 800 g is entirely consistent with the calculation and matches the option 800.00 g. Why Other Options Are Wrong: - 204.60 g: This mass would correspond to a much smaller molality and would not lower the freezing point by as much as 6 K. - 304.30 g: This is still well below the roughly 800 g required, so the resulting solution would have a freezing point significantly higher than minus 6 degree celsius. Why Other Options Are Wrong (continued): - 400.00 g: This is about half of the required mass. Doubling this value leads to the correct approximate mass, showing that 400 g is insufficient to reach minus 6 degree celsius. Common Pitfalls: Students sometimes confuse molarity and molality or forget that the Kf formula uses molality based on kilograms of solvent, not solution. Another mistake is to use the mass of solution instead of the mass of water when calculating moles of solute. Rounding too early in the calculation can also cause noticeable discrepancies. It is best to carry at least three or four significant figures through the calculations and round only at the end. Finally, some learners may forget that ethylene glycol does not dissociate, so the van hoff factor remains 1. Final Answer: 800.00 g of ethylene glycol should be added to 4 kg of water to lower the freezing point to approximately minus 6 degree celsius under the given conditions.

Question 7

Which of the following general statements correctly describes a key feature of catabolic reactions in biochemistry?

Accepted Answer

Introduction / Context: This biology and biochemistry question tests conceptual understanding of catabolism, one of the two main categories of metabolic reactions in living organisms. Catabolic reactions break down larger molecules into smaller ones. They are closely linked to the release of energy that cells capture in the form of ATP or other high energy molecules. Distinguishing catabolic reactions from anabolic reactions, which build up molecules and generally require energy, is a core idea in metabolism. Given Data / Assumptions: - Four statements are provided about catabolic reactions. - The options involve location, body weight changes, energy release, and energy consumption. - We assume standard textbook definitions of catabolism and anabolism in human and general biology. Concept / Approach: Catabolic reactions are those that break down complex molecules such as carbohydrates, fats, and proteins into simpler molecules like carbon dioxide, water, and ammonia. During this breakdown, chemical bond energy is released and captured in forms such as ATP and reduced coenzymes. Therefore, a defining feature of catabolic processes is that they are exergonic at the cellular level. They are not limited to any single organelle such as mitochondria, nor are they restricted to times of weight loss. Those extra statements reflect misunderstandings rather than core definitions. Thus, the correct identifying feature is that catabolic reactions involve release of energy. Step-by-Step Solution: Step 1: Recall the definition of catabolism. It refers to metabolic pathways that break down complex molecules into simpler products. Step 2: During this breakdown, chemical potential energy stored in bonds is released and partially captured as ATP, NADH, or other energy carriers that cells use for work. Step 3: Because catabolic reactions are typically exergonic, a general statement that they involve release of energy is correct. Step 4: Evaluate the idea that catabolic reactions occur only in mitochondria. Some catabolic pathways, such as glycolysis, occur in the cytoplasm, so this statement cannot be universally correct. Step 5: Evaluate the claim that catabolic reactions occur only during loss of body weight. Catabolism is ongoing in every cell even when body weight is stable, as nutrients are constantly being used for energy. Step 6: Anabolic reactions, not catabolic reactions, are more strongly associated with net consumption of energy, so calling energy consumption a feature of catabolic reactions is misleading. Verification / Alternative check: Biochemistry textbooks classify metabolic pathways into catabolic and anabolic based on whether they release or require energy overall. Examples of catabolic pathways include glycolysis, beta oxidation of fatty acids, and the citric acid cycle, all of which generate ATP and reduced coenzymes. Anabolic pathways such as protein synthesis and DNA synthesis require ATP. This consistent pattern confirms that energy release is the hallmark of catabolic reactions, not energy consumption or weight loss alone. Why Other Options Are Wrong: - They occur only in mitochondria: Many catabolic reactions occur in the cytosol, peroxisomes, and other cell regions. Glycolysis, for instance, is cytosolic, so catabolism is not restricted to mitochondria. Why Other Options Are Wrong (continued): - They occur only during loss of body weight: Catabolic processes occur continuously as cells generate energy from nutrients. Body weight changes depend on the balance between catabolism and anabolism, not on the presence of catabolism alone. - They involve net consumption of energy: This description fits anabolic processes better. While some catabolic steps have activation energy requirements, the overall catabolic pathway releases more energy than it consumes. Common Pitfalls: Learners may associate catabolism only with dieting or weight loss and forget that all cells need continual energy production. Another confusion is between local energy input required to start certain steps and the overall energy balance of a pathway. It is useful to remember the simple rule that catabolism breaks down molecules and releases energy, while anabolism builds molecules and generally consumes energy. Final Answer: They involve release of energy. is the correct general statement about catabolic reactions in living systems.

Question 8

In advanced wastewater treatment, tertiary sewage treatment is specifically designed to remove which pair of nutrients to prevent eutrophication of natural water bodies?

Accepted Answer

Introduction / Context: This environmental chemistry and biology question concerns the purpose of tertiary sewage treatment. Primary and secondary stages remove solids and much of the organic matter, but tertiary treatment targets specific dissolved pollutants. Two nutrients, phosphorus and nitrogen, are major contributors to eutrophication, which is the excessive growth of algae and plants in natural water bodies. Knowing that tertiary treatment focuses on these nutrients helps learners understand water quality management and pollution control. Given Data / Assumptions: - Several pairs of elements are listed, including combinations with calcium, phosphorus, and nitrogen. - The question asks which pair is targeted in tertiary treatment to prevent eutrophication. - We assume typical municipal wastewater treatment plant design and standard environmental science terminology. Concept / Approach: Eutrophication occurs when lakes, rivers, or coastal waters receive excess nutrients, especially phosphates and nitrates, leading to dense algal blooms. When these algae die and decompose, they deplete dissolved oxygen, harming aquatic life. Primary and secondary treatment remove most suspended solids and biodegradable organic matter but may leave high levels of dissolved nitrogen compounds and phosphates. Tertiary treatment includes specific chemical, biological, or physical processes to remove these nutrients. Calcium is not generally a target of tertiary nutrient removal in this context. Therefore, the pair phosphorus and nitrogen is the correct answer. Step-by-Step Solution: Step 1: Identify the main nutrients that cause eutrophication. These are typically nitrogen in forms such as nitrate and ammonia and phosphorus in the form of phosphate. Step 2: Recall that primary treatment removes large solids and some suspended matter, while secondary treatment uses biological processes to break down organic matter. Step 3: Understand that tertiary treatment is an additional step used when higher water quality is needed. It may employ chemical precipitation, advanced biological processes, or filtration to remove remaining nutrients, particularly nitrogen and phosphorus. Step 4: Examine the options. Only the pair phosphorus and nitrogen directly corresponds to the key eutrophication causing nutrients. Step 5: Recognise that calcium is not considered a pollutant contributing to eutrophication and is not the focus of tertiary nutrient removal. Verification / Alternative check: Environmental science sources often describe tertiary treatments such as phosphorus removal by chemical precipitation and nitrogen removal by nitrification and denitrification processes. Case studies of upgraded sewage plants frequently mention reductions in total nitrogen and total phosphorus to meet environmental standards. Calcium, on the other hand, may influence hardness but is not a major eutrophication driver. This confirms that phosphorus and nitrogen are the targets of tertiary nutrient removal. Why Other Options Are Wrong: - Calcium and Phosphorus: While phosphorus is a key nutrient pollutant, calcium is not a significant eutrophication driver. This pair does not capture the full set of nutrients targeted. Why Other Options Are Wrong (continued): - Nitrogen and Calcium: Nitrogen is important, but calcium is not the second critical nutrient causing eutrophication, so this option is incomplete and incorrect. - Calcium and Nitrogen: Again, calcium is not a key nutrient pollutant in this context, so this pair does not match the objective of tertiary treatment. Common Pitfalls: Learners might confuse water hardness, which involves calcium and magnesium, with nutrient pollution. Hardness can be a water quality concern but is not typically addressed by tertiary nutrient removal. Another mistake is to focus only on nitrogen and forget the equally important role of phosphorus in algal growth. Remember that environmental regulations often set limits for both total nitrogen and total phosphorus, highlighting that both must be controlled. Final Answer: Phosphorus and Nitrogen are the two nutrients that tertiary sewage treatment is designed to remove in order to reduce eutrophication of natural water bodies.

Question 9

Which of the following common substances is also known in chemistry as carbolic acid?

Accepted Answer

Introduction / Context: This simple chemistry question checks knowledge of an older common name for a widely used organic compound. Carbolic acid is a historical name that appears in many texts, especially in discussions of antiseptics and the history of surgery. Knowing that carbolic acid refers to phenol helps learners recognise the same compound under different names and connect past and present terminology. Given Data / Assumptions: - The options are phenol, hydroxide, sulphuric acid, and ethanol. - The task is to identify which of these is also called carbolic acid. - We assume standard nomenclature as used in general chemistry and history of science. Concept / Approach: Carbolic acid is the common historical name for phenol, which has the formula C6H5OH. It is an aromatic hydroxy compound derived from benzene with one hydroxyl group. Phenol was famously used by Joseph Lister as an antiseptic in surgery. Hydroxide refers generally to the OH minus ion or to metal hydroxides, not to a specific organic compound. Sulphuric acid is a strong mineral acid with formula H2SO4 and bears no relation to the term carbolic acid. Ethanol is an alcohol used in beverages and as a solvent. Only phenol matches the name carbolic acid. Step-by-Step Solution: Step 1: Recall that phenol is an aromatic compound consisting of a benzene ring and one hydroxyl group, historically called carbolic acid due to its acidic nature and carbon rich aromatic ring. Step 2: Remember that the term carbolic was used in the nineteenth century and early twentieth century when phenol was widely used as an antiseptic and disinfectant. Step 3: Consider hydroxide. This term generally refers to the hydroxide ion OH minus or compounds like sodium hydroxide and does not describe a specific organic acid. Step 4: Sulphuric acid is a strong mineral acid with formula H2SO4 and is never called carbolic acid. Step 5: Ethanol is a simple alcohol, C2H5OH, and is also not associated with the term carbolic acid. Step 6: Therefore, the substance also known as carbolic acid is phenol. Verification / Alternative check: Historical accounts of antiseptic surgery describe the use of carbolic acid, later identified as phenol, to clean wounds and sterilise instruments. Modern safety data sheets and chemical dictionaries list phenol with the synonym carbolic acid. No such synonym exists for hydroxide, sulphuric acid, or ethanol. These references confirm that phenol is uniquely associated with the name carbolic acid. Why Other Options Are Wrong: - Hydroxide: This term denotes the OH minus ion or its metal salts and is not the name for a specific organic compound known as carbolic acid. Why Other Options Are Wrong (continued): - Sulphuric acid: A strong inorganic acid with formula H2SO4, commonly called oil of vitriol historically, but never referred to as carbolic acid. - Ethanol: A simple alcohol used in beverages and fuel, sometimes called ethyl alcohol, but not carbolic acid. Common Pitfalls: Some learners may confuse different historical names such as oil of vitriol, spirit of wine, and carbolic acid. It is helpful to link each old name with the modern IUPAC related compound: oil of vitriol with sulphuric acid, spirit of wine with ethanol, and carbolic acid with phenol. Making such associations helps prevent mixing up these old terms in exams. Final Answer: Phenol is the substance that is also known as carbolic acid.

Question 10

Which of the following gases has a density greater than that of oxygen gas under the same conditions of temperature and pressure?

Accepted Answer

Introduction / Context: This question tests basic gas behaviour under the ideal gas approximation and relates gas density to molar mass. At the same temperature and pressure, gases with higher molar mass will have higher density. Learners must compare the molar masses of four common gases and identify which one is heavier than oxygen gas. This concept is useful in practical situations such as gas leakage and ventilation safety. Given Data / Assumptions: - The gases in the options are carbon dioxide, ammonia, methane, and helium. - We compare their densities with that of oxygen gas at the same temperature and pressure. - We assume ideal gas behaviour, so density is proportional to molar mass when conditions are equal. Concept / Approach: Under the same temperature and pressure, one mole of any ideal gas occupies the same volume. Therefore, density, which is mass per unit volume, is directly proportional to molar mass. Oxygen gas O2 has a molar mass of about 32 g mol minus1. Carbon dioxide CO2 has a molar mass of 44 g mol minus1, ammonia NH3 is 17 g mol minus1, methane CH4 is 16 g mol minus1, and helium He is 4 g mol minus1. The gas with molar mass greater than 32 g mol minus1 will be denser than oxygen under identical conditions, so carbon dioxide is the correct choice. Step-by-Step Solution: Step 1: Calculate or recall the molar mass of oxygen gas O2. Each oxygen atom has atomic mass about 16, so O2 has molar mass 2 multiplied by 16 equals 32 g mol minus1. Step 2: For carbon dioxide CO2, carbon contributes 12 and each oxygen contributes 16, giving total molar mass 12 plus 2 multiplied by 16 equals 44 g mol minus1. Step 3: For ammonia NH3, nitrogen contributes 14 and hydrogen contributes 3 multiplied by 1, giving molar mass 17 g mol minus1. Step 4: For methane CH4, carbon contributes 12 and hydrogen contributes 4 multiplied by 1, giving molar mass 16 g mol minus1. Step 5: For helium He, the molar mass is 4 g mol minus1, as it is a monoatomic noble gas. Step 6: Since at the same temperature and pressure gas density is proportional to molar mass, only carbon dioxide with molar mass 44 g mol minus1 is denser than oxygen gas with molar mass 32 g mol minus1. Verification / Alternative check: One way to verify this is to compare known common facts about these gases. Helium balloons rise in air because helium is lighter than both oxygen and nitrogen. Methane and ammonia are also often mentioned as lighter gases. Carbon dioxide is known to accumulate in low lying areas in mines and volcanic regions because it is heavier than air, which is mainly nitrogen and oxygen. These observations confirm the molar mass comparisons and show that carbon dioxide is indeed heavier than oxygen under similar conditions. Why Other Options Are Wrong: - Ammonia: With molar mass 17 g mol minus1, it is significantly lighter than oxygen and will be less dense under the same conditions. Why Other Options Are Wrong (continued): - Methane: With molar mass 16 g mol minus1, it is lighter than oxygen and therefore has lower density. - Helium: It has a very low molar mass of 4 g mol minus1 and is much lighter than oxygen, which is why helium balloons float. Common Pitfalls: Some students may rely on everyday impressions, thinking methane or ammonia are heavy due to their odour or flammability, but density is determined by molar mass, not smell or reactivity. Others may forget that oxygen itself is not extremely heavy compared to gases like carbon dioxide. Always calculate or recall molar masses and compare them directly when dealing with relative gas densities under equal conditions. Final Answer: Carbon dioxide has a higher molar mass and density than oxygen gas at the same temperature and pressure.

Question 11

Which of the following statements about natural rubber is incorrect?

Accepted Answer

Introduction / Context: This polymer chemistry question asks about the composition and properties of natural rubber. Natural rubber is an important industrial material and a classic example of a natural polymer. The learner must distinguish between correct statements about its structure and processing and an incorrect statement that actually describes a different synthetic rubber. Knowing that natural rubber is based on cis isoprene and that chloroprene rubber is a separate synthetic material is central to answering this question correctly. Given Data / Assumptions: - Four statements describe natural rubber in different ways. - The task is to identify which statement is false. - We assume standard polymer chemistry definitions and the distinction between natural rubber and synthetic rubbers such as neoprene. Concept / Approach: Natural rubber is obtained from latex sap of rubber trees and is chemically a polymer of 2 methyl 1,3 butadiene, commonly called isoprene. The polymerisation primarily yields the cis isomer, so natural rubber is cis polyisoprene. Because the polymer chains can stretch and return to their original shape, natural rubber is classed as an elastomer. To improve its mechanical properties such as elasticity, durability, and resistance to temperature, natural rubber is vulcanised by heating with sulphur or sulphur based compounds. On the other hand, chloroprene, which is 2 chloro 1,3 butadiene, is the monomer for neoprene, a synthetic rubber. Thus, saying that natural rubber is a polymer of chloroprene is incorrect. Step-by-Step Solution: Step 1: Recall that natural rubber is polyisoprene, and more specifically, a polymer of cis isoprene units. This supports statement that it is a polymer formed from cis isoprene units. Step 2: Recognise that an elastomer is a polymer with elasticity that can be stretched and will return to its original dimensions. Natural rubber clearly behaves this way, so it is correctly described as an elastomer. Step 3: Understand that natural rubber is often vulcanised by heating with sulphur and accelerators, which forms crosslinks between polymer chains and improves properties such as tensile strength and resistance to deformation. Step 4: Chloroprene is a different monomer, 2 chloro 1,3 butadiene, and its polymerisation produces neoprene, a synthetic rubber. Natural rubber does not arise from chloroprene. Step 5: Therefore, the statement that natural rubber is a polymer of chloroprene is incorrect, while the other three statements accurately describe natural rubber. Verification / Alternative check: Textbooks and reference materials on polymers list natural rubber as cis polyisoprene obtained from Hevea brasiliensis. They also describe its classification as an elastomer and note that vulcanisation with sulphur is a key process discovered by Charles Goodyear. Neoprene is separately described as a synthetic rubber made from chloroprene. This clear separation confirms that the statement linking natural rubber with chloroprene is wrong. Why Other Options Are Wrong: - It is an elastomer: This is true because natural rubber can be stretched and then returns to its original shape due to long chain polymer behaviour. Why Other Options Are Wrong (continued): - It is a polymer formed from cis isoprene units: This is correct and matches the chemical structure of natural rubber as cis polyisoprene. - It is heated with sulphur compounds to improve its properties: This describes vulcanisation, a standard treatment for natural rubber to enhance mechanical strength and elasticity, so it is correct. Common Pitfalls: Students sometimes confuse different monomers with similar names such as isoprene, butadiene, and chloroprene. The presence of the term prene in both isoprene and chloroprene may cause mixing of facts, leading to the wrong association of chloroprene with natural rubber. To avoid this, remember that natural rubber is from isoprene, while neoprene is from chloroprene. Keeping a small table linking each monomer with its corresponding rubber type can help in revising these topics. Final Answer: Natural rubber is a polymer of chloroprene. is the incorrect statement about natural rubber.

Question 12

A chemical bond formed by the complete transfer of one or more electrons from one atom to another is called what type of bond?

Accepted Answer

Introduction / Context: This basic chemistry question covers the classification of chemical bonds based on how electrons are distributed between atoms. The three common types are ionic, covalent, and coordinate bonds. Understanding the difference between electron transfer and electron sharing is a foundation for predicting compound properties, such as melting point, solubility, and electrical conductivity. Given Data / Assumptions: - The question describes a bond formed by the transfer of electrons from one atom to another. - The options are ionic bond, covalent bond, coordinate bond, and none of these. - We assume simple textbook definitions of these bonding types. Concept / Approach: In an ionic bond, one atom loses electrons to become a positively charged ion, while another atom gains those electrons to become a negatively charged ion. The electrostatic attraction between the oppositely charged ions holds them together. In covalent bonding, atoms share electron pairs rather than transfer them completely. A coordinate bond is a special type of covalent bond where both shared electrons in the bond pair originate from the same atom. Therefore, a bond described explicitly as being formed by transfer of electrons fits the definition of an ionic bond. Step-by-Step Solution: Step 1: Recall that ionic bonding occurs when one atom completely transfers one or more valence electrons to another atom, forming ions. Step 2: After the transfer, the atom that loses electrons becomes a cation and the atom that gains electrons becomes an anion. Step 3: The electrostatic force of attraction between the cation and anion is what constitutes the ionic bond. Step 4: In covalent bonding, atoms share electron pairs so that each can achieve a stable electron configuration. No full transfer takes place. Step 5: A coordinate bond also involves sharing, but both electrons in the shared pair come from one atom. This is still a form of covalent bonding, not electron transfer. Step 6: Since the question clearly emphasises complete transfer of electrons, the correct name is ionic bond. Verification / Alternative check: Standard examples help verify the classification. Sodium chloride NaCl is formed when sodium transfers one electron to chlorine, producing Na plus and Cl minus. The attraction between these ions forms an ionic lattice. On the other hand, the bond between two hydrogen atoms in H2 is covalent because they share electrons. Ammonium ion NH4 plus contains a coordinate bond from nitrogen to hydrogen. These well known cases align with the definitions and confirm that electron transfer is characteristic of ionic bonds. Why Other Options Are Wrong: - Covalent bond: This bond involves sharing of electrons between atoms, not complete transfer, so it does not match the description in the question. Why Other Options Are Wrong (continued): - Coordinate bond: This is a type of covalent bond where a lone pair from one atom forms a shared pair with another atom. It still involves sharing, not transfer. - None of these: This is incorrect because ionic bond is a standard category that fits the description. Common Pitfalls: Some learners may confuse electron transfer and sharing, especially in polar covalent bonds where electron density is uneven. Another mistake is to think that coordinate bonds are fundamentally different from covalent bonds. It is helpful to remember that ionic bonds arise from full electron transfer and electrostatic attraction, whereas covalent and coordinate bonds arise from shared electron pairs. Keeping these distinctions clear reduces confusion in more advanced topics like Lewis structures and molecular geometry. Final Answer: Ionic bond is the term used for a bond formed by the complete transfer of one or more electrons from one atom to another.

Question 13

The standard electrode potential of a half cell is measured experimentally using which electrical instrument in combination with a reference electrode?

Accepted Answer

Introduction / Context: This electrochemistry question focuses on how standard electrode potentials are measured. Standard electrode potential is a voltage difference between a given half cell and the standard hydrogen electrode under defined conditions. To determine this potential, an instrument that measures electrical potential difference is required. Identifying the correct instrument reinforces understanding of how electrochemical data is obtained in practice. Given Data / Assumptions: - The options include pyrometer, electrometer, thermometer, and voltmeter. - The question asks specifically about measuring standard electrode potential of a half cell. - We assume standard laboratory practice where a half cell is connected to a reference electrode in a galvanic cell and the potential difference is measured. Concept / Approach: Standard electrode potential is expressed in volts and represents the tendency of a half cell to gain or lose electrons relative to the standard hydrogen electrode. To measure such a potential difference, an instrument that reads voltage is needed. A voltmeter is designed to measure electrical potential differences in volts. A pyrometer measures high temperatures, a thermometer measures temperature, and an electrometer is a very sensitive apparatus for measuring small electric charges, but standard laboratory determination of electrode potentials uses a voltmeter. Therefore, the correct instrument is a voltmeter. Step-by-Step Solution: Step 1: Recognise that standard electrode potential is defined as an electric potential difference between a half cell and a reference standard hydrogen electrode. Step 2: Electric potential difference is measured in units of volts. Step 3: An instrument that directly measures potential difference in volts is called a voltmeter. Step 4: A pyrometer is used to measure very high temperatures, for example in furnaces, and does not measure electric potentials. Step 5: A thermometer measures temperature in degrees and also does not measure potential difference. Step 6: An electrometer is a specialised instrument for detecting very small electric charges or potentials, but in standard electrochemistry experiments a voltmeter is commonly specified for measuring electrode potentials. Step 7: Therefore, when the question refers to the instrument for measuring standard electrode potential, the appropriate answer is voltmeter. Verification / Alternative check: Typical electrochemistry laboratory manuals describe setting up a galvanic cell by connecting the half cell under study to a standard hydrogen electrode. The output terminals of this cell are then connected to a high impedance voltmeter, which reads the potential difference. The value obtained under standard conditions is reported as the standard electrode potential. These descriptions consistently mention a voltmeter, confirming that it is the correct instrument here. Why Other Options Are Wrong: - Pyrometer: This instrument measures high temperatures, not electrical quantities, so it is unrelated to electrode potential measurements. Why Other Options Are Wrong (continued): - Thermometer: It measures temperature and cannot measure voltage, so it is not used for standard electrode potentials. - Electrometer: While it can detect small electric charges or potentials, the standard basic instrument referred to in most educational contexts for electrode potential measurement is a voltmeter, not an electrometer. Common Pitfalls: Some learners may be distracted by the similarity between the words electrometer and electrode and choose electrometer without considering the practical context. Others may not distinguish instruments by what physical quantity they measure. A useful approach is to always link an instrument name to the quantity it measures: voltmeter for voltage, ammeter for current, thermometer for temperature, and so on. This habit makes such questions straightforward. Final Answer: Voltmeter is the electrical instrument used to measure standard electrode potentials of half cells.

Question 14

In industrial chemistry, which of the following acids is popularly known as the "King of Chemicals" because of its enormous use in fertilizers, petroleum refining, and many other industries?

Accepted Answer

Introduction / Context: In basic general chemistry and industrial chemistry, some substances are given special titles because of how widely they are used. One such famous title is the "King of Chemicals". Examination questions often test whether students know which common industrial acid receives this nickname. The acid in question plays a central role in manufacturing fertilizers, in petroleum refining, in many chemical syntheses, and even in metallurgy and battery manufacture. Identifying this acid helps you connect textbook chemistry with real world industrial practice. Given Data / Assumptions: We are comparing common acids and compounds such as nitric acid, hydrochloric acid, silver nitrate solution, sulphuric acid, and phosphoric acid. The phrase "King of Chemicals" refers to the scale of production, diversity of applications, and importance to the chemical industry. We assume a general knowledge level appropriate for school or entry level competitive examinations. The answer must be one specific chemical that is widely accepted as holding this title in standard chemistry references. Concept / Approach: Sulphuric acid is produced in larger quantities worldwide than almost any other individual chemical. It is used to make phosphate fertilizers, to refine petroleum, to clean metal surfaces, to manufacture detergents, dyes, explosives, and numerous other chemicals. Because so many industries depend on sulphuric acid, its production volume is sometimes used as an indicator of a country overall industrial strength. For this reason, sulphuric acid has earned the nickname "King of Chemicals" in textbooks and competitive exam guides. Other acids like nitric and hydrochloric acid are also important but are not given this particular title in the same universal way. Step-by-Step Solution: Step 1: List the options and recall their major uses. Nitric acid is important for explosives and fertilizers, hydrochloric acid is used in pickling and cleaning, silver nitrate is a salt used in laboratories, and phosphoric acid is used in fertilizers and soft drinks. Step 2: Think about which chemical is produced on the largest industrial scale and used in the greatest variety of processes. Step 3: Remember that sulphuric acid is used in the manufacture of phosphate fertilizers, which are essential for agriculture around the world. Step 4: Note that sulphuric acid is also vital in petroleum refining, metal processing, lead acid batteries, and the production of many other chemicals. Step 5: Conclude that sulphuric acid has the broadest and most critical industrial role, justifying its nickname as the "King of Chemicals". Verification / Alternative check: If you look at lists of top bulk chemicals by production volume, sulphuric acid usually appears near the very top. Many school level chemistry books explicitly state that sulphuric acid is called the "King of Chemicals". In contrast, nitric acid is sometimes described as important in the fertilizer and explosives industries, but it does not receive the same royal title. Hydrochloric acid is very useful but more specialized. Silver nitrate is far too expensive and limited in use to be considered. Phosphoric acid is important but still not as central as sulphuric acid. These comparisons confirm that sulphuric acid is the intended answer. Why Other Options Are Wrong: Nitric acid is a key chemical but is usually described as a strong oxidizing acid rather than the "King of Chemicals". Hydrochloric acid has major uses in pickling steel and cleaning but its production volume is lower than that of sulphuric acid. Silver nitrate is a laboratory reagent and photographic chemical, not a bulk industrial chemical on the same scale. Phosphoric acid is significant in fertilizers but is not as universally central across different industries. Therefore, none of these alternatives match the widely accepted nickname. Common Pitfalls: Students sometimes confuse nitric acid with sulphuric acid because both are strong acids and both appear frequently in reaction equations. Another common mistake is to focus on laboratory familiarity rather than industrial importance. To avoid confusion, connect the phrase "King of Chemicals" directly with sulphuric acid and remember its role in fertilizers and petroleum refining, which are huge global industries. Recalling that sulphuric acid production is a rough measure of industrial development can also help fix this association in memory. Final Answer: The acid popularly known as the "King of Chemicals" is sulphuric acid.

Question 15

Leakage in domestic LPG (liquefied petroleum gas) cylinders is easily detected because a small amount of which strongly smelling substance is added to the otherwise odourless gas?

Accepted Answer

Introduction / Context: Liquefied petroleum gas, commonly called LPG, is widely used as domestic cooking gas. In its pure form LPG is almost odourless, which would make gas leaks very dangerous because people could not detect them by smell. To improve safety, gas companies deliberately add a substance with a strong, unpleasant odour so that even a small leak can be noticed quickly. This question tests whether you know the type of compound that is added to LPG and the reason behind it, which is an important part of everyday chemistry awareness and safety education. Given Data / Assumptions: LPG consists mainly of hydrocarbons such as propane and butane that have very little smell in pure form. The additive must have a strong and characteristic odour detectable at very low concentrations. The options include mercaptans, propylene, carbon disulphide, phosgene, and chloroform. We assume typical domestic gas cylinders used for cooking and heating. Concept / Approach: The compounds most commonly added to LPG to give it a warning smell are called mercaptans, also known as thiols. A typical example is ethyl mercaptan, chemically written as C2H5SH. Thiols contain a sulphur hydrogen group that gives them a very strong, unpleasant odour even at extremely low concentrations. This makes them ideal as odorants in fuel gases. Other substances listed in the options either do not have a suitable smell profile, are too dangerous, or are not used for this specific purpose. Therefore, the correct answer is mercaptans such as ethyl mercaptan. Step-by-Step Solution: Step 1: Recognise that LPG itself is nearly odourless, so a special additive is required for safety. Step 2: Recall that gas companies add a substance with a characteristic rotten cabbage or rotten egg like smell so that leaks can be detected quickly. Step 3: Identify mercaptans, especially ethyl mercaptan, as classic odorants added to LPG and natural gas supplies. Step 4: Compare this with propylene, which is another hydrocarbon gas without a strong warning odour, so it is not used as an odorant. Step 5: Note that carbon disulphide, phosgene, and chloroform are toxic industrial chemicals and solvents, not standard safety additives in household LPG. Verification / Alternative check: Information from gas utilities and safety leaflets usually states that LPG is odorised with ethyl mercaptan or similar thiol compounds. Safety posters often mention that if you smell a strong and unfamiliar gas odour in the kitchen, you should suspect an LPG leak and open doors and windows. Technical descriptions of gas distribution systems also list mercaptans as the class of compounds used for odorisation. There is no mention of propylene, carbon disulphide, phosgene, or chloroform being used as odorants in domestic LPG. This evidence supports mercaptans as the correct choice. Why Other Options Are Wrong: Propylene is a hydrocarbon similar to propane and would not add a strong identifying odour. Carbon disulphide has a smell but is highly toxic and flammable, and it is not the standard choice for LPG odorisation. Phosgene is a very toxic choking gas once used as a chemical weapon, so it would never be added deliberately to domestic gas. Chloroform is a volatile liquid with its own hazards and is not suitable or used as an LPG odorant. Thus, these alternatives do not match the safe, standard practice adopted worldwide. Common Pitfalls: Some learners may choose carbon disulphide or phosgene simply because they recognise them as smelly or toxic and assume that any strong smelling chemical could be used. Others might pick propylene because it looks chemically similar to gases already present in LPG. To avoid such mistakes, remember that the odorant must be effective at very low concentrations, reasonably stable, and compatible with fuel systems. Mercaptans meet these requirements and are specifically mentioned in textbooks and safety training materials. Final Answer: Leakage in LPG cylinders is easily detected because a small amount of mercaptans such as ethyl mercaptan is added to the otherwise odourless gas.

Question 16

In chemical bonding, a bond formed mainly by complete transfer of one or more electrons from one atom to another is known as which type of bond?

Accepted Answer

Introduction / Context: Atoms can join together through different types of chemical bonds depending on how they share or transfer electrons. One very important class of bond involves one atom losing electrons and another atom gaining those electrons, resulting in oppositely charged ions that attract each other. This question asks you to name the type of bond that is formed predominantly by the transfer of electrons between atoms, a foundational concept in chemical bonding and ionic compounds. Given Data / Assumptions: The bond described involves transfer, not sharing, of electrons. One atom becomes positively charged and the other becomes negatively charged. Options include ionic, covalent, co-ordinate, hydrogen, and metallic bonds. We assume standard definitions from introductory chemistry. Concept / Approach: When an atom of a metal with low ionisation energy transfers an electron to a non metal atom with high electron affinity, a cation and an anion are formed. The electrostatic attraction between these oppositely charged ions constitutes an ionic bond. In contrast, covalent bonds involve sharing electron pairs between atoms, co-ordinate bonds involve a shared pair donated entirely by one atom, hydrogen bonds are weak intermolecular attractions, and metallic bonds involve a lattice of positive ions in a sea of delocalised electrons. Therefore, the bond formed mainly by transfer of electrons is called an ionic bond. Step-by-Step Solution: Step 1: Identify key words in the question, such as "transfer of electrons" and "between atoms of the elements". Step 2: Recall that in ionic bonding, one atom loses electrons to become a cation and another gains electrons to become an anion. Step 3: Recognise that the resulting electrostatic attraction between these oppositely charged ions is what we call an ionic bond. Step 4: Contrast this with covalent bonding, where electrons are shared rather than completely transferred. Step 5: Note that co-ordinate, hydrogen, and metallic bonds all involve different mechanisms and do not fit the simple transfer description given in the question. Verification / Alternative check: Take a common example such as sodium chloride. A sodium atom loses one electron to form Na plus, and a chlorine atom gains one electron to form Cl minus. The bond between Na plus and Cl minus is purely electrostatic and is described as ionic. The process clearly involves electron transfer rather than sharing. By contrast, in a chlorine molecule Cl2, each atom shares one electron pair in a covalent bond. In ammonia, nitrogen shares electrons with hydrogen in covalent bonds. None of these examples contradict the definition that transfer of electrons leads to ionic bonding. Why Other Options Are Wrong: A covalent bond is formed by sharing of electron pairs between atoms, so it does not involve complete transfer. A co-ordinate bond is a special case of covalent bond where one atom donates both electrons in the shared pair, but again the electrons are shared, not fully transferred. A hydrogen bond is a weak attraction between a hydrogen atom already covalently bonded to an electronegative atom and another electronegative atom nearby; it is not a primary electron transfer bond. A metallic bond is the attraction between metal cations and a sea of delocalised electrons in a metal lattice. None of these fit the phrase "formed by transfer of electrons" as precisely as an ionic bond. Common Pitfalls: Students sometimes confuse ionic and covalent bonds because both involve valence electrons and can appear together in complex compounds. Another mistake is to think that co-ordinate bonding is somehow more "ionic" because one atom donates electrons, but it is still a shared pair. To avoid confusion, remember the key distinction: in ionic bonding, electrons are completely transferred, creating ions; in covalent and co-ordinate bonding, electrons are shared. Focus on the word "transfer" in the question to quickly identify the bond as ionic. Final Answer: A bond formed mainly by transfer of electrons from one atom to another is called an ionic bond.

Question 17

In the modern periodic table, most of the artificially prepared radioactive elements, especially the transuranic elements, are placed in which block of elements?

Accepted Answer

Introduction / Context: The periodic table is arranged into blocks based on which type of orbital is being filled by the valence electrons: s-block, p-block, d-block, and f-block. Some elements are naturally radioactive, while many heavier elements have been produced artificially in laboratories and nuclear reactors. These artificially prepared radioactive elements, especially the transuranic elements beyond uranium, occupy a particular region of the periodic table. This question asks you to identify which block contains most of these artificial radioactive elements. Given Data / Assumptions: The periodic table is divided into s, p, d, and f blocks based on electron configuration. Heavy elements, especially those with atomic number greater than 92, are typically artificially produced and radioactive. The lanthanides and actinides together form the f-block of the periodic table. We are concerned mainly with where artificial radioactive elements are placed in standard representations of the table. Concept / Approach: The f-block consists of the lanthanide series and the actinide series. Many lanthanides are naturally occurring, while almost all of the higher actinides and transuranic elements are synthetic and radioactive. Elements such as neptunium, plutonium, americium, curium, and so on are placed in the actinide series, which is part of the f-block. Although some radioactive isotopes also exist in the s, p, and d blocks, the majority of artificially prepared heavy radioactive elements belong to the actinide part of the f-block. Hence the f-block is the best answer in the sense of this general knowledge question. Step-by-Step Solution: Step 1: Recall how the blocks are defined. s-block elements fill s orbitals, p-block fill p orbitals, d-block fill d orbitals, and f-block fill f orbitals. Step 2: Remember that the two rows usually shown separately at the bottom of the periodic table are the lanthanides and actinides, which together form the f-block. Step 3: Note that uranium has atomic number 92 and is found among the actinides; transuranic elements with atomic number greater than 92 are also placed in this series. Step 4: Recognise that many of these transuranic elements, such as neptunium, plutonium, americium, and curium, are artificially prepared radioactive elements. Step 5: Conclude that since these synthetic radioactive elements are located in the actinide series, the correct block is the f-block. Verification / Alternative check: If you look at a detailed periodic table, you will see that the f-block includes elements from cerium to lutetium (lanthanides) and from thorium to lawrencium (actinides). While some d-block and p-block elements have radioactive isotopes, the set of elements that are inherently artificial and mostly radioactive, such as elements with atomic numbers 95 and above, are in the actinide region of the f-block. This pattern is consistently shown in school textbooks and periodic table charts, confirming that f-block is the site of most artificially prepared radioactive elements. Why Other Options Are Wrong: The s-block includes alkali metals and alkaline earth metals, most of which are stable and not predominantly artificial radioactive elements. The p-block contains many common non metals, metalloids, and some metals, again mostly stable at least in their lighter isotopes. The d-block consists of transition metals like iron, copper, and zinc, which are mainly stable or naturally occurring, even though some heavier d-block elements have radioactive isotopes. There is no g-block in the currently accepted periodic table for known elements. Therefore, these options do not match the location of most synthetic radioactive elements. Common Pitfalls: A common misunderstanding is to associate radioactivity only with very heavy elements and then guess randomly among d-block or f-block without recalling how the actinides are classified. Some learners also forget that the two separated rows at the bottom of the periodic table are actually part of the main table and belong to the f-block. To avoid confusion, remember that the actinide series, which contains many synthetic radioactive elements, is part of the f-block, making this block the correct answer. Final Answer: Most artificially prepared heavy radioactive elements are placed in the f-block elements (lanthanides and especially actinides) of the periodic table.

Question 18

The atoms of different elements that contain the same number of neutrons in their nuclei, but may have different atomic numbers and mass numbers, are called what?

Accepted Answer

Introduction / Context: In nuclear and atomic chemistry, several related terms describe specific relationships between atomic number, mass number, and neutron count. Students often encounter isotopes, isobars, and isotones and need to distinguish among them clearly. This question focuses on the situation where atoms of different elements share the same number of neutrons in their nuclei, even though their atomic numbers and mass numbers can differ. Knowing the correct term for this relationship is important for understanding nuclear stability and nuclide classification. Given Data / Assumptions: We are looking at atoms of different elements, so their atomic numbers are not the same. These atoms have the same number of neutrons in their nuclei. Mass numbers may be different because mass number equals protons plus neutrons. The terms available include isobars, isotopes, isotones, and isoelectronic species. Concept / Approach: Isotopes are atoms of the same element with the same atomic number but different mass numbers, meaning they have the same number of protons but different numbers of neutrons. Isobars are atoms of different elements with the same mass number but different atomic numbers. Isotones, on the other hand, are atoms of different elements that have the same number of neutrons but different numbers of protons and usually different mass numbers. Isoelectronic species have the same number of electrons. The question describes atoms with equal neutron count but not necessarily equal atomic number or mass number, which is the definition of isotones. Step-by-Step Solution: Step 1: Recall that the number of neutrons in an atom is given by N = A − Z, where A is mass number and Z is atomic number. Step 2: Identify that the question mentions atoms of elements which have the same number of neutrons, so N is the same for all such atoms. Step 3: Recognise that the atoms belong to different elements, so their atomic numbers Z are different. Step 4: Match this description to the term isotones, which refers to nuclides that share the same neutron number but have different proton numbers. Step 5: Confirm that isotopes and isobars do not fit this description because they relate to equal atomic number and equal mass number respectively, not equal neutron count. Verification / Alternative check: Consider two specific nuclides, for example carbon 14 and nitrogen 15. Carbon 14 has atomic number 6 and mass number 14, so it has 8 neutrons. Nitrogen 15 has atomic number 7 and mass number 15, also giving 8 neutrons. These two nuclides belong to different elements but share the same neutron count, so they are classic examples of isotones. By contrast, isotopes would be pairs like carbon 12 and carbon 14, which share atomic number 6 but have different neutron numbers. This comparison confirms that equal neutron number across different elements corresponds to isotones. Why Other Options Are Wrong: Isobars are nuclides that have the same mass number A but different atomic numbers Z, so they may not have the same number of neutrons. Isotopes are nuclides of the same element with the same atomic number Z but different mass numbers, which means they differ in neutrons rather than share neutron count. Isoelectronic species are ions or atoms with the same number of electrons, which is a different concept independent of neutron count. The option "None of these" is incorrect because the specific term isotones does exist and matches the description in the question. Common Pitfalls: Students often mix up isotopes, isobars, and isotones because the terms sound similar. A typical mistake is to choose isotopes whenever they see a question about similar nuclear properties, forgetting that isotopes must belong to the same element. Another error is to think that isobars and isotones are interchangeable. A helpful memory aid is to remember that all three words start with "iso" meaning same, and the second part hints at what is the same: "topes" refers to place in the periodic table (same atomic number), "bars" refers to mass (same mass number), and "tones" can be linked to neutrons. Using this association will help you select the right term. Final Answer: Atoms of different elements that have the same number of neutrons are called isotones.

Question 19

The atoms of the same element that have the same atomic number but different mass numbers, due to different numbers of neutrons, are known by what name?

Accepted Answer

Introduction / Context: A single chemical element can exist in several nuclear forms that share the same number of protons but differ in the number of neutrons in the nucleus. These different forms have the same chemical behaviour but may have different physical properties, such as mass and nuclear stability. Understanding the terminology for these variants is essential in both chemistry and physics, especially in topics like radioactivity, nuclear medicine, and isotopic analysis. This question asks you to identify the term used for atoms that share an atomic number but differ in mass number. Given Data / Assumptions: The atoms belong to the same element, so their atomic number Z is the same. They have different mass numbers A, which means different numbers of neutrons. Their electronic configurations in the neutral state are identical, leading to similar chemical behaviour. Available terms include isobars, isotopes, isoelectronic species, isotones, and radioisomers. Concept / Approach: Isotopes are defined as atoms of the same element that have the same atomic number but different mass numbers. This implies that the number of protons is constant while the number of neutrons varies. Isobars have the same mass number but different atomic numbers. Isotones have the same number of neutrons but different atomic and mass numbers. Isoelectronic species share the same number of electrons but may be different elements or ions. Radioisomers refer to different nuclear energy states. The scenario described in the question clearly matches the definition of isotopes. Step-by-Step Solution: Step 1: Recognise that atomic number Z equals the number of protons and defines the element identity. Step 2: Mass number A equals the sum of protons and neutrons in the nucleus. Step 3: If Z is the same but A is different, the difference must come from neutrons, so the nuclei have varying neutron counts. Step 4: Recall that such nuclei belonging to the same element but having different neutron numbers are called isotopes. Step 5: Confirm that none of the other listed terms better describe same Z but different A for the same element. Verification / Alternative check: Classic examples of isotopes include hydrogen 1, hydrogen 2, and hydrogen 3, often known as protium, deuterium, and tritium. All three have atomic number 1 because they contain one proton each, but their mass numbers are 1, 2, and 3 respectively due to different neutron counts. Similarly, carbon 12 and carbon 14 are isotopes of carbon; each has six protons, but carbon 12 has six neutrons while carbon 14 has eight. These examples match the description given in the question and are universally referred to as isotopes in textbooks. Why Other Options Are Wrong: Isobars are nuclides that have the same mass number but different atomic numbers, such as argon 40 and calcium 40, so they do not satisfy the condition of same atomic number. Isoelectronic species have the same number of electrons but can involve different elements or ions, and their atomic numbers may differ. Isotones have equal neutron numbers but different proton numbers. Radioisomers refer to different energy states of the same nucleus rather than simply differences in neutron count. Therefore, none of these options fits the simple relationship of same atomic number but different mass numbers as directly as isotopes. Common Pitfalls: Students sometimes confuse isotopes and isobars because both involve mass numbers. A common mistake is to think that any pair with the same mass number must be isotopes, even if they are different elements. Another confusion arises between isotopes and isotones when working with nuclear equations. To avoid these problems, remember the root meanings: "iso" means same, "topes" refers to place in the periodic table (same element and atomic number), "bars" hints at equal mass, and "tones" reminds you of neutrons. With this mental map, you can quickly identify isotopes as same atomic number but different mass numbers. Final Answer: Atoms of the same element that have the same atomic number but different mass numbers are called isotopes.

Question 20

In Earth atmosphere, the natural ozone layer that absorbs most of the Sun harmful ultraviolet radiation is mainly concentrated in which atmospheric layer?

Accepted Answer

Introduction / Context: Ozone in the atmosphere plays a crucial protective role by absorbing a significant portion of the Sun harmful ultraviolet radiation, particularly UV-B. This region of enhanced ozone concentration is commonly called the ozone layer. Knowing where this layer is located within the structure of Earth atmosphere is important in environmental science, climate studies, and general knowledge. The atmosphere is divided into several layers, and this question asks you to identify the specific layer where ozone is most concentrated. Given Data / Assumptions: Earth atmosphere is divided into layers such as the troposphere, stratosphere, mesosphere, thermosphere, and ionosphere (which overlaps some of these layers). Ozone is present in small amounts throughout the atmosphere but is highly concentrated in a specific region known as the ozone layer. The ozone layer is responsible for absorbing much of the Sun ultraviolet radiation. The question is about where this concentration is highest, not about ground level ozone pollution. Concept / Approach: The ozone layer is located primarily in the stratosphere, roughly between 15 and 35 kilometres above Earth surface. Within this region, the concentration of ozone is much higher than in other atmospheric layers, although still small in absolute terms. Ozone forms and decomposes through photochemical reactions involving oxygen molecules and ultraviolet light. While small amounts of ozone can exist in the troposphere, especially as a pollutant near the ground, the natural protective ozone layer that shields life from ultraviolet radiation is a feature of the stratosphere. Step-by-Step Solution: Step 1: Recall the basic order of atmospheric layers from the ground upwards: troposphere, stratosphere, mesosphere, thermosphere, and exosphere, with the ionosphere overlapping parts of the upper layers. Step 2: Remember that most weather phenomena occur in the troposphere and that commercial jets often fly at the lower stratosphere. Step 3: Identify that the maximum concentration of ozone, known as the ozone layer, lies in the stratosphere, typically between about 15 and 35 kilometres altitude. Step 4: Recognise that this stratospheric ozone strongly absorbs ultraviolet radiation and thus protects living organisms on the surface. Step 5: Conclude that among the given options, stratosphere is the correct atmospheric layer for the natural ozone layer. Verification / Alternative check: Environmental science texts and climate reports consistently show graphs of ozone concentration versus altitude, with a pronounced peak in the stratosphere. International agreements such as the Montreal Protocol were specifically concerned with protecting stratospheric ozone from depletion by chlorofluorocarbons. Although the ionosphere is associated with charged particles and radio communication, it is not the region defined by maximum ozone concentration. The mesosphere and thermosphere are too high and contain very little ozone. These cross checks confirm that the ozone layer is primarily a stratospheric feature. Why Other Options Are Wrong: The troposphere is the lowest layer where weather occurs and where we live; it contains only small amounts of ozone, often as a pollutant near ground level, not the protective ozone layer. The ionosphere is a region of ionised gases that overlaps the mesosphere, thermosphere, and exosphere but is defined by charged particles rather than ozone. The mesosphere is higher than the stratosphere and has very low ozone content. The thermosphere is even higher and is dominated by highly energetic particles and ionisation, not by ozone. Therefore, none of these layers match the main ozone concentration region. Common Pitfalls: Some students confuse the troposphere and stratosphere because they are adjacent layers, leading them to choose the wrong one. Others might pick the ionosphere because they associate it with radiation from the Sun, forgetting that its key feature is ionisation, not ozone. To avoid these mistakes, remember a simple phrase such as "Ozone shield in the stratosphere" to connect the ozone layer with the correct atmospheric layer. Final Answer: The natural ozone layer is mainly concentrated in the stratosphere of Earth atmosphere.

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