Question 1

Considering electronegativity differences, which hydrogen bond among the following has the greatest ionic character?

Accepted Answer

Introduction / Context: This question examines your understanding of bond polarity and ionic character based on electronegativity differences between bonded atoms. Covalent bonds can vary in how evenly electrons are shared. When the electronegativity difference between two atoms is large, the bond becomes strongly polar and may be described as having significant ionic character. Here, you must identify which hydrogen bond, H–X, has the greatest ionic character by comparing electronegativities of the second element in each pair. Given Data / Assumptions: The bonds compared are H–F, H–Cl, H–N, H–O and H–S. Hydrogen has moderate electronegativity, roughly around 2.1 on the Pauling scale. Fluorine is the most electronegative element, followed by oxygen, chlorine, nitrogen and sulfur among the listed atoms. Greater electronegativity difference implies stronger bond polarity and more ionic character. Concept / Approach: Bond ionic character is an approximate concept that increases with the difference in electronegativity between the two atoms in a bond. The larger the difference, the more the electrons are drawn toward the more electronegative atom, making the bond more polar and closer in behaviour to an ionic bond. On the Pauling scale, fluorine has the highest electronegativity of about 4.0, while hydrogen is about 2.1, giving a difference near 1.9. Oxygen, chlorine and nitrogen have lower electronegativities than fluorine, so their differences with hydrogen are smaller. Therefore, among the listed bonds, H–F has the largest electronegativity difference and thus the greatest ionic character. Step-by-Step Solution: Step 1: Recall approximate electronegativities: H ~ 2.1, F ~ 4.0, O ~ 3.5, Cl ~ 3.0, N ~ 3.0, S ~ 2.5. Step 2: Compute rough differences with hydrogen: H–F ~ 1.9, H–O ~ 1.4, H–Cl ~ 0.9, H–N ~ 0.9, H–S ~ 0.4. Step 3: Recognise that the largest difference among these is for the H–F bond. Step 4: A greater electronegativity difference means the electron pair is more strongly attracted to the more electronegative atom, creating a larger partial charge separation. Step 5: This greater charge separation corresponds to higher bond polarity and greater ionic character. Step 6: Therefore, among the given options, H–F has the greatest ionic character. Verification / Alternative check: You can verify this by looking at properties like bond dipole moments and acid strength. Hydrogen fluoride has a very strong bond dipole because fluorine draws electron density extremely strongly. In water, HF behaves as a weak acid compared with HCl, but this is due to bond strength and solvation, not lack of polarity. Comparing other hydrogen compounds, such as H2O, HCl and NH3, shows that their bonds are polar but none exhibit as extreme a charge separation as H–F. This is also reflected in the fact that fluorine forms the most polar covalent bonds of all elements, reinforcing that H–F has high ionic character among the options. Why Other Options Are Wrong: H–Cl has a significant electronegativity difference, but chlorine is less electronegative than fluorine, so H–Cl is less polar than H–F. H–N and H–O are also polar, but the electronegativity differences are smaller than that for H–F, giving them less ionic character. H–S has the smallest difference among the options; sulfur is only slightly more electronegative than hydrogen, so the bond is comparatively less polar. Therefore, none of these bonds surpass H–F in ionic character. Common Pitfalls: Students sometimes misremember electronegativity order, thinking oxygen might be the most electronegative element rather than fluorine. This can lead them to incorrectly choose H–O. To avoid this, remember the simple rule that fluorine is always the most electronegative element in the periodic table. Another mistake is to confuse bond strength with ionic character; while H–F has a strong bond, the question is about polarity, not necessarily about how easily the bond breaks. Always focus on electronegativity differences when comparing ionic character of covalent bonds. Final Answer: The hydrogen bond with the greatest ionic character among the options is H–F.

Question 2

Bulletproof jackets and similar high-strength protective materials are commonly made from which type of synthetic polymer?

Accepted Answer

Introduction / Context: This question links polymer chemistry with real world applications in safety and defence. Bulletproof materials used in body armour and helmets must combine high tensile strength, toughness and relatively low weight. Chemists have developed specialised polymers that meet these requirements. Recognising which class of polymer is used in such applications helps you understand how chemical structure translates into material properties. Given Data / Assumptions: The focus is on bulletproof materials, especially those used in protective jackets. Options include polycarbonates, polyvinyl polymers, polyamides (aramids like Kevlar), PVC and polystyrene. The polymer should have very high tensile strength and impact resistance. We assume knowledge of common engineering plastics and fibres. Concept / Approach: Bulletproof jackets are often made from high performance fibres such as Kevlar, which is an aramid, a specialised polyamide. Polyamides have repeating amide linkages in their backbone, and aramids in particular contain aromatic rings that provide stiffness and high strength. Kevlar fibres have very strong intermolecular hydrogen bonding and highly oriented chains, giving them exceptional tensile strength and the ability to absorb and dissipate the energy of bullets and shrapnel. Polycarbonates and PVC are useful plastics, but they do not usually form the main fibres in bullet resistant vests. Polystyrene is brittle and unsuitable for this purpose. Step-by-Step Solution: Step 1: Identify the polymer family known for high strength fibres used in body armour. Kevlar is a classic example and is chemically an aramid, which belongs to the polyamide family. Step 2: Polyamides contain amide linkages (-CONH-) and can form strong hydrogen bonds between chains, leading to high mechanical strength. Step 3: Aramids like Kevlar also include aromatic rings in the backbone, which increase stiffness and help the chains pack closely. Step 4: These structural features allow polyamide fibres to have very high tensile strength, making them ideal for bullet resistant fabrics. Step 5: Polycarbonates are tough, transparent plastics used in safety glasses and some impact resistant parts but are not the primary fibres in bulletproof vests. Step 6: Polyvinyl polymers such as PVC and polystyrene are widely used plastics but lack the combination of strength and flexibility needed for ballistic protection. Step 7: Therefore, the correct choice is polyamides, specifically high performance aramids like Kevlar. Verification / Alternative check: If you look into the materials used by defence and law enforcement agencies for bullet resistant vests, names like Kevlar, Twaron and other aramid fibres commonly appear. These are all polyamides with aromatic rings. They are woven into layers to form flexible yet highly resistant panels that can stop bullets and shrapnel. While polycarbonates may be used for transparent shields, visors or windows, and rigid plastics like polycarbonate and acrylic are used for bullet resistant glass laminates, the soft body armour itself relies on polyamide fibres. This reinforces that polyamides are the class of polymers associated with bulletproof materials. Why Other Options Are Wrong: Polycarbonates are tough and impact resistant but are usually used as rigid plastics rather than fibres forming the core of bulletproof fabrics. Polyvinyl polymers such as PVC do not offer the necessary combination of tensile strength and low weight; they are more suitable for pipes, cables and flooring. PVC (polyvinyl chloride) is relatively rigid and may become brittle under certain conditions. Polystyrene is brittle and shatters easily under impact, making it the opposite of what is needed for bullet resistance. None of these materials match the performance of high strength polyamide fibres in body armour applications. Common Pitfalls: A common error is to choose polycarbonates because they are associated with safety glasses and impact resistant applications. While they are indeed tough, the question is specifically about bulletproof material in the sense of soft armour, which is dominated by Kevlar and related aramids. Another source of confusion is generalising from the term plastic, assuming any strong plastic could be used for body armour. To answer correctly, it is important to recall specific examples taught in polymer chemistry: nylon and Kevlar as polyamides, with Kevlar featuring prominently in discussions of bulletproof vests. Final Answer: Bulletproof jacket materials are commonly made from high strength polyamides (aramids such as Kevlar).

Question 3

Among the following elements, which one is the most electronegative?

Accepted Answer

Introduction / Context: Electronegativity is a measure of the tendency of an atom to attract shared electrons toward itself in a chemical bond. It plays a key role in determining bond polarity, molecular structure and chemical reactivity. This question asks you to identify the most electronegative element among a small set of choices, which is a common type of periodic trends problem in exams. Given Data / Assumptions: The elements listed are oxygen, chlorine, nitrogen, sodium and aluminium. You should compare their relative electronegativities on the Pauling scale. Fluorine, though the most electronegative element overall, is not among the options. We assume a standard trend: electronegativity increases across a period and decreases down a group. Concept / Approach: Across a period in the periodic table, electronegativity generally increases from left to right, while down a group it decreases. Nonmetals in the upper right corner (excluding the noble gases) are the most electronegative. Fluorine is the most electronegative element, but since it is not an option, we must compare oxygen, chlorine, nitrogen, sodium and aluminium. Among these, oxygen has a higher electronegativity than nitrogen and chlorine. Sodium and aluminium are metals with much lower electronegativity values. Thus, oxygen is the most electronegative element among the choices. Step-by-Step Solution: Step 1: Recall approximate electronegativity values on the Pauling scale: O ~ 3.5, Cl ~ 3.0, N ~ 3.0, Na ~ 0.9, Al ~ 1.5. Step 2: Identify that sodium and aluminium are metals and generally have low electronegativities, so they can be excluded as candidates for the highest value. Step 3: Compare the nonmetals: oxygen, chlorine and nitrogen. Among these, oxygen has an electronegativity of about 3.5, which is higher than the values for chlorine and nitrogen. Step 4: Therefore, oxygen is the most electronegative element in the given list. Step 5: Confirm this by periodic trends: oxygen is above sulfur and to the right of nitrogen, positioned closer to fluorine, the most electronegative element. Verification / Alternative check: You can verify this result by examining common chemical behaviours. Oxygen tends to form strongly polar covalent bonds with many elements, such as in water, metal oxides and oxoacids. The large electronegativity difference between oxygen and metals leads to ionic character and basic or amphoteric oxides. Chlorine is also highly electronegative, but when comparing bond polarities such as H–O versus H–Cl, the H–O bond is less polar than H–F but similar or slightly different compared with H–Cl. Nonetheless, numerical tables of electronegativity consistently place oxygen above both nitrogen and chlorine. Sodium and aluminium, being metals, readily lose electrons to form cations, which is characteristic of low electronegativity. Why Other Options Are Wrong: Chlorine is indeed highly electronegative, but its value is slightly lower than that of oxygen. Nitrogen has a high electronegativity but still slightly less than oxygen. Sodium has a very low electronegativity; it tends to lose its valence electron to form Na+ rather than attract electrons. Aluminium has a moderate but still relatively low electronegativity, behaving as a typical metal that can form positive ions and covalent compounds with more electronegative elements. Thus, none of these options surpass oxygen in electronegativity among the choices given. Common Pitfalls: A common mistake is to think that chlorine, being a halogen and located further to the right on the periodic table, must always be more electronegative than oxygen. However, electronegativity also depends on the period. Oxygen is in period 2, closer to the nucleus and with less shielding than chlorine in period 3, which increases its effective nuclear attraction for bonding electrons. Another pitfall is confusing ionisation energy with electronegativity; while they are related, they are not identical. To avoid these errors, memorise that fluorine is the most electronegative, followed by oxygen as one of the next highest among common elements. Final Answer: Among the listed elements, the most electronegative is oxygen (O).

Question 4

In sewage and wastewater treatment, the process specifically used to remove floating oil and grease from wastewater is known as what?

Accepted Answer

Introduction / Context: This question connects environmental chemistry with civil engineering and public health. Sewage and wastewater contain various types of impurities, including suspended solids, dissolved substances, and floating materials like oil and grease. Different treatment steps are designed to remove different categories of pollutants. You are asked to identify the specific process used to remove floating oil and grease from wastewater, which is an important pre-treatment step before further purification. Given Data / Assumptions: The focus is on removal of oil and grease from sewage or wastewater. Options include skimming, filtration, screening, sedimentation and chlorination. Oil and grease are usually lighter than water and tend to float on the surface. Standard wastewater treatment terminology is assumed. Concept / Approach: Skimming is the process by which floating oil, grease and scum are removed from the surface of wastewater. In a skimming tank or chamber, wastewater is allowed to stand or flow slowly so that oils and greases rise to the surface due to their lower density. Mechanical scrapers or simple overflow systems then remove this floating layer. Screening, in contrast, removes larger solid objects; filtration removes fine suspended particles by passing water through porous media; sedimentation allows heavier solids to settle; and chlorination is used for disinfection. Therefore, the appropriate technique for removing floating oil and grease is skimming. Step-by-Step Solution: Step 1: Recognise that oil and grease are less dense than water and therefore tend to float on the surface of sewage. Step 2: In a treatment plant, wastewater is allowed to flow through a chamber where flow velocity is reduced, giving time for oils and greases to rise to the surface. Step 3: The surface layer of oil and grease is then removed mechanically; this removal process is called skimming. Step 4: Screening refers to the removal of large floating debris such as sticks, leaves, plastics and rags using metal screens, not specifically oil. Step 5: Filtration is used at later stages to remove finer suspended particles and is not the primary method for oil and grease removal. Step 6: Sedimentation removes heavier suspended solids that settle at the bottom, which is the opposite behaviour to floating oil and grease. Step 7: Chlorination is a disinfection step using chlorine and does not physically remove oil and grease. Thus skimming is the correct process. Verification / Alternative check: Descriptions of standard sewage treatment plants often mention primary treatment including screening, grit removal, sedimentation and skimming. Skimming tanks are specifically designed for the removal of grease and oil. They may be located after bar screens and grit chambers. Textbook diagrams show skimming arms or mechanisms that sweep the surface to collect foam, oil and scum. In contrast, filtration units and chlorination facilities are located downstream, after most physical impurities have been removed. This alignment of processes confirms that skimming is the named method for removing oil and grease. Why Other Options Are Wrong: Screening removes large floating solids, not emulsified or free oil and grease. Filtration is aimed at finer suspended material and will clog quickly if used to remove large amounts of oil. Sedimentation is used to settle out heavy particles like sand and silt that sink, not those that float. Chlorination kills microorganisms but does not physically eliminate floating oils and greases from the water. Therefore, these options do not describe the specific process used to handle oil and grease in sewage treatment. Common Pitfalls: Students sometimes confuse screening and skimming because both occur early in treatment and involve removing materials from wastewater. The key difference is that screening uses bars or meshes to catch large solids, whereas skimming removes a floating scum layer from the surface. Another mistake is to think that sedimentation covers all physical removal, ignoring the fact that light materials float instead of sink. To avoid such confusion, remember: heavy particles settle (sedimentation), large debris is caught (screening), and floating oils and greases are taken off the top (skimming). Final Answer: The process used to remove floating oil and grease from sewage is called skimming.

Question 5

White (yellow) phosphorus is stored and kept immersed under water because of which characteristic property?

Accepted Answer

Introduction / Context: This question concerns the physical and chemical properties of white (also called yellow) phosphorus and the safety precautions required when handling it. Some substances are so reactive with air that they ignite or burn spontaneously at or near room temperature. Chemists must therefore store them under special conditions. Here you are asked to identify why phosphorus is kept under water rather than left exposed to air. Given Data / Assumptions: The form of phosphorus considered is white or yellow phosphorus, not red or black phosphorus. It is known to be highly reactive and flammable. Options mention ignition temperature and critical temperature, as well as solubility. Ignition temperature is the minimum temperature at which a substance catches fire spontaneously in air without an external flame. Concept / Approach: White phosphorus has a very low ignition temperature, meaning it can catch fire in air at temperatures only slightly above room temperature. It reacts with oxygen to form phosphorus pentoxide, releasing heat and often glowing in the dark due to slow oxidation, a phenomenon called chemiluminescence. To prevent accidental ignition, white phosphorus is stored underwater, where it cannot contact atmospheric oxygen. Water acts as a physical barrier, and because phosphorus is insoluble in water, it remains safe and stable. Critical temperature, by contrast, relates to gas liquefaction and is not the main issue here. Step-by-Step Solution: Step 1: Recall that ignition temperature is the lowest temperature at which a substance begins to burn in air spontaneously. Step 2: White phosphorus has a very low ignition temperature, around room temperature or slightly above, making it prone to catching fire if left in air. Step 3: Because of this, to handle white phosphorus safely, it is kept under water to isolate it from oxygen in the air. Step 4: A high ignition temperature would make a substance less likely to burn, which is the opposite of the situation with phosphorus. Step 5: Critical temperature is a property related to the liquefaction of gases and is not directly responsible for spontaneous ignition of a solid like phosphorus. Step 6: Solubility in water is low for white phosphorus, so water is simply used as a protective layer rather than a solvent. Step 7: Therefore, the correct reason is that white phosphorus has a very low ignition temperature. Verification / Alternative check: Descriptions of phosphorus in textbooks note that white phosphorus glows faintly in the dark when exposed to air, due to slow oxidation, and can easily ignite, producing dense white fumes of phosphorus pentoxide. Safety data sheets for white phosphorus require that it be stored under water, usually in glass containers, precisely to keep it away from air. Red phosphorus, on the other hand, is more stable and does not require storage under water. This difference emphasises that the key property involved is the low ignition temperature of white phosphorus, not any special behaviour related to critical temperature or solubility. Why Other Options Are Wrong: If the ignition temperature were very high, phosphorus would be difficult to ignite and would not need special precautions, which contradicts known behaviour. Critical temperature arguments apply to gases near their liquefaction point and do not explain why a solid element ignites so easily in air. Saying that phosphorus is kept under water because its critical temperature is high or low is therefore incorrect. Phosphorus is also not stored under water because it is highly soluble; in fact, it is only sparingly soluble in water. Water acts as a protective barrier, not as a solvent in this context. Common Pitfalls: Students sometimes confuse ignition temperature with melting or boiling point, assuming that a low melting point automatically means a substance catches fire easily. These are different concepts; ignition temperature specifically refers to the temperature needed for combustion in air. Another mistake is to overuse the idea of solubility and think that any substance stored under water must be highly soluble, which is not true for white phosphorus. To avoid such errors, remember key textbook examples: white phosphorus has low ignition temperature and is stored under water, while substances with high ignition temperatures require intense heating or a flame to burn. Final Answer: White phosphorus is kept under water because its ignition temperature is very low, so it would otherwise ignite easily in air.

Question 6

All isotopes of the same chemical element have which of the following in common?

Accepted Answer

Introduction / Context: This question deals with the fundamental definitions of isotopes, which are variants of the same chemical element. Understanding isotopes is crucial for topics such as radioactivity, nuclear chemistry and applications like radiocarbon dating. The key is to know which subatomic particles are the same and which differ among isotopes of a given element, and how this relates to atomic number and atomic mass. Given Data / Assumptions: Isotopes are being compared within the same element. Atomic number is defined as the number of protons in the nucleus. Atomic mass (or mass number) is the total number of protons and neutrons. Options describe different possible relationships between atomic number and atomic mass for isotopes. We assume a basic understanding of protons, neutrons and electrons. Concept / Approach: By definition, isotopes of an element have the same number of protons in the nucleus, and therefore the same atomic number, but differ in the number of neutrons. Because the atomic mass (mass number) equals protons plus neutrons, different neutron counts lead to different mass numbers. The number of electrons in a neutral atom equals the number of protons, but isotopes can be neutral or form ions. The constant property across isotopes of the same element is therefore atomic number, while atomic mass can vary. Step-by-Step Solution: Step 1: Recall that the atomic number (Z) is the number of protons in an atom and uniquely identifies the element. Step 2: For isotopes of the same element, the number of protons is the same, so the atomic number is the same. Step 3: The mass number (A) is the sum of protons and neutrons: A = Z + number of neutrons. Step 4: Isotopes differ in their number of neutrons, so they have different mass numbers while sharing the same atomic number. Step 5: Therefore, isotopes of a single element have the same atomic number but different atomic masses. Step 6: This matches option C. Verification / Alternative check: Consider carbon isotopes as a familiar example. Carbon-12 has 6 protons and 6 neutrons, mass number 12. Carbon-14 has 6 protons and 8 neutrons, mass number 14. Both are carbon because they each have 6 protons, so their atomic number is 6. The difference is in the neutron count and therefore the mass. Similar patterns hold for hydrogen isotopes: protium (1H) has 1 proton and 0 neutrons, deuterium (2H) has 1 proton and 1 neutron and tritium (3H) has 1 proton and 2 neutrons. These examples clearly show constant atomic number and varying mass number among isotopes. Why Other Options Are Wrong: Option A suggests that isotopes have different atomic numbers and different masses, which would make them entirely different elements, not isotopes of the same element. Option B suggests different atomic numbers with the same mass, which again would indicate two different elements accidentally having the same mass number, not isotopes. Option D claims the same atomic number and the same atomic mass, which would describe identical atoms, not isotopes. Option E mentions different numbers of protons and electrons, which could describe ions but does not capture the defining feature of isotopes. Thus, only option C matches the precise definition of isotopes. Common Pitfalls: A common misunderstanding is to confuse mass number with atomic weight or to think that isotopes differ in both protons and neutrons. Remember that changing the number of protons changes the element itself, so isotopes must keep the proton count constant. Another pitfall is ignoring neutrons altogether and focusing only on protons and electrons. To avoid this, always remember the simple rule: isotopes are same element (same atomic number), different mass number due to different neutrons. Final Answer: All isotopes of the same element have the same atomic number but different atomic masses.

Question 7

Concentrated nitric acid does not attack or dissolve which of the following metals under normal conditions?

Accepted Answer

Introduction / Context: This question relates to the reactivity of metals with acids, a core topic in inorganic chemistry. Concentrated nitric acid is a strong oxidising agent and can react with many metals, often passivating them by forming protective oxide layers. However, some noble metals are resistant to attack by nitric acid alone and require special mixtures such as aqua regia for dissolution. Identifying which metal among the options is not attacked by nitric acid illustrates the concept of noble metals and chemical stability. Given Data / Assumptions: The acid considered is concentrated nitric acid, HNO3. Metals listed include gold, copper, zinc, iron and aluminium. We are considering reaction under typical laboratory conditions, not in special mixtures. Nitric acid can act both as an acid and as an oxidising agent. Concept / Approach: Gold is a noble metal with very low reactivity. It does not dissolve in most single acids, including concentrated nitric acid. Instead, gold requires aqua regia, a mixture of concentrated nitric acid and concentrated hydrochloric acid, to dissolve. In contrast, base metals such as copper, zinc and iron do react with nitric acid, often producing metal nitrates and nitrogen oxides. Aluminium can be passivated by nitric acid but is still not considered completely inert the way gold is. The key point is that nitric acid alone does not dissolve gold, so gold is the correct answer. Step-by-Step Solution: Step 1: Recall that gold is a noble metal, highly resistant to corrosion and chemical attack. Step 2: Nitric acid is a strong oxidising acid but is unable to dissolve gold by itself under normal conditions. Step 3: Aqua regia, a mixture of concentrated nitric acid and concentrated hydrochloric acid, is required to dissolve gold, highlighting its resistance to single acids. Step 4: Copper reacts with concentrated nitric acid to form copper nitrate, nitrogen oxides and water, so it is attacked by nitric acid. Step 5: Zinc and iron also react with nitric acid, though sometimes they may become passivated due to protective oxide or nitrate layers. Step 6: Therefore, among the listed metals, gold is the one that nitric acid does not react with under ordinary conditions. Verification / Alternative check: This fact is emphasised in many general chemistry textbooks: gold and platinum are classic examples of noble metals that resist attack by single mineral acids. The historical name aqua regia, meaning royal water, comes from its ability to dissolve royal metals like gold and platinum, unlike nitric or hydrochloric acid alone. Laboratory demonstrations often show copper reacting vigorously with nitric acid, producing brown fumes of nitrogen dioxide, while a piece of gold remains unchanged in the same acid. These observations confirm that gold is the metal not attacked by nitric acid alone. Why Other Options Are Wrong: Copper is well known to react with concentrated nitric acid, forming a blue solution of copper nitrate and releasing nitrogen oxides. Zinc, being a more reactive metal, also dissolves in nitric acid, though concentrated acid can sometimes produce passivation effects. Iron reacts with nitric acid but may likewise become passivated, especially with concentrated acid, yet it is still chemically attacked. Aluminium can form a protective oxide layer, but nitric acid does interact with its surface; it is not as inert as gold. Thus, none of these metals exhibits the complete resistance to nitric acid that gold does. Common Pitfalls: Students sometimes confuse passivation, where a protective layer slows or stops further reaction, with complete inertness. Metals like aluminium and iron can appear to resist further attack after an initial reaction, but that does not make them equivalent to gold. Another mistake is to assume that strong acids must react with all metals, ignoring the special stability of noble metals. To avoid confusion, remember that gold and platinum are classic examples of metals that do not dissolve in nitric acid alone and require aqua regia for dissolution. Final Answer: Concentrated nitric acid does not attack or dissolve gold under normal conditions.

Question 8

In everyday chemistry, what is the common name of the compound calcium oxychloride, CaOCl2?

Accepted Answer

Introduction / Context: This question tests your knowledge of common names of important industrial and household chemicals. Many compounds are better known by their everyday names than by their systematic chemical names. Calcium oxychloride, CaOCl2, is widely used as a disinfectant and bleaching agent, especially for bleaching fabrics and disinfection of drinking water. Recognising its common name is a standard exam question in basic chemistry. Given Data / Assumptions: The chemical formula given is CaOCl2, calcium oxychloride. Options include baking powder, baking soda, bleaching powder, washing soda and lime powder. Each name corresponds to a different well known household or industrial chemical. We assume familiarity with basic uses of these compounds. Concept / Approach: Bleaching powder is the common name for calcium oxychloride, CaOCl2. It is produced by passing chlorine gas over dry slaked lime (calcium hydroxide). Bleaching powder is used to bleach cotton and linen in the textile industry and to disinfect drinking water and sewage due to its ability to release chlorine. Baking soda is sodium bicarbonate, NaHCO3; baking powder is a mixture containing sodium bicarbonate and acid salts; washing soda is sodium carbonate decahydrate, Na2CO3·10H2O; and lime usually refers to calcium oxide or calcium hydroxide. These are different compounds with different formulas and uses. Step-by-Step Solution: Step 1: Recall that bleaching powder is chemically calcium oxychloride and is represented approximately by the formula CaOCl2. Step 2: Bleaching powder is manufactured by passing chlorine gas over dry slaked lime, which is calcium hydroxide, Ca(OH)2. Step 3: It is widely used for bleaching fabrics and wood pulp and for disinfecting water because it slowly releases chlorine. Step 4: Baking soda is NaHCO3, sodium bicarbonate, used in baking and as an antacid, not the same as CaOCl2. Step 5: Baking powder is a mixture containing baking soda and an acid salt, again unrelated to calcium oxychloride. Step 6: Washing soda is Na2CO3·10H2O, sodium carbonate decahydrate, used in cleaning and softening water. Step 7: Lime powder usually refers to CaO or Ca(OH)2, not to CaOCl2. Thus, the correct common name for CaOCl2 is bleaching powder. Verification / Alternative check: You can verify this association by remembering typical exam tables: Bleaching powder (CaOCl2 or more exactly Ca(OCl)Cl·H2O), Washing soda (Na2CO3·10H2O), Baking soda (NaHCO3), Baking powder (mixture based on NaHCO3) and Plaster of Paris (CaSO4·1/2H2O). Bleaching powder is always listed as a chlorine releasing disinfectant and bleaching agent. Its application in municipal water disinfection, particularly in rural or emergency situations, is widely discussed. None of the other names match the formula CaOCl2 in standard chemical lists. Why Other Options Are Wrong: Baking powder is a mixture, not a single compound, and its key component is sodium bicarbonate, not calcium oxychloride. Baking soda is specifically NaHCO3, with a very different composition and use. Washing soda is sodium carbonate decahydrate, a water softener and cleaning agent, not a disinfectant like bleaching powder. Lime powder refers to calcium oxide or calcium hydroxide and does not contain chlorine. Hence these options do not correspond to the formula CaOCl2 and are incorrect for this question. Common Pitfalls: Students sometimes confuse the names baking powder and baking soda, or mix up washing soda and bleaching powder because both are white powders used in household contexts. To avoid such confusion, it helps to associate each compound with its formula and main use: bleaching powder (CaOCl2) for bleaching and disinfection; washing soda (Na2CO3·10H2O) for laundry and water softening; baking soda (NaHCO3) for baking and digestion; and baking powder as a mixture that releases CO2 when heated. Keeping a small table of these common names and formulas in mind can prevent mistakes in multiple choice questions. Final Answer: The common name of CaOCl2 (calcium oxychloride) is bleaching powder.

Question 9

Activated charcoal is used in purification processes to remove colouring matter from solutions by which principal mechanism?

Accepted Answer

Introduction / Context: This question explores the role of activated charcoal (also called activated carbon) in purification and decolourisation processes. Activated charcoal is used in laboratories and industries to remove coloured impurities from solutions, in gas masks to remove toxic vapours and in water filters to improve taste and odour. Understanding the mechanism by which it works is important in physical chemistry and environmental applications. Given Data / Assumptions: The substance under discussion is activated charcoal or activated carbon. It is used to remove colouring matter and other impurities from pure substances. Options include bleaching, oxidation, adsorption, reduction and filtration. Activated charcoal has extremely high internal surface area due to its porous structure. Concept / Approach: Activated charcoal purifies by adsorption, not by absorption or simple filtration. Adsorption is the process in which molecules from a gas or liquid adhere to the surface of a solid. Because activated charcoal has a vast internal surface area with many pores, it can adsorb large quantities of coloured and odorous molecules from solutions or gases. This removes them from the bulk phase without necessarily changing their chemical identity. Bleaching by chemical reaction, oxidation or reduction would change the chemical structure of the impurities, which is not the primary mechanism here. Filtration alone would only remove suspended solids, not dissolved coloured molecules. Step-by-Step Solution: Step 1: Recall that activated charcoal is produced by heating carbonaceous materials to create a highly porous structure with enormous surface area. Step 2: When a coloured solution is shaken with activated charcoal, the coloured molecules are attracted to and held on the surface of the charcoal particles. Step 3: This surface attachment is called adsorption and depends on physical forces such as van der Waals interactions and sometimes specific chemical affinities. Step 4: After adsorption, the charcoal with attached impurities can be removed by filtration, leaving behind a decolourised or purified solution. Step 5: Bleaching in this context does not mean a chemical reaction with oxidising agents like chlorine; instead, it is primarily a physical process of adsorption. Step 6: Oxidation or reduction reactions may occur in specialised cases but are not the general mechanism in simple decolourisation with activated charcoal. Step 7: Therefore, the correct description is that activated charcoal removes colouring matter by adsorption onto its large surface area. Verification / Alternative check: The behaviour of activated charcoal in gas masks and water filters supports this explanation. Toxic vapours, organic compounds and odours are removed because they bind to the surface of the charcoal rather than being chemically destroyed. When the charcoal becomes saturated with adsorbed species, its effectiveness decreases and it must be replaced or regenerated. The concept of adsorption is widely discussed in physical chemistry, especially in the context of adsorption isotherms and surface phenomena. These topics describe how the amount of solute adsorbed depends on concentration and temperature, fitting the observed behaviour of activated charcoal. Why Other Options Are Wrong: Bleaching through chemical reaction usually involves specific oxidising agents such as chlorine or hydrogen peroxide, which alter the structure of coloured compounds. Activated charcoal does not act primarily as a strong oxidiser. Oxidation and reduction would require suitable reagents and reaction conditions, which are not provided by charcoal alone in simple decolourisation experiments. Simple filtration would only remove suspended solids, not dissolved coloured molecules; the key step is adsorption onto charcoal followed by filtration of the solid charcoal. Therefore, the main mechanism is adsorption, not bleaching, oxidation, reduction or mere filtration. Common Pitfalls: Students sometimes confuse adsorption with absorption because the words sound similar. Absorption refers to a substance being taken up into the bulk of another (like a sponge absorbing water), whereas adsorption involves accumulation on the surface. Another mistake is to assume that any removal of colour must be a chemical bleaching reaction. In fact, physical adsorption can effectively remove dyes and other coloured molecules without changing their chemical identity. To avoid confusion, remember that activated charcoal works mainly through adsorption due to its large surface area and porous structure. Final Answer: Activated charcoal removes colouring matter from solutions mainly by adsorption onto its large surface area.

Question 10

In the nitrogen cycle, nitrogen fixation refers specifically to which of the following processes?

Accepted Answer

Introduction / Context: This question belongs to environmental and biological chemistry, focusing on the nitrogen cycle. Nitrogen fixation is a key step that makes atmospheric nitrogen available to living organisms. Although nitrogen gas is abundant in the atmosphere, most organisms cannot use it directly. Understanding the precise meaning of nitrogen fixation helps you distinguish it from other processes in the nitrogen cycle, such as nitrification, assimilation and ammonification. Given Data / Assumptions: The process in question is nitrogen fixation. Options include assimilation of nitrate, utilisation of nitrogen gas, conversion of organic nitrogen to proteins, conversion of N2 to NH3 and conversion of ammonia to nitrate. We assume standard textbook definitions for steps in the nitrogen cycle. Biological nitrogen fixation is typically carried out by certain bacteria and cyanobacteria. Concept / Approach: Nitrogen fixation is defined as the conversion of atmospheric molecular nitrogen, N2, into ammonia (NH3) or related forms that can be assimilated by plants. This process can occur biologically via nitrogen fixing bacteria or industrially in processes like the Haber–Bosch synthesis of ammonia. Once ammonia is formed, nitrifying bacteria can convert it into nitrite and nitrate, and plants can assimilate these ions to build amino acids and proteins. Assimilation of nitrate, conversion of ammonia to nitrate and conversion of organic nitrogen to proteins are different steps, not nitrogen fixation itself. Step-by-Step Solution: Step 1: Recall that atmospheric nitrogen exists mostly as N2, a very stable molecule with a triple bond between nitrogen atoms. Step 2: Most plants and animals cannot directly use N2 because breaking the triple bond requires a large amount of energy. Step 3: Nitrogen fixation refers to the process of converting N2 into ammonia (NH3) or ammonium ions (NH4+), which can then enter biological pathways. Step 4: Biological nitrogen fixation is performed by specialised microorganisms, often in association with leguminous plants, using the enzyme nitrogenase. Step 5: Industrial nitrogen fixation in the Haber process converts N2 and H2 into NH3 under high temperature and pressure with a catalyst. Step 6: Other steps such as assimilation (uptake of nitrate by plants), nitrification (conversion of ammonia to nitrate) and protein synthesis (conversion of organic nitrogen to proteins) are distinct from nitrogen fixation. Step 7: Therefore, nitrogen fixation is best described as the conversion of molecular nitrogen (N2) into ammonia. Verification / Alternative check: Diagrams of the nitrogen cycle in textbooks show a specific arrow labelled nitrogen fixation from atmospheric N2 to ammonia or related compounds in soil. Further arrows represent nitrification (NH3 to NO2− and NO3−), assimilation (uptake of nitrate by plants and conversion into organic nitrogen), and denitrification (return of nitrogen to the atmosphere). Only the first step, capturing N2 from the atmosphere and reducing it to ammonia, is called nitrogen fixation. The fact that nitrogenase enzymes and the Haber process are described as nitrogen fixing also supports this definition. Why Other Options Are Wrong: Assimilation of nitrate by plants is a step in which plants absorb nitrate from soil and reduce it to ammonium to incorporate into amino acids; it is not nitrogen fixation. Direct utilisation of nitrogen gas by animals does not occur, as animals cannot use N2 directly, so this option is incorrect by definition. Conversion of organic nitrogen to proteins occurs within organisms and involves biosynthesis, not fixation of atmospheric nitrogen. Conversion of ammonia into nitrate is nitrification, a separate process carried out by nitrifying bacteria, not nitrogen fixation. Therefore these options do not match the definition of nitrogen fixation. Common Pitfalls: Students sometimes loosely use the term nitrogen fixation to refer to any step that moves nitrogen through the cycle, leading to confusion between fixation, nitrification, assimilation and ammonification. Another mistake is to think that nitrogen fixation is plant uptake of nitrate, ignoring the crucial role of bacteria in converting N2 to ammonia. To avoid these errors, remember that fixation always involves capturing atmospheric N2 and reducing it to ammonia, while other terms describe transformations of nitrogen once it is already in biologically available forms. Final Answer: Nitrogen fixation is the conversion of molecular nitrogen (N2) from the atmosphere into ammonia (NH3).

Question 11

The biogas commonly used as a fuel for cooking is mainly a mixture of which two gases?

Accepted Answer

Introduction / Context: This question is about renewable energy and environmental chemistry. Biogas is a fuel produced from the anaerobic decomposition of organic matter such as animal dung, plant waste and sewage. It is widely used in rural areas as a clean cooking fuel. Understanding its composition helps explain its energy content, combustion behaviour and environmental impact. Given Data / Assumptions: The fuel in question is biogas used for cooking. Options list combinations such as carbon dioxide with oxygen, isobutane with propane, methane with carbon monoxide, methane with carbon dioxide and ethane with hydrogen. We assume standard descriptions of biogas composition. Biogas is produced in biogas plants under anaerobic conditions. Concept / Approach: Biogas typically contains about 50–70 percent methane (CH4), which is the main combustible component, and 30–50 percent carbon dioxide (CO2), with small amounts of hydrogen sulfide, water vapour and other gases. Methane provides the energy when biogas is burned, similar to natural gas. Carbon dioxide is non combustible but is produced as a by product in the digestion process and dilutes the fuel. Other options, such as mixtures of isobutane and propane or ethane and hydrogen, correspond more to liquefied petroleum gas or industrial mixtures rather than biogas. Carbon monoxide may be present only in traces, not as a major component of biogas used for cooking. Step-by-Step Solution: Step 1: Recall that biogas is produced by anaerobic digestion of organic waste by bacteria in a biogas plant. Step 2: The main gas produced in this process is methane, which is the primary energy carrier. Step 3: Carbon dioxide is also produced in significant amounts and forms the second largest component of biogas. Step 4: Other gases such as hydrogen sulfide and nitrogen may be present in small quantities but are not the primary components. Step 5: Therefore, biogas used for cooking is best described as a mixture of methane and carbon dioxide. Step 6: Compare this with LPG (liquefied petroleum gas), which mainly contains propane and butane, not methane and carbon dioxide. Step 7: Carbon monoxide is a toxic gas and is not a major intentional component of biogas mixtures used in households. Verification / Alternative check: Information from renewable energy resources and environmental science texts lists typical biogas composition as around 60 percent methane and 40 percent carbon dioxide, with minor fractions of other gases. This composition explains why biogas burns with a bluish flame and produces less soot than solid fuels, while also having a lower calorific value compared with pure methane or natural gas due to the diluting effect of carbon dioxide. Biogas digesters are designed to capture and use this methane rich gas for cooking and lighting, reducing reliance on wood and fossil fuels and lowering greenhouse gas emissions compared with uncontrolled decomposition. Why Other Options Are Wrong: Carbon dioxide and oxygen would be a mixture more similar to air enriched with CO2 but would not be a combustible fuel. Isobutane and propane are components of LPG, not biogas. Methane and carbon monoxide together describe producer gas or other industrial mixtures, but not standard biogas composition, and carbon monoxide is highly toxic. Ethane and hydrogen are not the main components of biogas; ethane is more associated with natural gas composition. Thus, these options do not match the well established composition of biogas used for cooking. Common Pitfalls: A frequent mistake is to confuse biogas with LPG or natural gas and therefore choose options containing propane or butane. Another error is to assume that carbon monoxide is a major combustible component, since it can burn; however, it is not produced in large amounts in typical biogas digesters and is highly poisonous. To avoid confusion, remember that biogas comes from biological digestion and is dominated by methane and carbon dioxide, while LPG is a petroleum product primarily containing propane and butane. Final Answer: Biogas used for cooking is mainly a mixture of methane and carbon dioxide.

Question 12

On a clean glass plate, a drop of water spreads into a thin layer but a drop of mercury remains almost spherical. Which explanation best accounts for this difference in behaviour?

Accepted Answer

Introduction / Context: This question explores surface tension and intermolecular forces, specifically cohesion and adhesion. Liquids on solid surfaces exhibit characteristic shapes depending on the balance between cohesive forces within the liquid and adhesive forces between the liquid and the solid. Understanding why water tends to wet glass and spread out, while mercury forms nearly spherical droplets, illustrates these concepts and is a classic example in physics and chemistry. Given Data / Assumptions: A drop of water on clean glass spreads to form a thin layer. A drop of mercury on the same glass remains almost spherical. Options mention cohesion, adhesion, density and metallic nature. Cohesion refers to attractive forces between molecules of the same substance. Adhesion refers to attractive forces between molecules of different substances. Concept / Approach: When adhesion between a liquid and a solid surface is stronger than cohesion within the liquid, the liquid tends to spread and wet the surface. When cohesion within the liquid dominates over adhesion to the surface, the liquid tends to bead up and form droplets with minimal contact area, often appearing spherical due to surface tension. Water has strong adhesive interactions with glass because both involve polar molecules and hydrogen bonding, so water spreads. Mercury has strong cohesive metallic bonding within the liquid, while its adhesion to glass is relatively weak, so droplets minimise contact with the glass and stay nearly spherical. Step-by-Step Solution: Step 1: Consider water on glass. Water molecules are polar and can form hydrogen bonds and strong attractions to the polar surface of glass (silica). Step 2: Adhesive forces between water and glass exceed cohesive forces between water molecules at the surface, so water spreads out to maximise contact with glass. Step 3: Now consider mercury on glass. Mercury atoms are held together by strong metallic bonding, producing very strong cohesive forces within the liquid metal. Step 4: Adhesion between mercury and glass is relatively weak compared with the cohesion within mercury. Step 5: As a result, the mercury droplet tries to minimise its surface area and contact with the glass, forming an almost spherical shape. Step 6: Therefore, the correct explanation is that cohesion of mercury is greater than its adhesion with glass. Verification / Alternative check: Capillary action experiments support this reasoning. Water in a narrow glass tube rises above the external water level, forming a concave meniscus because adhesion to glass pulls water up along the walls. Mercury in a glass capillary shows a convex meniscus and actually falls below the external mercury level, indicating that cohesion in mercury dominates and the liquid pulls away from the glass walls. These behaviours are textbook evidence that mercury has stronger cohesion than adhesion with glass, while water shows the opposite trend, explaining the shapes of drops on a glass plate. Why Other Options Are Wrong: Saying mercury is a metal and therefore must be spherical is not a correct explanation; metallic nature alone does not guarantee spherical shape on all surfaces. Density differences affect gravitational forces but do not directly determine whether a drop spreads or beads up on a surface; surface tension and adhesion are more important. The statement that cohesion of water is greater than its adhesion with glass is opposite to reality; if that were true, water would form droplets rather than spreading. The idea that water has much higher surface tension than mercury is also incorrect; mercury actually has a higher surface tension, which is one reason its drops are so spherical. Common Pitfalls: Students sometimes focus on density or metal versus nonmetal classification instead of thinking about intermolecular forces. Another common error is to assume that higher surface tension alone explains everything, ignoring the balance between cohesion and adhesion. To avoid these mistakes, always analyse whether the liquid wets the surface or not. If it wets, adhesion dominates; if it beads up, cohesion dominates. Remember that water wets clean glass (strong adhesion), while mercury does not (strong cohesion), which is exactly what this question illustrates. Final Answer: A drop of mercury remains almost spherical on glass because the cohesion of mercury is greater than its adhesion with glass, whereas water has stronger adhesion to glass and spreads.

Question 13

The property of metals that allows them to be beaten or hammered into thin sheets is known as what?

Accepted Answer

Introduction / Context: This question checks your understanding of basic mechanical properties of metals, which are commonly introduced in school chemistry and physics. Metals have characteristic properties such as malleability and ductility that make them useful for shaping into various forms. Being able to match these property names with their definitions is essential for interpreting everyday uses of materials like aluminium foil and metal wires. Given Data / Assumptions: The specific behaviour described is that metals can be beaten or hammered into thin sheets. Options include ductility, malleability, viscosity, tensile strength and elasticity. We assume standard textbook definitions of these mechanical properties. Examples include gold leaf, aluminium foil and metal sheets for construction. Concept / Approach: Malleability is defined as the property of a material, especially metals, to be hammered or rolled into thin sheets without breaking or cracking. Ductility, by contrast, refers to the ability to be drawn into thin wires. Viscosity describes resistance to flow in liquids, tensile strength measures resistance to being pulled apart and elasticity refers to the ability to return to original shape after deformation. Thus, the word that specifically matches the ability to form sheets is malleability. Step-by-Step Solution: Step 1: Identify the behaviour described: a metal can be beaten or hammered into thin sheets. Step 2: Recall that malleability is the property that allows metals to be hammered or rolled into sheets. Step 3: Note that ductility refers to drawing metals into wires, which is a different process involving tension rather than compression or hammering. Step 4: Viscosity applies mainly to fluids and describes thickness or resistance to flow, not shaping into sheets. Step 5: Tensile strength measures how much pulling force a material can withstand before breaking, which is related but not the same as the ability to form sheets. Step 6: Elasticity concerns the ability to return to original shape after deforming, such as in springs, and is not specific to sheet formation. Step 7: Therefore, the property described in the question is malleability. Verification / Alternative check: Examples in textbooks and everyday life support this definition. Gold is extremely malleable and can be beaten into very thin leaves used for decoration and jewellery. Aluminium is malleable enough to be rolled into thin foil for food packaging. Copper and steel are rolled into sheets for roofing, containers and structural parts. These applications all rely on the malleability of metals. In contrast, drawing copper into electrical wires relies on ductility, another property often mentioned alongside malleability. Remembering these paired examples helps verify the correct meanings. Why Other Options Are Wrong: Ductility involves pulling a metal into wires and is often associated with tensile forces rather than hammering or rolling. Viscosity has nothing to do with solid metals; it is a property of liquids such as honey or oil. Tensile strength is a mechanical parameter that describes how much tensile stress a material can bear, which may influence ductility but does not define the ability to form sheets. Elasticity is a general property for metals, polymers and other materials but does not specifically refer to sheet formation. None of these terms match the definition given in the question as precisely as malleability does. Common Pitfalls: Students often confuse malleability and ductility because they are usually taught together as properties of metals. A practical way to distinguish them is to remember that malleability relates to making sheets (think of aluminium foil), while ductility relates to making wires (think of copper wire). Writing the first letters S for sheets and W for wires next to M (malleability) and D (ductility) in your notes can help cement the difference. Being clear on these definitions will help you answer similar questions quickly and accurately. Final Answer: The property of metals that allows them to be beaten into thin sheets is called malleability.

Question 14

In electrochemistry, the passage of an electric current through a conducting liquid (an electrolyte solution or molten salt) generally produces what kind of effect in the liquid?

Accepted Answer

Introduction / Context: This general chemistry question is about what happens when an electric current passes through a conducting liquid, such as an aqueous electrolyte or a molten salt. The concept belongs to electrochemistry and is closely related to electrolysis, electroplating and the production of elements like chlorine, sodium and aluminium. Understanding that the current does not merely heat the liquid but can cause definite chemical changes is essential for many practical and industrial applications. Given Data / Assumptions: A conducting liquid is present. This can be an aqueous solution of an electrolyte or a molten ionic compound. An electric current is passed through this conducting liquid between suitable electrodes. We are asked about the overall effect produced inside the liquid. We assume normal laboratory or industrial conditions where significant current is passed for some time. Concept / Approach: When an electric current passes through a conducting liquid containing ions, the positive and negative ions move towards the oppositely charged electrodes. At these electrodes, the ions can gain or lose electrons. This gain or loss of electrons is a redox (reduction–oxidation) process and leads to the formation of new substances, such as gases or deposited metals. This process is called electrolysis. Therefore, the effect of the current is not limited to physical heating or movement, but a true chemical reaction in the electrolyte and at the electrodes. Step-by-Step Solution: 1) Identify the nature of a conducting liquid: it contains free ions (cations and anions) that can move under the influence of an electric field.2) When a potential difference is applied, cations move towards the cathode and anions move towards the anode.3) At the cathode, cations can gain electrons (reduction), forming neutral atoms or molecules, such as metal deposits or hydrogen gas.4) At the anode, anions can lose electrons (oxidation), producing neutral atoms or molecules, for example chlorine gas from chloride ions.5) These oxidation and reduction processes change the chemical composition of the solution or molten salt.6) Because new substances are formed and the original ions are consumed or transformed, the overall effect is a chemical reaction, specifically electrolysis. Verification / Alternative check: Consider the electrolysis of molten sodium chloride. When current passes, sodium ions are reduced at the cathode to form liquid sodium metal, and chloride ions are oxidised at the anode to form chlorine gas. Both products are chemically different from the starting ionic compound. In the electrolysis of aqueous copper sulphate with copper electrodes, copper is deposited at the cathode and dissolves from the anode. These examples clearly show that electric current through a conducting liquid leads to chemical transformations rather than mere physical changes such as warming or simple mixing. Why Other Options Are Wrong: Evaporation of the liquid only: Some heating may occur, but evaporation is not the main or defining effect of current in an electrolyte. Galvanisation of the container metal: Galvanisation refers to coating iron or steel with zinc, which is a different process and not the general effect of current in any conducting liquid. Mainly a physical change without any reaction: The movement of ions leads directly to redox reactions and formation of new substances, which is chemical, not purely physical. Only heating of the liquid with no change in composition: Joule heating may occur, but electrolysis clearly changes composition, so this description is incomplete and misleading. Common Pitfalls: A frequent mistake is to think of current in liquids as similar to current in metallic wires, where the conductor itself does not change chemically. In electrolytes, however, the charge carriers are ions, and their discharge at electrodes produces new substances. Another confusion is between simple heating of the solution and electrolysis; while the solution may warm up, the hallmark of electrolysis is the appearance of gases, metal deposits or other products at the electrodes. Remember that whenever ions are discharged at electrodes, a chemical reaction has taken place. Final Answer: The passage of an electric current through a conducting liquid primarily causes a chemical reaction in the electrolyte (electrolysis).

Question 15

Polyethylene terephthalate, commonly abbreviated as PET, is a very familiar form of which type of synthetic polymer that is widely used for making plastic bottles and food containers?

Accepted Answer

Introduction / Context: This general knowledge chemistry question asks about the nature of PET, which most people encounter in the form of soft drink bottles, food packaging and synthetic fibres. Knowing what type of polymer PET is helps connect everyday materials with fundamental concepts in organic and polymer chemistry, especially the classification of plastics such as polyesters, polyamides and others. Given Data / Assumptions: The polymer in question is PET, which stands for polyethylene terephthalate. PET is widely used for making bottles and many transparent plastic containers. We need to identify the broad class of synthetic material (for example, nylon, acrylic, polyester or rayon) to which PET belongs. We assume introductory level knowledge about different families of polymers. Concept / Approach: PET is formed by the condensation polymerisation of ethylene glycol with terephthalic acid or dimethyl terephthalate. The repeating units are linked by ester bonds (–COO–) formed between carboxylic acid and alcohol groups. A polymer whose backbone contains such ester linkages in the main chain is called a polyester. PET can be spun into fibres, used as a film and moulded into bottles, making it one of the best known polyester materials in daily life. Step-by-Step Solution: 1) Recall that PET stands for polyethylene terephthalate, an important synthetic polymer.2) Identify the functional group linking the repeating units: an ester group formed by reaction of an acid and an alcohol.3) Polymers that have ester linkages (–COO–) in the main chain are classified broadly as polyesters.4) Nylon, on the other hand, has amide linkages (–CONH–) and belongs to the polyamide family.5) Acrylic materials are usually based on derivatives of acrylic or methacrylic acid, such as polymethyl methacrylate (PMMA), and do not specifically match PET structure.6) Rayon is regenerated cellulose obtained from natural cellulose and is not the same as PET, even though both can be used as fibres.7) Therefore, the correct classification of PET is that it is a polyester. Verification / Alternative check: In textbooks and packaging codes, PET is grouped under polyesters. Its recycling symbol often shows the number 1 with the word PET, indicating the material class. Clothing labels may mention polyester fibres, some of which are made from PET. Industrial literature also describes PET as a thermoplastic polyester used for fibres, films and bottles. These references confirm that polyester is the correct category, not nylon, acrylic, polyethylene or rayon. Why Other Options Are Wrong: Nylon (polyamide): This polymer family contains amide linkages and is used in products like nylon fibres and engineering plastics, but it is structurally different from PET. Acrylic polymer: Acrylic plastics such as PMMA are based on esters of acrylic acid, not on terephthalic acid and ethylene glycol as in PET. Rayon (regenerated cellulose): Rayon is made from natural cellulose that has been chemically treated, whereas PET is a purely synthetic polyester. Polyethylene: Although the name of PET starts with polyethylene, simple polyethylene is a different polymer made only from ethene units and contains no ester linkages. Common Pitfalls: Some students are misled by the word polyethylene in the full name and may incorrectly think PET is just polyethylene. However, polyethylene terephthalate contains aromatic units and ester linkages that place it firmly in the polyester category. Another confusion is between rayon and polyester because both can be used as textile fibres. Remember that rayon comes from cellulose, while polyester like PET is synthetic and based on ester linkages involving terephthalic acid. Keeping track of the functional groups in the polymer backbone helps avoid such mistakes. Final Answer: PET (polyethylene terephthalate), used for making many plastic bottles, is a form of polyester.

Question 16

In the alcohol methanol (CH3OH), the O–H bond between the oxygen atom and the hydrogen atom is best described as which type of chemical bond?

Accepted Answer

Introduction / Context: This question is about the nature of the O–H bond in methanol (CH3OH), a simple alcohol. It tests understanding of how differences in electronegativity between atoms influence bond polarity and how we classify bonds as ionic, polar covalent or non polar covalent. Recognising that bonds involving highly electronegative atoms such as oxygen are often polar is essential in predicting solubility, hydrogen bonding and many physical properties of molecules. Given Data / Assumptions: The molecule in question is methanol, CH3OH. We focus on the bond between oxygen and hydrogen, usually written as the O–H bond. We assume knowledge of electronegativity differences between oxygen and hydrogen. The options include polar covalent, ionic, non polar covalent and other less suitable descriptions. Concept / Approach: Bonds are classified based on how electrons are shared between atoms. If electrons are completely transferred, the bond is ionic. If they are shared equally, the bond is non polar covalent. If they are shared unequally, due to a difference in electronegativity, the bond is polar covalent. Oxygen is significantly more electronegative than hydrogen, so in an O–H bond, the shared electron pair is drawn closer to oxygen. As a result, oxygen acquires a partial negative charge and hydrogen a partial positive charge, making the bond polar covalent and allowing hydrogen bonding between molecules. Step-by-Step Solution: 1) Note that methanol is a covalent molecule composed of carbon, hydrogen and oxygen atoms.2) Identify the two atoms forming the bond in question: oxygen and hydrogen.3) Recall that oxygen has a higher electronegativity than hydrogen, meaning it attracts shared electrons more strongly.4) In the O–H bond, electrons are shared between O and H, so the bond is covalent, not ionic, because no complete transfer of electrons occurs.5) Because oxygen pulls the shared electrons closer to itself, the sharing is unequal, giving the bond a partial negative charge on oxygen and a partial positive charge on hydrogen.6) Unequal sharing of electrons in a covalent bond corresponds to a polar covalent bond.7) Therefore, the O–H bond in CH3OH is classified as a polar covalent bond. Verification / Alternative check: The polarity of the O–H bond in alcohols and water leads to many well known properties. For example, methanol and water are miscible because their molecules can form hydrogen bonds, which require polar O–H bonds with partial charges. Spectroscopic and physical property data, such as high boiling points compared with non polar molecules of similar size, also support the presence of strong polar covalent O–H bonds. These observations confirm that the bond is not ionic or non polar covalent but clearly polar covalent. Why Other Options Are Wrong: Ionic bond: An ionic bond would involve complete transfer of an electron from hydrogen to oxygen or vice versa, leading to H+ and O2− ions, which is not the case in neutral methanol. Non polar covalent bond: This would require equal sharing of electrons and usually occurs between identical atoms, such as H2 or Cl2, not between oxygen and hydrogen. Purely cationic interaction: This phrase does not describe a standard type of chemical bond in neutral molecules and is not appropriate for the O–H bond. Metallic bond: Metallic bonding occurs between metal atoms in a metal lattice and involves delocalised electrons, which is completely different from the O–H bond in an organic molecule. Common Pitfalls: Students sometimes assume that any bond involving hydrogen is non polar, confusing it with the H–H bond in a hydrogen molecule. Another confusion is between ionic and polar covalent bonds when electronegativity differences are moderate. A useful guideline is that bonds between hydrogen and significantly more electronegative atoms like oxygen, nitrogen or fluorine are polar covalent and can participate in hydrogen bonding. Remembering this helps correctly classify the O–H bond in methanol and similar molecules. Final Answer: The O–H bond in methanol (CH3OH) is a polar covalent bond.

Question 17

In descriptive organic chemistry, ethane (C2H6) is often said to have a slightly ______ character in its taste and odour when detected in high concentration. Which word best completes this statement?

Accepted Answer

Introduction / Context: This general knowledge chemistry question is about a descriptive property of ethane gas, a simple alkane with formula C2H6. Although pure ethane is usually described as colourless and almost odourless, when such small hydrocarbons are detected at higher concentrations, they can be perceived as having a faintly sweet or gasoline like character. Examinations often use this traditional description to distinguish ethane from substances associated with sour or salty tastes. Given Data / Assumptions: The compound in question is ethane gas, a small saturated hydrocarbon. The question describes ethane as having a slightly characteristic taste or odour. Possible descriptive words include sweet, bitter, sour, salty and pungent. We are using conventional qualitative descriptions used in basic chemistry texts, not encouraging tasting of chemicals in practice. Concept / Approach: In everyday chemistry language, many low molecular mass hydrocarbons and related organic solvents are described as having a mild, sweetish smell or taste when compared with strongly bitter or sour compounds. Ethane is used as a fuel gas and is often present in natural gas mixtures. While it is not advisable to taste laboratory gases, the traditional description is that ethane has a faintly sweet character rather than a bitter, sour or salty one. This convention is reflected in many objective type questions in chemistry examinations. Step-by-Step Solution: 1) Recognise that ethane is a simple alkane consisting only of carbon and hydrogen atoms.2) Many alkanes are colourless gases at room temperature and are described as having mild or faintly sweet odours, especially when compared to sharp or irritating gases like ammonia or chlorine.3) Bitter or sour characteristics are usually associated with certain ionic compounds, acids or specific organic substances, not with neutral hydrocarbons like ethane.4) Salty taste is characteristic of many ionic solids in solution, particularly sodium chloride, and does not apply to a non polar gas like ethane.5) Pungent odours are more consistent with gases such as ammonia, sulphur dioxide or chlorine, which are very different chemically from ethane.6) Therefore, the most suitable word among the given options to describe the slight character of ethane is sweet. Verification / Alternative check: Descriptive tables of common gases often summarise ethane as colourless and nearly odourless, sometimes noting a faintly sweet smell at higher concentrations, similar to other light alkanes. While modern safety practice forbids tasting chemicals, historic descriptions still use the term sweet for hydrocarbons such as ethane, propane and butane, in contrast to the clearly sour taste of acids or the salty taste of ionic solutions. These references support choosing sweet rather than any of the other adjectives listed in the options. Why Other Options Are Wrong: Bitter: Bitter tastes are more typical of certain alkaloids or specific organic molecules, not simple neutral hydrocarbons like ethane. Sour: Sour taste is characteristic of acids that produce hydrogen ions in solution, and ethane is a non acidic hydrocarbon. Salty: Salty taste comes from ionic compounds in water, especially sodium and potassium salts, which is not relevant for gaseous ethane. Pungent: Strongly irritating gases such as ammonia or sulphur dioxide are labelled pungent, whereas ethane is relatively mild in odour. Common Pitfalls: Because modern laboratory practice discourages tasting or smelling chemicals directly, many learners are unfamiliar with these traditional qualitative descriptions and may guess randomly. A practical approach for exam questions is to associate sour with acids, salty with ionic salt solutions, pungent with strongly irritating gases and sweet with many volatile organic liquids and gases. Using this pattern, ethane is most reasonably described as slightly sweet in character among the given options. Final Answer: Ethane gas is described as having a slightly sweet character in its taste and odour in traditional chemistry descriptions.

Question 18

In biochemistry, lecithin (often found in egg yolk and cell membranes) is best classified as which type of lipid based compound?

Accepted Answer

Introduction / Context: This question belongs to biochemistry and general chemistry. Lecithin is a common term encountered in nutrition, biology and food technology, and it is an important component of biological membranes. The question asks you to identify the correct class of compound to which lecithin belongs, reinforcing knowledge of different lipid categories such as phospholipids, sterols, glycolipids and lipoproteins. Given Data / Assumptions: The compound is lecithin, sometimes known as phosphatidylcholine. Lecithin is found in egg yolk, soybeans and cell membranes. We need to identify whether lecithin is a sterol, phospholipid, lipoprotein, glycolipid or another type of compound. We assume basic familiarity with major classes of lipids. Concept / Approach: Lipids are a broad group that includes triglycerides, phospholipids, sterols, glycolipids and lipoprotein complexes. Phospholipids are characterised by a glycerol backbone esterified to two fatty acids and one phosphate group, which is often linked to an additional polar head group such as choline, ethanolamine or serine. Lecithin is the common name for phosphatidylcholine, in which the phosphate is bonded to choline. This structure clearly places lecithin in the phospholipid category, rather than among sterols or simple fats. Step-by-Step Solution: 1) Recall that lecithin is a major component of cell membranes and is often extracted from egg yolk or soy for use as an emulsifier.2) Examine the structure of lecithin: it has a glycerol backbone attached to two fatty acids and one phosphate group, with choline as the polar head.3) Molecules that have this pattern of two hydrophobic fatty acid chains and a hydrophilic phosphate head group are classified as phospholipids.4) Sterols, in contrast, are ring structured lipids such as cholesterol and do not have the typical glycerol plus phosphate plus two fatty acid structure of phospholipids.5) Lipoproteins are complexes of lipids and proteins that transport fats in the blood and are not single molecules like lecithin.6) Glycolipids are lipids with carbohydrate groups attached, mainly in nerve cell membranes, rather than the choline head group seen in lecithin.7) Therefore, lecithin fits squarely into the phospholipid category. Verification / Alternative check: Biochemistry textbooks describe lecithin explicitly as phosphatidylcholine, a major phospholipid in biological membranes. Its amphipathic nature, with a hydrophilic head and hydrophobic tails, contributes to membrane bilayer formation. Food technology sources also mention lecithin as a phospholipid emulsifier used in chocolate, baked goods and margarine. These consistent descriptions verify that the correct classification is phospholipid, not sterol, lipoprotein or glycolipid. Why Other Options Are Wrong: Sterol: Sterols such as cholesterol are four ring structures with a hydroxyl group and side chain, very different from glycerol based phospholipids like lecithin. Lipoprotein: Lipoproteins are large complexes of lipids and specific proteins that transport lipids; lecithin is one type of lipid, not an entire lipoprotein particle. Glycolipid: Glycolipids have sugar or carbohydrate groups attached to lipids, especially in nerve tissues, whereas lecithin has a choline head group rather than a sugar. Simple triglyceride: Simple fats or triglycerides are glycerol with three fatty acids and no phosphate group, so they are structurally distinct from lecithin. Common Pitfalls: Students sometimes confuse lecithin with cholesterol and incorrectly label it a sterol, because both are lipids associated with membranes. Others may pick lipoprotein because they have seen lecithin mentioned together with blood lipids. A good way to avoid this confusion is to remember that the term phosphatidylcholine directly signals a phospholipid (phosphatidyl) carrying a choline head group. When you see lecithin or phosphatidylcholine, think of a phospholipid component of the membrane bilayer. Final Answer: Lecithin is best classified as a phospholipid.

Question 19

In the crust of the Earth, which metal is the most abundant by weight, occurring mainly in the form of oxides and silicate minerals such as bauxite and feldspars?

Accepted Answer

Introduction / Context: This question belongs to geochemistry and general science. It asks about the most abundant metal in the crust of the Earth. Knowing which elements are most common in the crust helps in understanding mineral resources, industrial metallurgy and why certain metals are widely used in construction and manufacturing. Given Data / Assumptions: We are concerned with the composition of the Earth crust, not of the whole Earth or the core. The options include several common metals: iron, magnesium, calcium, aluminium and copper. We assume average crustal composition by weight as reported in standard geology and chemistry references. We focus on metals, not non metals like oxygen and silicon, which are also very abundant in the crust. Concept / Approach: The crust of the Earth is composed primarily of oxygen and silicon, but when we look specifically at metals, aluminium stands out as the most abundant by weight. Aluminium occurs mainly in the form of silicate minerals and in bauxite ore (hydrated aluminium oxides). Iron is another abundant metal, but in the crust it is less abundant than aluminium. Therefore, although iron dominates the core of the Earth, aluminium is the most abundant metal in the crust itself. Step-by-Step Solution: 1) Recall that oxygen and silicon are the most abundant elements in the Earth crust, forming silicate minerals.2) When considering metals only, common candidates include aluminium, iron, calcium, sodium, potassium and magnesium.3) Aluminium makes up a significant percentage of the crust by weight, often quoted around eight percent, mostly in the form of aluminosilicates and bauxite.4) Iron is also abundant but typically present in lower percentage than aluminium in the crust and is concentrated more in the core of the Earth.5) Calcium and magnesium are important constituents of rocks but are less abundant than aluminium when compared by total crustal content.6) Copper is present only in trace amounts compared with aluminium and iron.7) Therefore, among the metals listed, aluminium is the most abundant metal in the crust of the Earth. Verification / Alternative check: Standard tables of crustal abundance show that oxygen is first, followed by silicon, then aluminium as the third most abundant element overall and the most abundant metal. Iron usually appears slightly lower in percentage for the crust. Mining and metallurgical data also reflect the huge importance of aluminium ore deposits such as bauxite. These independent sources are consistent with the statement that aluminium is the most abundant metal in the Earth crust, confirming the correctness of the answer. Why Other Options Are Wrong: Iron: Very abundant and essential in geology and industry, but in the crust its abundance is lower than that of aluminium. Magnesium: An important component of many silicate minerals, especially in the mantle, but not as abundant as aluminium in the crust. Calcium: Significant in limestone and some silicates, but its overall crustal abundance is lower than aluminium. Copper: Economically important but geochemically much less abundant than aluminium, existing mainly in smaller ore deposits. Common Pitfalls: Students sometimes confuse the composition of the crust with that of the whole Earth. Because iron is abundant in the core, they may wrongly assume iron is the most abundant metal overall. It is helpful to distinguish clearly between crust (outer layer) and core (inner region). In the crust, aluminium dominates among metals, while iron dominates in the core. Remembering that bauxite and aluminosilicate minerals are widespread in rocks helps fix aluminium as the correct answer for crustal abundance questions. Final Answer: The most abundant metal in the crust of the Earth is aluminium.

Question 20

In experiments with discharge tubes, cathode rays (streams emitted from the cathode) are known to possess which of the following properties?

Accepted Answer

Introduction / Context: This question is about cathode rays, which were studied in early discharge tube experiments and were crucial in the discovery of the electron. Understanding whether cathode rays have mass, charge or both helps clarify the nature of electrons as fundamental particles of matter and charge, and explains why they are deflected by electric and magnetic fields. Given Data / Assumptions: Cathode rays are produced in a discharge tube when a high voltage is applied between electrodes in a low pressure gas. They travel from the cathode towards the anode. We are asked whether these rays have mass, charge, both or neither. We assume standard school level understanding that cathode rays consist of electrons. Concept / Approach: Cathode rays are streams of electrons. An electron is a subatomic particle with a definite mass (although very small compared with atoms) and a definite negative electric charge. Because electrons carry charge, cathode rays are deflected by electric and magnetic fields. Because they have mass, they can impart momentum, rotate light paddle wheels in the tube and produce mechanical effects when they strike objects. Therefore, cathode rays clearly have both mass and charge, not one without the other. Step-by-Step Solution: 1) Recall J. J. Thomson experiments which showed that cathode rays are deflected by electric and magnetic fields in a manner consistent with negatively charged particles.2) The direction of deflection indicates that these particles carry a negative charge.3) Thomson also measured the charge to mass ratio (e/m) of these particles, demonstrating that they have a finite mass as well as charge.4) In discharge tubes, cathode rays can turn a small paddle wheel placed in their path, showing that they transfer momentum and therefore have mass.5) At the same time, their deflection in electric and magnetic fields proves the presence of electric charge.6) These observations confirm that cathode rays consist of electrons, which possess both mass and electric charge. Verification / Alternative check: The modern model of the atom includes electrons as negatively charged particles with a rest mass of about 9.11 × 10^−31 kg and a charge of about −1.6 × 10^−19 coulomb. Cathode rays are nothing but streams of such electrons. If cathode rays had no mass, they would not convey mechanical effects to a paddle wheel; if they had no charge, they would not be deflected in fields. The fact that experiments show both deflection and mechanical effects supports the conclusion that they have both mass and charge. Why Other Options Are Wrong: Mass only but no electric charge: This would not explain the observed deflection in electric and magnetic fields. Electric charge only but no mass: Pure charge without mass would not provide mechanical effects such as moving a paddle wheel and would not have a measurable e/m ratio. Neither mass nor charge: This would correspond to neutral radiation like some forms of electromagnetic radiation, not to cathode rays. Only magnetic moment without mass: This is not an accepted description of cathode rays; electrons indeed have mass and charge, and any magnetic moment is secondary to these properties. Common Pitfalls: Students sometimes confuse cathode rays with electromagnetic radiation such as X rays, which have energy but no rest mass and no net charge. It is important to remember that cathode rays are material particles (electrons), not light waves. Another confusion is mixing up cathode rays with anode rays (positive rays). Focusing on the key experimental facts of deflection and mechanical impact helps reinforce that cathode rays have both mass and electric charge. Final Answer: Cathode rays possess both mass and electric charge, because they are streams of electrons.

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