Question 1

A chemical bond formed by the sharing of one or more pairs of electrons between atoms is known as which type of bond?

Accepted Answer

Introduction / Context: This question tests your understanding of basic chemical bonding. Different types of bonds, such as ionic, covalent, metallic and hydrogen bonds, arise from different ways in which atoms interact with electrons. The question focuses on the bond formed specifically by the sharing of electron pairs between atoms. Given Data / Assumptions: The bond in question is formed by sharing electrons between atoms. We assume neutral atoms initially that achieve stable configurations by sharing. Several possible bond types are listed: ionic, covalent, coordinate, metallic and hydrogen bonds. We use standard textbook definitions of each bond type. Concept / Approach: In covalent bonding, atoms share pairs of electrons so that each atom can attain a stable electron configuration, often an octet. This is different from ionic bonding, where electrons are transferred completely from one atom to another, forming ions. Coordinate bonds are a special case where both electrons in a shared pair come from one atom, but the bond is still fundamentally covalent. Metallic bonds involve a sea of delocalised electrons, and hydrogen bonds are weak intermolecular attractions. Therefore, the general term for a bond formed by sharing electron pairs is covalent bond. Step-by-Step Solution: 1) Recall that in covalent bonding, two atoms contribute one electron each to form a shared pair, resulting in a covalent bond.2) Consider common examples like H2, Cl2 and H2O, where atoms share electrons to complete their valence shells.3) In ionic bonding, one atom loses electrons and another gains electrons, resulting in electrostatic attraction between ions, not sharing.4) Coordinate bonding involves sharing as well, but both electrons in the shared pair originate from the same atom; still, the bond is covalent in nature.5) Metallic bonding involves many atoms sharing a pool of delocalised electrons throughout the metal lattice rather than localised pairs between two specific atoms.6) Hydrogen bonding is an intermolecular attraction between a hydrogen atom and a more electronegative atom, not a primary bond formed by sharing electrons.7) Therefore, the basic bond formed by sharing electron pairs between atoms is called a covalent bond. Verification / Alternative check: Lewis structures and valence bond theory both describe covalent bonds as shared electron pairs between specific atoms. For example, in methane (CH4), each C–H bond is a covalent bond formed by a shared pair of electrons. This concept is reinforced by many introductory chemistry texts, which define covalent bonding exactly as electron sharing between atoms. No other bond type fits the simple description in the stem as clearly as covalent bonding. Why Other Options Are Wrong: Ionic bond: Formed by complete transfer of electrons from one atom to another, resulting in ions held together by electrostatic forces, not by sharing pairs. Coordinate (dative) bond: A special case of covalent bond where the shared pair comes from one atom, but the general term the question seeks is covalent bond. Metallic bond: Involves a lattice of positive metal ions and a sea of delocalised electrons, rather than discrete shared pairs between two atoms. Hydrogen bond: A weak intermolecular attraction between an H atom bonded to a highly electronegative atom and a lone pair on another electronegative atom; not a primary sharing bond. Common Pitfalls: Students sometimes choose coordinate bond because it is also described using shared electrons, but the question does not mention that the pair comes from the same atom. It simply says that electrons are shared between atoms, which is the general definition of a covalent bond. To avoid confusion, remember that coordinate bonds are a subtype of covalent bonds, and unless the question specifically highlights donation from one atom, the general term covalent bond is expected. Final Answer: A bond formed by sharing pairs of electrons between atoms is called a covalent bond.

Question 2

Which of the following nitrogen containing substances can act both as an oxidising agent and as a reducing agent, depending on the reaction conditions?

Accepted Answer

Introduction / Context: This question explores redox chemistry and the concept of a compound that can behave both as an oxidising agent and as a reducing agent. These so called intermediate oxidation state compounds can either gain or lose electrons depending on the reacting partner. We focus here on nitrogen oxy compounds and related substances. Given Data / Assumptions: The candidates are nitric acid (HNO3), ammonia (NH3), nitrous acid (HNO2), nitrogen dioxide (NO2) and dinitrogen gas (N2). We need to identify which substance can show both oxidising and reducing behaviour. We assume aqueous or typical laboratory conditions where these substances commonly react. Oxidising agents accept electrons, reducing agents donate electrons. Concept / Approach: Compounds of nitrogen exhibit many oxidation states, and those in intermediate states often can act both as oxidising and reducing agents. In nitrous acid, nitrogen has an oxidation number of +3, which lies between higher states like +5 in nitric acid and lower states like +1 in nitrous oxide or -3 in ammonia. Because of this intermediate position, HNO2 can be oxidised to nitrate or reduced to products like NO. Therefore, it can either gain electrons (behaving as an oxidising agent) or lose electrons (behaving as a reducing agent) depending on the reaction partner. Step-by-Step Solution: 1) Determine approximate oxidation states: in nitric acid (HNO3), nitrogen is in +5; in nitrous acid (HNO2), nitrogen is in +3; in ammonia (NH3), nitrogen is in -3; in nitrogen dioxide (NO2), nitrogen is around +4; in dinitrogen gas (N2), nitrogen is in 0.2) A species at a very high oxidation state like +5 in HNO3 usually acts as an oxidising agent and tends not to behave as a reducing agent in ordinary reactions.3) A species at a very low oxidation state like -3 in NH3 typically acts as a reducing agent and rarely as an oxidising agent.4) Nitrous acid with nitrogen at +3 can be oxidised to nitrate (nitrogen at +5) or reduced to NO or other lower oxidation state species, so it can both accept and donate electrons.5) Therefore, nitrous acid behaves as both an oxidising and a reducing agent in different chemical reactions.6) Nitrogen dioxide (NO2) can undergo disproportionation, but in typical introductory chemistry and exam questions, nitrous acid is more commonly highlighted for dual behaviour. Verification / Alternative check: Example reactions demonstrate this dual behaviour. As an oxidising agent, HNO2 can oxidise iodide ions to iodine while itself being reduced. As a reducing agent, it can be oxidised to nitrate in the presence of strong oxidisers such as KMnO4. Textbooks often explicitly state that nitrous acid is capable of acting either as an oxidising agent or as a reducing agent, confirming that it has amphoteric redox behaviour typical of an intermediate oxidation state compound. Why Other Options Are Wrong: Nitric acid (HNO3): Nitrogen is in the highest common oxidation state of +5 and mainly acts as a powerful oxidising agent, not as a reducing agent. Ammonia (NH3): Nitrogen is in a very low oxidation state of -3, so ammonia usually acts as a reducing agent and is not used as an oxidising agent. Nitrogen dioxide (NO2): While it can participate in complex redox chemistry, standard exam answers emphasise nitrous acid as the classical example of a species acting as both oxidising and reducing agent. Dinitrogen gas (N2): Molecular nitrogen is very stable and relatively inert under normal conditions and is not commonly described as both oxidising and reducing in basic chemistry questions. Common Pitfalls: Students sometimes select nitric acid simply because they know it is a strong oxidising agent, forgetting that the question asks for a substance that also shows reducing behaviour. Others may be unsure and pick ammonia, knowing it is reducing, but they overlook that it does not act as an oxidiser. A good strategy is to check the oxidation state of the central atom: intermediate values often indicate potential for both oxidation and reduction. Remember that nitrogen in nitrous acid (oxidation state +3) can move either up to +5 or down to lower states, making HNO2 a classic example of a compound that acts both as oxidising and reducing agent. Final Answer: The nitrogen compound that can act as both an oxidising and a reducing agent is nitrous acid (HNO2).

Question 3

An aqueous solution that turns blue litmus paper red is classified as which type of substance?

Accepted Answer

Introduction / Context: This simple chemistry question concerns acid base indicators, specifically litmus paper. Blue and red litmus papers are common tools used in laboratories and classrooms to identify whether a solution is acidic or basic. Understanding these colour changes is a foundation for more advanced acid base concepts. Given Data / Assumptions: We are told that blue litmus paper turns red when dipped into a solution. We need to classify the solution as acidic, basic, alkaline, neutral or something else. We assume that the solution is aqueous and that litmus behaves in its usual way. Alkali refers to a soluble base, usually a hydroxide of an alkali metal or alkaline earth metal. Concept / Approach: Litmus is a natural dye used as an indicator. In acidic solutions, blue litmus turns red, and in basic or alkaline solutions, red litmus turns blue. Neutral solutions usually do not change the colour of litmus paper. Therefore, if blue litmus paper becomes red upon immersion, the solution must be acidic in nature. This rule is widely used as a quick and qualitative test of acidity or basicity in introductory chemistry. Step-by-Step Solution: 1) Recall the basic rule: acids turn blue litmus red, and bases turn red litmus blue.2) The question states that blue litmus paper is converted into red when placed in the solution.3) This colour change matches the behaviour of an acidic solution, not of a base or an alkali.4) Neutral salt solutions such as sodium chloride solution typically leave litmus unchanged, so they do not fit the described behaviour.5) Organic solvents like alcohols often have little effect on litmus in the absence of water and are not classified simply as acid or base on this test.6) Therefore, the solution that turns blue litmus red must be an acid. Verification / Alternative check: In basic laboratory practice, tests with both red and blue litmus papers are often used. An unknown acidic solution will turn blue litmus red, but leaves red litmus unchanged. Conversely, a basic solution will turn red litmus blue, but leave blue litmus unchanged. Neutral solutions leave both colours unchanged. Since only the behaviour of blue litmus turning red is specified, this clearly corresponds to an acidic solution according to standard indicator rules. Why Other Options Are Wrong: Base: A base would cause the opposite effect, turning red litmus paper blue rather than blue litmus red. Alkali: An alkali is a soluble base, so it shares the same indicator behaviour as bases and would not turn blue litmus red. Neutral salt solution: Neutral solutions do not change the colour of either blue or red litmus paper in a noticeable way. Organic solvent: Many organic solvents do not affect litmus strongly and are not simply categorised as acids or bases based on a litmus test alone. Common Pitfalls: Students sometimes remember only part of the litmus rules and confuse which colour change indicates acid or base. A helpful memory aid is the phrase "acids are red," reminding you that acidic solutions will ultimately cause red colour on litmus. Another pitfall is assuming that terms base and alkali mean different indicator behaviour, but in practice both bases and alkalis turn red litmus blue. Paying attention to which litmus colour is changing and in which direction is key to answering such questions correctly. Final Answer: A solution that turns blue litmus paper red is an acid.

Question 4

In radioactivity, the group displacement law describing how atomic number and mass number change during alpha and beta decay was proposed by which scientists?

Accepted Answer

Introduction / Context: This question involves the group displacement law in radioactivity. The law explains how the position of an element in the periodic table changes when it undergoes alpha or beta decay. It was an important step in linking nuclear transformations to chemical periodicity and helped chemists correctly identify newly formed radioactive elements. Given Data / Assumptions: The subject is the group displacement law in radioactive decay. We are asked which scientists proposed this law. Options include well known figures like Becquerel, Rutherford, Mendeleev, and the team of Frederick Soddy and Kasimir Fajans. We assume knowledge of basic radioactive processes: alpha decay and beta decay. Concept / Approach: The group displacement law states that when an atom emits an alpha particle, its atomic number decreases by 2 and its mass number decreases by 4, so it moves two places to the left in the periodic table. When an atom emits a beta particle (electron), its atomic number increases by 1 while its mass number remains unchanged, so it moves one place to the right in the periodic table. Frederick Soddy and Kasimir Fajans independently studied radioactive series and proposed this law, explaining how radioactive decay shifts elements between groups in the periodic table. Step-by-Step Solution: 1) Identify that the group displacement law specifically connects nuclear decay (alpha and beta emission) with changes in atomic number and position in the periodic table.2) Recall that Henri Becquerel discovered radioactivity, but did not formulate the displacement law.3) Dmitri Mendeleev created the periodic table and predicted unknown elements, but his work predates the discovery of radioactivity.4) Ernest Rutherford investigated nuclear structure and alpha scattering but is not primarily associated with the group displacement law.5) Frederick Soddy and Kasimir Fajans independently analysed decay series and formulated the group displacement law summarising how alpha and beta emissions shift elements in the periodic table.6) Therefore, the law is correctly attributed to Soddy and Fajans. Verification / Alternative check: Textbooks on nuclear chemistry summarise the law as follows: emission of an alpha particle (helium nucleus) reduces atomic number by 2 and mass number by 4; emission of a beta particle increases atomic number by 1 without changing mass number. These predictable shifts in Z explain how elements in a radioactive series move across groups. The name Soddy Fajans displacement law appears in many references, confirming that these two scientists are credited with this formulation rather than Becquerel, Rutherford or Mendeleev. Why Other Options Are Wrong: Henri Becquerel: Discovered radioactivity in uranium salts but did not establish the group displacement law. Ernest Rutherford: Known for the nuclear model of the atom and identification of alpha particles, not for the specific group law linking decay to periodic position. Dmitri Mendeleev: Developed the periodic table, but his work came before the discovery of radioactivity. James Chadwick and Niels Bohr: Chadwick discovered the neutron and Bohr formulated a model of the hydrogen atom; neither is associated with the group displacement law. Common Pitfalls: Students sometimes choose Rutherford or Becquerel because they strongly associate those names with radioactivity in general. However, the key is to link the specific term group displacement law with the names Soddy and Fajans. A useful memory tip is that Soddy worked extensively with radioactive series, and Fajans is known for both the displacement law and Fajans rules in bonding. Remembering these associations helps you avoid confusion when similar names appear together in nuclear chemistry questions. Final Answer: The group displacement law in radioactivity was proposed by Frederick Soddy and Kasimir Fajans.

Question 5

In a nuclear reactor, which material is commonly used as a moderator to slow down fast neutrons so that they can efficiently cause further fission?

Accepted Answer

Introduction / Context: This question relates to nuclear reactor design and the role of moderators. In many types of reactors, neutrons produced in fission are fast and must be slowed down to thermal energies to efficiently cause further fission in fuel such as uranium 235. The substance used to slow down these neutrons without capturing too many of them is called a moderator. Given Data / Assumptions: We are asked which material slows down neutrons in a nuclear reactor. Options include heavy water, ordinary water, zinc rods, molten caustic soda and liquid sodium metal. We are considering typical commercial and research reactors discussed in general science courses. A good moderator should slow neutrons effectively while having low neutron absorption. Concept / Approach: Moderators reduce the kinetic energy of fast neutrons through elastic scattering collisions without capturing too many of them. Light nuclei such as hydrogen and deuterium are effective for this purpose. Ordinary water (H2O) can act as a moderator, but it absorbs more neutrons than heavy water. Heavy water (D2O), which contains deuterium instead of hydrogen, is an excellent moderator with a very low neutron absorption cross section. For this reason, some reactor types, especially heavy water reactors, use D2O as both coolant and moderator. Step-by-Step Solution: 1) Identify that the question is about slowing neutrons, which is the task of a moderator, not of a coolant or control rod alone.2) Zinc rods are metals and are not used as standard moderators; they can capture neutrons and are not efficient for slowing down fast neutrons.3) Molten caustic soda is a corrosive medium and is not a typical moderator in reactor designs.4) Liquid sodium metal is used as a coolant in some fast breeder reactors because it transfers heat efficiently but does not serve as a neutron moderator in thermal reactors.5) Ordinary water can act as a moderator in light water reactors, but the textbook answer that appears most often in general science questions is heavy water, particularly in the context of heavy water reactors.6) Therefore, heavy water (D2O) is the correct choice in this question as the moderator that slows down neutrons. Verification / Alternative check: Descriptions of pressurised heavy water reactors (PHWRs) used in several countries state clearly that heavy water acts as a moderator to thermalise neutrons, allowing natural uranium fuel to sustain a chain reaction. While some reactors use ordinary water as both coolant and moderator, heavy water is unique because it slows down neutrons efficiently while absorbing relatively few, making it ideal when natural or low enriched fuel is used. This widely discussed role of heavy water confirms it as the expected answer. Why Other Options Are Wrong: Ordinary water (H2O): It can be a moderator in light water reactors, but the classic exam answer for this question is heavy water, especially in the context of moderators highlighted in school level textbooks. Zinc rods: These are not used as moderators; they would absorb neutrons and are not suited to slow down neutrons efficiently. Molten caustic soda: Highly corrosive and not used as a reactor moderator. Liquid sodium metal: Used primarily as a coolant in some fast reactors, not as a thermal neutron moderator. Common Pitfalls: Students sometimes pick ordinary water because they remember that water plays an important role in reactors. It is true that light water acts as a moderator in many designs, but heavy water is often emphasised in exam syllabi as the specialised moderator used in certain reactor types, especially for natural uranium fuel. To avoid confusion, remember that both H2O and D2O can moderate, but heavy water is the classic, low absorption moderator often highlighted in general knowledge questions. Final Answer: In many nuclear reactors, especially heavy water reactors, the speed of neutrons is slowed down by heavy water (D2O) acting as a moderator.

Question 6

In atmospheric environmental science, the term “ozone hole” refers specifically to which phenomenon in the Earth's stratosphere?

Accepted Answer

Introduction / Context: This question belongs to environmental chemistry and climate science. It asks what scientists mean by the term “ozone hole.” Understanding this term is important for discussions about ultraviolet (UV) radiation, global environmental protection policies and the role of chlorofluorocarbons (CFCs) in ozone depletion. Given Data / Assumptions: The term in focus is “ozone hole.” We are dealing with the Earth atmosphere, particularly the ozone layer. Options suggest different interpretations involving the stratosphere and troposphere. We assume the standard scientific meaning used in environmental reports and textbooks. Concept / Approach: The ozone layer is a region in the stratosphere, roughly 10 to 50 kilometres above the Earth surface, where ozone (O3) concentration is relatively high and provides protection by absorbing harmful UV radiation. The “ozone hole” is not a literal hole but a region where stratospheric ozone concentration drops dramatically below normal levels, especially over the Antarctic in spring. This thinning of the ozone layer results in increased UV radiation reaching the surface and is mainly caused by chemical reactions involving CFCs and other halogen compounds. Step-by-Step Solution: 1) Recognise that the ozone layer is located in the stratosphere, not in the troposphere.2) The term “ozone hole” refers to a significant depletion or thinning of this layer, not an increase in thickness.3) While ozone can also be present in the troposphere as a pollutant, the “ozone hole” problem is specifically about stratospheric ozone.4) The word “hole” is a metaphor used to describe areas where the ozone concentration has become much lower than the long term average, often by more than half.5) Therefore, the most accurate description among the options is the decrease in the thickness or concentration of the ozone layer in the stratosphere. Verification / Alternative check: Reports from organisations such as the World Meteorological Organization and the United Nations Environment Programme describe the Antarctic ozone hole as a region of exceptionally low stratospheric ozone concentration. Satellite and balloon measurements show that ozone levels there can drop to a fraction of normal values during certain seasons. The term does not imply a complete absence of ozone or a literal physical hole but is used to stress the severity of depletion. This description matches option stating a decrease in the thickness of the ozone layer in the stratosphere. Why Other Options Are Wrong: Increase in thickness of the ozone layer: This would be the opposite of the problem; it would mean more protection from UV, not less. Decrease in the ozone layer in the troposphere: Tropospheric ozone is a separate issue related to air pollution and smog, not the protective ozone layer high above the Earth. A literal physical hole in the ozone layer: The term “hole” is metaphorical; the layer is thinned, not physically torn open. Complete disappearance of ozone everywhere on Earth: Even in severe depletion, some ozone remains; the problem is reduction in concentration, not total loss worldwide. Common Pitfalls: Students sometimes imagine the ozone hole as an actual hole in the sky, which leads to confusion. It is helpful to think instead of a thinning blanket: the ozone layer is like a protective blanket that has become much thinner in certain regions. Another common mistake is mixing up tropospheric and stratospheric ozone. Tropospheric ozone is a pollutant at ground level, while stratospheric ozone is beneficial. The ozone hole specifically refers to depletion of stratospheric ozone over polar regions. Final Answer: The term “ozone hole” refers to a decrease in the thickness of the ozone layer in the stratosphere, especially over polar regions.

Question 7

In cell biology and physical chemistry, is osmosis classified as an active transport process or a passive transport process across a semipermeable membrane?

Accepted Answer

Introduction / Context: This question connects basic physical chemistry with biology. Osmosis is a fundamental process in cells, plant physiology and many industrial applications such as water purification. The question asks whether osmosis is an active or passive process, which relates to whether it requires metabolic energy such as ATP to occur. Given Data / Assumptions: We are dealing with osmosis: the movement of solvent molecules, usually water, through a semipermeable membrane. We want to classify it as active or passive transport. Active transport requires direct energy input from the cell, often in the form of ATP. Passive transport occurs spontaneously down a concentration or potential gradient and does not require metabolic energy. Concept / Approach: Osmosis is defined as the movement of solvent molecules from a region of lower solute concentration (higher solvent potential) to a region of higher solute concentration (lower solvent potential) through a semipermeable membrane. This flow is driven by differences in chemical potential or osmotic pressure and does not require energy expenditure by the membrane itself. Because water moves down its own free energy gradient and no ATP is directly consumed for the movement, osmosis is classified as a passive transport process. Step-by-Step Solution: 1) Recall the definition of passive transport: movement of particles or solvent down a concentration or electrochemical gradient without direct energy input from the cell.2) Recall the definition of active transport: movement of substances against their concentration or electrochemical gradient with energy input, typically using ATP driven pumps.3) In osmosis, water molecules move from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution).4) This movement occurs spontaneously due to the gradient in solvent chemical potential, without the need for cellular energy or ATP hydrolysis.5) Therefore, osmosis fits the definition of passive transport and not active transport.6) Although living cells can control osmotic balance using active transport of solutes, the actual flow of water across the membrane down its water potential gradient remains a passive process. Verification / Alternative check: In plant cells, osmosis causes water to enter roots and move into cells, generating turgor pressure. Cells may use active transport to move ions in or out to adjust internal solute concentration, but the subsequent water flow is still an automatic response to the new gradient. Similarly, in dialysis or reverse osmosis systems, external pressure or concentration differences drive water movement across membranes, not ATP. Textbooks consistently classify osmosis under passive transport mechanisms, alongside simple diffusion and facilitated diffusion. Why Other Options Are Wrong: Purely active transport process: Osmosis does not inherently require ATP or direct metabolic energy; it is driven by concentration and pressure differences. Both active and passive, depending on ATP availability: While cells may use active transport of solutes to create osmotic gradients, the water movement itself is always passive. Neither active nor passive transport: Osmosis clearly fits the definition of passive transport, so this option is incorrect. Only occurs during facilitated diffusion through protein pumps: Osmosis can occur through simple diffusion of water molecules through the membrane or aquaporins, which are channels, not ATP driven pumps. Common Pitfalls: Some students confuse the creation of osmotic gradients, which may involve active transport of ions, with the osmotic flow of water, which is passive. Another common misconception is that any process occurring across a biological membrane must use energy, but many vital processes like diffusion of gases and osmosis are passive. It is helpful to separate the movement of solutes (which can be active or passive) from the movement of water, which in osmosis always follows its potential gradient passively. Final Answer: Osmosis is classified as a purely passive transport process across a semipermeable membrane.

Question 8

Tetraethyl lead (TEL) has historically been used in the petroleum industry mainly as which of the following?

Accepted Answer

Introduction / Context: This question comes from applied chemistry and environmental science. It asks about the main historical use of tetraethyl lead (TEL), a once common but now largely banned compound in fuels. Knowing its purpose and hazards helps you understand why many countries phased out leaded petrol to protect public health and the environment. Given Data / Assumptions: The compound is tetraethyl lead, often abbreviated as TEL. We consider its primary historical industrial application. Options include uses as a fire extinguisher, gasoline additive, mosquito repellent, pain killer and food preservative. We assume modern awareness of TEL environmental and health impacts. Concept / Approach: Tetraethyl lead is an organolead compound that was added to gasoline to improve its octane rating, reducing engine knocking in internal combustion engines. It acts as an antiknock agent, allowing engines to run more smoothly and efficiently on lower cost fuel blends. However, combustion of leaded petrol released toxic lead compounds into the atmosphere, leading to serious health and environmental concerns. As a result, the use of TEL in motor fuels has been phased out in most parts of the world in favour of unleaded gasoline and alternative additives. Step-by-Step Solution: 1) Recognise tetraethyl lead as a compound historically associated with motor fuels and the term “leaded petrol.”2) Its primary function was to act as an antiknock additive, raising the octane number of gasoline and preventing premature ignition of the fuel air mixture.3) TEL was never used as a pain killer or in pharmaceuticals because of its toxicity.4) It was not used as a fire extinguisher chemical or as a household mosquito repellent since lead compounds are hazardous.5) TEL definitely was not a food preservative; lead contamination in food is dangerous and tightly regulated.6) Therefore, the correct description of its main historical use is as a gasoline additive to increase octane rating and reduce knocking. Verification / Alternative check: Historical accounts of petrol formulations describe “ethyl gasoline” or “leaded petrol” that contained small amounts of tetraethyl lead. These fuels were marketed as premium products that allowed high compression engines to run without knocking. Over time, evidence accumulated linking environmental lead levels and health issues, especially in children, to emissions from leaded petrol. This prompted a worldwide movement to phase out TEL as a fuel additive and switch to unleaded fuel, confirming that its primary use was indeed as a gasoline additive, not any of the other roles listed in the options. Why Other Options Are Wrong: Fire extinguisher chemical: Fire extinguishers typically use substances like carbon dioxide, dry chemical powders or halons, not organolead compounds. Mosquito repellent: TEL is highly toxic and would not be used in domestic insect control products. Pain killer: Pharmacological pain relievers are carefully tested; toxic metal based compounds like TEL are not used as analgesics. Food preservative: Lead compounds are poisonous; they are strictly excluded from food products. Common Pitfalls: Most students recognise tetraethyl lead from discussions of leaded petrol, but some may not recall the term “octane rating” or “antiknock agent.” A useful memory aid is that TEL allowed engines to operate with higher compression without knocking, which improved performance but at the cost of serious lead pollution. Remembering the link between TEL, leaded fuel, and environmental lead contamination helps you correctly identify its role as a gasoline additive in exam questions. Final Answer: Tetraethyl lead was historically used mainly as a gasoline additive to increase octane rating and reduce engine knocking.

Question 9

What is the correct IUPAC name of the alkyne with the condensed structural formula H3C–C≡C–H?

Accepted Answer

Introduction / Context: This organic chemistry question tests your ability to apply IUPAC nomenclature rules to a simple alkyne. The given condensed structural formula H3C–C≡C–H represents a linear chain of carbon atoms with a triple bond. Correctly counting the carbon atoms and numbering the chain allows you to select the correct IUPAC name. Given Data / Assumptions: The condensed formula is H3C–C≡C–H. We assume a straight chain with no branching. We must name the compound according to IUPAC rules for alkynes. Options include acetylene, propyne, propene, butan-1-ol and decanoic acid. Concept / Approach: Alkynes are hydrocarbons that contain at least one carbon carbon triple bond and are named by replacing the suffix “ane” with “yne” in the corresponding alkane name. First, count the total number of carbon atoms in the longest chain containing the triple bond. Then, number the chain from the end nearer to the triple bond to assign the lowest possible number to the triple bond position. For a three carbon chain with a triple bond starting at carbon 1, the IUPAC name is prop-1-yne, commonly shortened to propyne. Step-by-Step Solution: 1) Examine the formula H3C–C≡C–H and count the carbon atoms: CH3 (one carbon), then the central C in the triple bond and the terminal C≡C–H carbon, giving a total of 3 carbon atoms.2) Since there are three carbon atoms in a straight chain, the parent alkane name is propane.3) The presence of a carbon carbon triple bond means we replace “ane” with “yne,” so the base name becomes propyne.4) Number the carbon chain from the end nearer to the triple bond. If we start from the terminal –C≡C–H end, the triple bond starts at carbon 1.5) The detailed IUPAC name is therefore prop-1-yne, which can be written simply as propyne.6) Among the options, the correct IUPAC based choice is “Propyne (prop-1-yne).” Verification / Alternative check: Acetylene, also called ethyne, has the formula HC≡CH and contains only two carbon atoms. The given compound clearly has three carbons, so acetylene cannot be correct. Propene (prop-1-ene) has a double bond, not a triple bond. Butan-1-ol has four carbons and a hydroxyl group, and decanoic acid is a ten carbon carboxylic acid. None of these match the three carbon, triple bonded structure. Only propyne satisfies both the carbon count and the triple bond requirement, confirming that it is the correct IUPAC name. Why Other Options Are Wrong: Acetylene (ethyne): Contains two carbons and a triple bond (C2H2), not three carbons. Propene (prop-1-ene): Has three carbons but contains a double bond, not a triple bond. Butan-1-ol: Has a four carbon chain and an –OH group, so the functional group and carbon count do not match. Decanoic acid: A ten carbon carboxylic acid, completely different from a simple three carbon alkyne. Common Pitfalls: Some students see the triple bond and think of acetylene, but forget to count the carbon atoms. It is essential to count the carbon chain length before naming any hydrocarbon. Others may overlook the difference between a double bond (alkene) and triple bond (alkyne) and choose propene by mistake. Always identify the functional group (single, double or triple bond) and then apply the appropriate suffix and numbering for the chain. Final Answer: The correct IUPAC name of H3C–C≡C–H is propyne (prop-1-yne).

Question 10

In organic chemistry and materials science, long chain molecules made up of repeating carbon based units are collectively known as what?

Accepted Answer

Introduction / Context: This question tests your understanding of basic terminology related to large molecules in chemistry and biology. It asks what we call long chain molecules formed by repeating smaller carbon containing units. These substances are very important in plastics, synthetic fibres, rubber, proteins and many natural materials. Given Data / Assumptions: The focus is on long chain molecules composed of repeating units. The repeating units are carbon based, consistent with organic chemistry. Options include ribosomes, hydrocarbons, polymers, enzymes and none of these. We assume standard textbook definitions for each term. Concept / Approach: Polymers are substances made up of many repeating small units (monomers) linked together in long chains. The term “polymer” comes from Greek roots meaning “many parts.” Most polymers in organic chemistry are carbon based, including polyethylene, PVC, proteins, DNA and many synthetic materials. Hydrocarbons are compounds made only of hydrogen and carbon, which can be small or large, but the key concept of repeated units forming a chain is captured specifically by the term polymer. Ribosomes and enzymes are complex biological structures or proteins, not general names for long chain molecules. Step-by-Step Solution: 1) Recognise that the question highlights long chains of molecules built from repeating units, which is the defining feature of polymers.2) Polymers can be natural, like cellulose and proteins, or synthetic, like nylon and polyethylene, and they consist of many monomer units joined by covalent bonds.3) Hydrocarbons are simply compounds of hydrogen and carbon; small molecules like methane and ethane are also hydrocarbons but are not long chains of repeating units.4) Ribosomes are cellular organelles involved in protein synthesis; they are structures made from RNA and protein, not what the question is describing.5) Enzymes are proteins that act as catalysts in biochemical reactions, and while they are polymers of amino acids, the general term the question is asking for is more basic and broader: polymers.6) Therefore, the correct answer is polymers, the general name for long chain molecules formed by repeating units. Verification / Alternative check: Textbooks in polymer chemistry define a polymer as a very large molecule composed of many repeating subunits, called monomers. Examples include polystyrene, made from styrene monomers, and proteins, made from amino acid monomers. While many polymers are predominantly carbon based, the key feature in the question is the concept of “long chains” and “repeating units,” which fits the definition of polymers exactly. Hydrocarbons may form long chains but do not necessarily consist of repeated identical units, and the term hydrocarbon does not emphasise repetition the way polymer does. Why Other Options Are Wrong: Ribosomes: These are complex cellular organelles built from RNA and proteins that assemble amino acids into proteins; they are not long chain molecules of repeating monomers. Hydrocarbons: This term covers all compounds containing only hydrogen and carbon, including small molecules like methane; it is not restricted to long chain, repeating unit molecules. Enzymes: These are specific proteins that catalyse biochemical reactions; they are examples of biological polymers but not the general term the question is asking for. None of these: This would be correct only if none of the listed terms matched, but “polymers” clearly does. Common Pitfalls: Some students may be tempted to choose hydrocarbons because they associate long chains of carbon atoms with this word. However, hydrocarbons include both short and long molecules and do not imply repeating unit structures. Another trap is to focus on biological examples and select ribosomes or enzymes, but those refer to specific cellular components or proteins rather than the general category of long chain, repeating unit molecules. To avoid confusion, always link the idea of many repeating monomers with the term polymer. Final Answer: Long chain molecules made up of repeating carbon based units are called polymers.

Question 11

Which metal is generally used for making the filament of an electric incandescent light bulb because of its very high melting point?

Accepted Answer

Introduction / Context: This question relates to applied physics and materials science. It asks which metal is used to make the filament in an incandescent electric bulb. The choice of filament material is critical because it must withstand very high temperatures without melting or evaporating too quickly while emitting visible light efficiently. Given Data / Assumptions: The context is a traditional incandescent light bulb. We are choosing a metal suitable for its filament. Options include tungsten, silver, iron, rhodium and copper. We assume typical operating conditions where filament temperatures can exceed 2000 degrees Celsius. Concept / Approach: A filament in an incandescent bulb must have an extremely high melting point, good tensile strength at high temperature and relatively low vapour pressure to minimise evaporation. Tungsten (chemical symbol W) is well known for having one of the highest melting points among metals, around 3400 degrees Celsius. This makes it ideal for bulb filaments. Other metals like silver, copper and iron melt at much lower temperatures and would fail under normal bulb operating conditions. Rhodium has high melting point but is rare and expensive; in practice, tungsten is the standard choice. Step-by-Step Solution: 1) Consider the requirements: high melting point, mechanical strength at high temperature and low vapour pressure.2) Tungsten has a melting point around 3400 degrees Celsius, significantly higher than those of silver, copper or iron.3) Silver and copper are good electrical conductors but have much lower melting points and would quickly melt or evaporate as filaments at incandescent temperatures.4) Iron has a higher melting point than silver or copper but still not high enough for efficient long term filament use and would oxidise easily if exposed to air.5) Rhodium has good properties but is far more costly and rarely used for common commercial light bulb filaments.6) In practice and in textbooks, tungsten is identified as the standard filament material in incandescent bulbs. Verification / Alternative check: Descriptions of incandescent lamp design in physics and electrical engineering references state that tungsten filaments are coiled inside evacuated or inert gas filled glass bulbs. The high melting point of tungsten allows it to glow white hot without melting. Its relatively low vapour pressure reduces evaporation, extending filament life. These practical reasons are why tungsten, and not other metals like silver or copper, is universally associated with bulb filaments. Why Other Options Are Wrong: Silver (Ag): Excellent electrical conductor but has a much lower melting point and would deform or melt at filament operating temperatures. Iron (Fe): Melts at around 1538 degrees Celsius, lower than tungsten; it also oxidises easily and is not used in standard lamp filaments. Rhodium (Rh): Though it has a high melting point, its cost and other material constraints mean it is not generally used for mass produced bulb filaments. Copper (Cu): Commonly used in electrical wiring for its conductivity but not suitable for filaments due to its relatively low melting point and tendency to oxidise. Common Pitfalls: Students sometimes mix up good electrical conductivity with suitability for filaments, and may wrongly choose silver or copper. While conductivity is important, the key property for filaments is high temperature resistance. Another mistake is to assume that any rare metal like rhodium must be used, but in reality cost and practicality matter. A simple memory aid is that tungsten lamps are often called “tungsten filament lamps” in technical descriptions, making it easy to associate tungsten with bulb filaments. Final Answer: The metal generally used for making the filament of an incandescent bulb is tungsten (W).

Question 12

The triple point of water is the unique condition at which ice, liquid water and water vapour coexist in equilibrium. Which of the following pressure–temperature pairs corresponds to this triple point?

Accepted Answer

Introduction / Context: This thermodynamics question concerns the triple point of water. The triple point is an important reference in temperature scales and phase diagrams because it represents the unique combination of pressure and temperature at which solid, liquid and gaseous phases of a substance coexist in equilibrium. For water, this point is used in defining the Kelvin temperature scale. Given Data / Assumptions: The question is specifically about water and its triple point. Several pressure–temperature combinations are offered, including values at 1 atmosphere and 4.58 Torr. We assume standard phase diagram information for pure water. Pressure units include atmosphere (atm) and Torr (approximately equivalent to mm of mercury). Concept / Approach: The triple point of water is experimentally determined to occur at a temperature of 0.01 degrees Celsius (273.16 K) and at a pressure of about 611 pascals, which is approximately 4.58 mm of mercury or 4.58 Torr. This point is not at 1 atmosphere; normal melting and boiling points at 1 atm are different from the triple point. Therefore, options that quote 1 atm as the pressure cannot represent the triple point of water. Only the combination of about 4.58 Torr and 0.01 degrees Celsius matches the physical definition. Step-by-Step Solution: 1) Recall that the triple point temperature of water is approximately 0.01°C, not exactly 0°C.2) The triple point pressure is much lower than standard atmospheric pressure and is roughly 611 Pa, which corresponds to about 4.58 Torr.3) Examine each option: pairs that include 1 atm cannot be correct, because triple point is at low pressure.4) Option a (1 atm, 0°C) corresponds roughly to the normal freezing point of water, not the triple point.5) Option b (1 atm, 0.01°C) is also at 1 atm and is not the triple point; at 1 atm, water would exist mainly as solid or liquid depending on the exact conditions.6) Option d (4.58 Torr, 0.01°C) matches both the known triple point pressure and temperature of water.7) Therefore, the triple point of water corresponds to 4.58 Torr and 0.01°C. Verification / Alternative check: International definitions of the Kelvin scale historically referenced the triple point of water at exactly 273.16 K (0.01°C) and at a specific equilibrium vapour pressure of about 611 Pa over ice water mixture. Converting 611 Pa to Torr gives roughly 4.58 Torr. Phase diagrams of water mark this point where the solid liquid and vapour coexist. It is distinct from the normal melting point at 0°C and 1 atm, and from the normal boiling point at 100°C and 1 atm. These references confirm that the correct pair is 4.58 Torr and 0.01°C. Why Other Options Are Wrong: 1 atm and 0°C: Represents the normal freezing point of water but not the triple point; only solid and liquid phases are relevant there. 1 atm and 0.01°C: Still at 1 atm, water will not have all three phases in equilibrium at this point. 1 atm and 0 K: This is physically impossible for liquid water; 0 K is absolute zero where all thermal motion stops. 760 Torr and 100°C: This pair corresponds approximately to the normal boiling point of water at 1 atm, not the triple point. Common Pitfalls: Students often confuse the triple point with the normal melting point at 0°C and 1 atm because both involve solid and liquid water. However, the triple point also includes vapour, and occurs at a much lower pressure. Another mistake is to memorise 0°C without remembering the 0.01°C correction and the specific low pressure. A good strategy is to recall that triple point is a special calibration point at 0.01°C and about 4.58 Torr, not at 1 atmosphere. Final Answer: The triple point of water occurs at 4.58 Torr and 0.01°C, where ice, liquid water and water vapour coexist in equilibrium.

Question 13

Which of the following gases is NOT considered one of the primary greenhouse gases in the Earth's atmosphere?

Accepted Answer

Introduction / Context: This environmental chemistry question concerns greenhouse gases, which trap outgoing infrared radiation and contribute to the greenhouse effect and climate change. The question asks you to identify which listed gas is not one of the primary greenhouse gases in the Earth atmosphere. Given Data / Assumptions: We are considering the major greenhouse gases in the troposphere and lower atmosphere. Options include methane, ozone, nitrous oxide, hydrogen and carbon dioxide. We assume standard climate science definitions of greenhouse gases. Water vapour is another major greenhouse gas but is not listed as an option here. Concept / Approach: Primary greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), and water vapour, along with man made gases such as CFCs. These gases absorb infrared radiation and warm the atmosphere. Hydrogen gas (H2), on the other hand, does not significantly absorb infrared radiation in the way greenhouse gases do and is not classified as a primary greenhouse gas. Therefore, among the options, hydrogen is the one that does not belong to the list of major greenhouse gases. Step-by-Step Solution: 1) Identify known greenhouse gases: carbon dioxide, methane, nitrous oxide, ozone and water vapour are all well established greenhouse gases.2) Methane (CH4) is a potent greenhouse gas with a strong warming effect per molecule compared with CO2.3) Ozone (O3) in the lower atmosphere acts as a greenhouse gas by absorbing infrared radiation, even though stratospheric ozone plays a different protective role.4) Nitrous oxide (N2O) is a long lived greenhouse gas emitted by agricultural activities and combustion.5) Hydrogen (H2) is a light gas that does not have significant infrared absorbing bands in the atmospheric window and is not counted among primary greenhouse gases.6) Carbon dioxide (CO2), although not listed as one of the first three options, is also an important greenhouse gas and is definitely not the correct answer here.7) Therefore, the gas that is not a primary greenhouse gas among the options is hydrogen. Verification / Alternative check: Climate science sources routinely list the main greenhouse gases as water vapour, carbon dioxide, methane, nitrous oxide, ozone and some halogenated gases like CFCs. Hydrogen is not on this list. Hydrogen does have some indirect climate effects when it leaks into the atmosphere and affects chemical pathways, but it is not itself a primary greenhouse gas. In contrast, methane, ozone and nitrous oxide are explicitly cited as greenhouse gases, confirming that hydrogen stands out as the non greenhouse gas in the options given. Why Other Options Are Wrong: Methane (CH4): A strong greenhouse gas produced from natural sources and human activities such as livestock and fossil fuel extraction. Ozone (O3): In the troposphere, ozone contributes to greenhouse warming and air pollution, even though in the stratosphere it shields us from UV radiation. Nitrous oxide (N2O): A potent greenhouse gas with long atmospheric lifetime, often associated with agriculture and combustion processes. Carbon dioxide (CO2): The most discussed greenhouse gas because of its large anthropogenic emissions from burning fossil fuels and deforestation. Common Pitfalls: Some students may confuse importance with abundance and think that only very abundant gases like nitrogen and oxygen matter for climate. However, these major constituents do not absorb infrared radiation in the way greenhouse gases do. Another mistake is to think that any gas present in the atmosphere must be a greenhouse gas, which is not true. The key is whether the gas has vibrational modes that interact with infrared radiation. Remember the core group: CO2, H2O, CH4, N2O and O3, and you will quickly recognise that hydrogen is not among them. Final Answer: The gas that is NOT a primary greenhouse gas in the Earth atmosphere is hydrogen (H2).

Question 14

In basic materials science and metallurgy, how is an alloy best described in terms of its composition and structure?

Accepted Answer

Introduction / Context: An alloy is a very common concept in everyday life and in basic materials science. Steel, brass, bronze and stainless steel are all examples of alloys that appear in buildings, machines, tools and household items. Exams often test whether learners understand that an alloy is not a new chemical compound with a fixed formula but a mixture of metals, sometimes with small amounts of non metals, designed to improve hardness, strength, corrosion resistance or other properties. Given Data / Assumptions: The question asks for the correct description of an alloy. Options mention compound, allotropic form, isomer and mixture. We assume standard school level definitions of mixture, compound, allotrope and isomer. Familiar examples include brass (copper and zinc) and bronze (copper and tin). Concept / Approach: A compound has a fixed composition and can be represented by a definite chemical formula, such as NaCl or H2O. A mixture, in contrast, can have variable composition. Allotropes are different structural forms of the same element, such as diamond and graphite for carbon. Isomers are different arrangements of atoms within a single molecular formula, mainly in organic chemistry. Alloys are best understood as mixtures, usually of metals melted and mixed together, then solidified. The exact proportions can vary, and their properties change with composition, which fits the definition of a mixture rather than a compound. Step-by-Step Solution: Step 1: Consider brass, which is made by mixing copper and zinc in different proportions to obtain desired hardness and colour.Step 2: Notice that brass does not have a single fixed formula; the percentage of zinc can vary from one grade of brass to another.Step 3: This variable composition is a key sign that brass is a mixture, not a compound.Step 4: Recognise that in alloys, the constituent elements usually retain their metallic character, such as electrical conductivity and metallic bonding.Step 5: Compare to the formal definitions: compounds must have fixed ratios, allotropes involve only one element, and isomers refer to molecules with the same formula but different structures.Step 6: Conclude that an alloy is best described as a mixture of a metal with one or more other elements, often other metals. Verification / Alternative check: If alloys were compounds, each alloy name would correspond to a single formula. In reality, steel is described by ranges of carbon content, alloy steels are specified by ranges of chromium, nickel or molybdenum, and so on. These ranges show that composition can change continuously, which is a hallmark of mixtures. Phase diagrams in metallurgy further support this, showing partial solubility and mixed phases rather than a single fixed compound composition over the entire range. Why Other Options Are Wrong: Option a is wrong because fixed stoichiometric compounds like NaCl are not called alloys. Option b is incorrect because an allotrope involves only one element in different structural forms, not mixtures of elements. Option c is a concept from molecular chemistry and does not describe metallic mixtures. Option e is incorrect because a perfectly pure metal contains only one element, whereas alloys always involve more than one element. Common Pitfalls: A common mistake is to think that because alloys have uniform appearance they must be compounds. In fact, many alloys are homogeneous mixtures called solid solutions, but they still allow variable composition. Another error is to mix up terms like allotrope, isomer and alloy because they all start with similar letters. The key is to remember that an alloy always involves more than one element and that its composition can be adjusted to tune its properties. Final Answer: An alloy is best described as a mixture of a metal with one or more other elements, often another metal rather than a fixed chemical compound.

Question 15

A subatomic particle has four times the mass of a proton and twice its positive charge. Which particle fits this description most accurately?

Accepted Answer

Introduction / Context: Understanding the basic nuclear and subatomic particles is an important part of general physics and chemistry. Protons, neutrons, electrons, alpha particles and other nuclei all differ in mass and charge. Many multiple choice questions ask you to compare particles using simple ratios of mass and charge. In this problem, we are told that an unknown particle has four times the mass of a proton and twice its positive charge, and we must identify which named particle matches that description. Given Data / Assumptions: Reference particle: proton with charge +1 and mass approximately 1 atomic mass unit. Unknown particle: mass is four times proton mass and charge is +2 relative to the proton. Options include helium atom, alpha particle, deuteron and tritium nucleus, plus a positron. We assume approximate masses and charges as taught at school level. Concept / Approach: An alpha particle is the nucleus of a helium atom. It contains two protons and two neutrons. Therefore, its charge is +2 (due to the two protons) and its mass is approximately 4 atomic mass units (two protons plus two neutrons). A deuteron is the nucleus of heavy hydrogen with one proton and one neutron, so its mass is about 2 units and its charge is +1. A tritium nucleus has one proton and two neutrons, giving a mass of about 3 units and charge +1. A helium atom includes two electrons and has net charge zero. A positron has the same mass as an electron, far smaller than a proton, and charge +1. Only the alpha particle has both mass about 4 and charge +2. Step-by-Step Solution: Step 1: Note that the unknown particle must have charge +2, which eliminates neutral atoms and particles with charge +1 or 0.Step 2: A helium atom has two protons and two electrons, so its net charge is zero, not +2.Step 3: A deuteron has charge +1 and mass about 2, so it does not match either the charge or mass given.Step 4: A tritium nucleus has charge +1 and mass about 3, still not equal to four times the proton mass or double its charge.Step 5: A positron has the same small mass as an electron and therefore cannot have four times the proton mass.Step 6: An alpha particle (helium nucleus) has two protons and two neutrons, so its mass is approximately 4 and its charge is +2.Step 7: Conclude that the particle described must be an alpha particle. Verification / Alternative check: You can check by recalling nuclear notation. The alpha particle is written as 4 2 He, where the top number 4 is the mass number and the bottom number 2 is the atomic number, which equals the charge in units of the proton charge. The proton is 1 1 H. Comparing these shows directly that the alpha particle has four times the mass number and twice the charge of the proton, matching the description in the question perfectly. Why Other Options Are Wrong: Option a, helium atom, is electrically neutral because the two positive charges of the protons are balanced by two electrons. Option c, deuteron, has charge +1 and mass 2, not mass 4 and charge +2. Option d, tritium nucleus, has mass 3 and charge +1, again not matching the required ratios. Option e, positron, has mass much smaller than the proton and cannot be four times heavier. Only the alpha particle fits both the mass and charge conditions together. Common Pitfalls: A common mistake is to confuse the helium atom with the helium nucleus. Although they involve the same element, the neutral atom has electrons and therefore net charge zero, while the alpha particle is just the nucleus with a net positive charge. Another pitfall is to remember only the mass or only the charge and not both. The question clearly specifies two ratios, and the correct answer must satisfy both at the same time. Final Answer: The particle with four times the mass of a proton and twice its positive charge is an alpha particle, the helium nucleus.

Question 16

In basic atomic structure, the atomic weight (approximately the mass number) of an element is given by which combination of subatomic particles?

Accepted Answer

Introduction / Context: Atomic weight, often approximated by the mass number for a given isotope, is a key idea in basic chemistry. It appears in periodic table data, stoichiometry calculations and nuclear chemistry. An exam question like this checks whether you understand which subatomic particles contribute most to the mass of an atom and therefore determine its atomic weight. This understanding is essential for balancing equations, using moles and interpreting isotopic notation. Given Data / Assumptions: The question asks what atomic weight (approximately the mass number) is based on. We consider protons, neutrons and electrons as the basic subatomic particles. We assume electron mass is negligible compared to proton and neutron mass at school level. We focus on the sum of whole numbers of particles, as used when defining mass number. Concept / Approach: Protons and neutrons reside in the nucleus and each has a mass close to 1 atomic mass unit. Electrons are much lighter, with a mass around 1 over 1836 of a proton, and their contribution to total atomic mass is usually neglected in introductory chemistry. The mass number, written as a whole number above the element symbol, is defined as the sum of protons and neutrons. Atomic weight listed on the periodic table is a weighted average of these mass numbers across natural isotopes, but in simple questions it is often approximated by the mass number. Thus, the sum of protons and neutrons in the nucleus is what gives the atomic weight in this simplified sense. Step-by-Step Solution: Step 1: Recall that the mass number A is defined as A = Z + N, where Z is the number of protons and N is the number of neutrons.Step 2: Recognise that each proton and neutron contributes roughly 1 unit of mass, while electrons contribute very little.Step 3: Understand that atomic weight on the periodic table is close to the mass number for each isotope, averaged over natural abundance.Step 4: Compare the options and look for the combination that matches Z + N.Step 5: Option b states that atomic weight is given by the sum of neutrons and protons, which is exactly the definition of mass number.Step 6: Therefore, select the sum of neutrons and protons as the correct basis for atomic weight in this context. Verification / Alternative check: Take a familiar example like carbon 12. Its atomic number is 6, meaning 6 protons, and its mass number is 12, so it must have 6 neutrons. The sum of protons and neutrons is 6 + 6 = 12, which matches the mass number and approximate atomic weight. For oxygen 16, there are 8 protons and 8 neutrons, again giving 16 as the mass number. In both cases, adding protons and neutrons yields the mass number, confirming the concept used in the question. Why Other Options Are Wrong: Option a, sum of neutrons and electrons, ignores the mass of protons and includes electrons, which are very light. Option c, only neutrons, clearly cannot be correct because it would give zero mass number for hydrogen 1, which has one proton and no neutrons. Option d, only protons, would give the same number for all isotopes of an element, which is not true in reality. Option e, sum of protons and electrons, again ignores neutrons, which contribute significantly to mass, and wrongly counts electrons that contribute very little to atomic weight. Common Pitfalls: Students sometimes confuse atomic number with atomic weight. Atomic number counts only protons, while atomic weight and mass number count both protons and neutrons. Another pitfall is to assume that because electrons are part of the atom they must contribute significantly to atomic weight, which is not accurate at this level. To avoid these errors, always remember that nucleus mass comes from protons plus neutrons, and that is what approximates atomic weight. Final Answer: In basic chemistry, the atomic weight or mass number of an element is approximately the sum of protons and neutrons in its nucleus.

Question 17

If 24 grams of carbon are completely burnt in excess oxygen to form carbon dioxide (CO2), how many moles of CO2 are produced?

Accepted Answer

Introduction / Context: Stoichiometry questions that connect mass, moles and balanced equations are central to basic chemistry. This problem asks how many moles of carbon dioxide are produced when a known mass of carbon is completely burnt in oxygen. It tests your understanding of molar mass, mole calculations and the simple one to one mole ratio in the combustion of carbon to carbon dioxide. Mastering this type of calculation is essential for more advanced topics such as limiting reagents and yield. Given Data / Assumptions: Mass of carbon burned: 24 g. Carbon is completely burnt in excess oxygen, so oxygen is not limiting. The main reaction is C + O2 → CO2. The molar mass of carbon is approximately 12 g/mol. The question asks for moles of CO2 formed, not its mass. Concept / Approach: The balanced chemical equation for complete combustion of carbon is C + O2 → CO2. This equation shows that one mole of carbon reacts with one mole of oxygen to form one mole of carbon dioxide. Therefore, the mole ratio between carbon and carbon dioxide is 1:1. First, you calculate how many moles of carbon are present in 24 g. Then, using the 1:1 ratio, you conclude that the same number of moles of CO2 is produced. This method uses the basic formula moles = mass / molar mass. Step-by-Step Solution: Step 1: Write the balanced equation for complete combustion of carbon: C + O2 → CO2.Step 2: Determine the molar mass of carbon. The atomic mass of carbon is about 12 g/mol.Step 3: Calculate the number of moles of carbon in 24 g using n = mass / molar mass.Step 4: n(C) = 24 g / 12 g/mol = 2 mol of carbon.Step 5: From the balanced equation, 1 mol of carbon gives 1 mol of CO2, so the mole ratio C:CO2 is 1:1.Step 6: Therefore, 2 mol of carbon will produce 2 mol of CO2 when combustion is complete and oxygen is in excess.Step 7: Conclude that the number of moles of CO2 formed is 2 mol. Verification / Alternative check: As a quick check, you can consider a smaller amount. If 12 g of carbon (1 mol) produces 1 mol of CO2, then doubling the mass of carbon to 24 g should double the amount of CO2 formed. That gives 2 mol of CO2, which matches the calculated result. You could also compute the mass of CO2 produced: 2 mol of CO2 have a mass of about 2 * 44 = 88 g, consistent with mass balance when you consider the addition of oxygen mass from the air. Why Other Options Are Wrong: Option a, 0.5 mol, would correspond to only 6 g of carbon reacting, which is much less than the given 24 g. Option b, 1 mol, would be correct for 12 g of carbon, not for 24 g. Option d, 0.25 mol, is even smaller and does not match the given mass. Option e, 4 mol, would require 48 g of carbon, which is double the provided mass. None of these match the 1:1 stoichiometric relationship and the correct mole calculation for 24 g of carbon. Common Pitfalls: A frequent error is to forget to divide the mass by molar mass and instead treat grams as if they were moles. Another common mistake is to misinterpret the mole ratio, for example thinking that one mole of carbon produces two moles of CO2, which is not supported by the balanced equation. To avoid these, always start by writing the balanced equation, then carefully convert mass to moles, and then apply the mole ratio step by step. Final Answer: When 24 g of carbon is completely burnt in excess oxygen, it produces 2 mol of CO2.

Question 18

Chloroform (CHCl3) used as an anaesthetic must be stored in tightly stoppered dark bottles. Otherwise, what harmful change can occur on exposure to air and light?

Accepted Answer

Introduction / Context: Chloroform is a volatile organic compound that was historically used as an anaesthetic. However, it is chemically reactive when exposed to oxygen and light. Understanding why chloroform must be stored in dark, tightly stoppered bottles links basic organic chemistry with practical safety. Exam questions often ask why certain chemicals require special storage conditions and what dangerous products may form if they are not handled correctly. Given Data / Assumptions: The substance is chloroform, CHCl3. It is used as an anaesthetic and therefore must be safe when administered. The storage recommendation is to keep it in tightly closed, dark bottles. The question asks what happens if it is not protected from air and light. Concept / Approach: When chloroform is exposed to air and sunlight, it can undergo a slow oxidation reaction with oxygen. This reaction produces phosgene, COCl2, a very toxic gas that was used as a chemical warfare agent. Phosgene can severely damage lungs if inhaled. To minimise this risk, chloroform is stored in dark containers, sometimes with a small amount of ethanol to inhibit phosgene formation. The key point is that the danger is not simple evaporation or harmless colour change, but chemical oxidation to a poisonous compound. Step-by-Step Solution: Step 1: Recall the formula of chloroform, CHCl3, and its use as a solvent and anaesthetic.Step 2: Remember that in the presence of oxygen and sunlight, chloroform can react to form new products.Step 3: One of the main products of this oxidation is phosgene, with the formula COCl2, which is highly toxic.Step 4: Phosgene is a gas that can cause severe lung damage and is unfit for any medical use.Step 5: To prevent this, chloroform is stored in dark, well stoppered bottles so that air and light do not easily reach it.Step 6: Therefore, the correct description of what happens if chloroform is not protected is that it gets oxidised to phosgene. Verification / Alternative check: Safety guidelines for handling chloroform mention the risk of phosgene formation and recommend storing it in amber glass bottles, away from sunlight. Some formulations include ethanol as a stabiliser because ethanol can react with phosgene and reduce its concentration. These details confirm that oxidation to phosgene is a recognised hazard and that the storage advice is designed to prevent exactly this reaction. Why Other Options Are Wrong: Option a, that chloroform simply becomes coloured, is incomplete and misleading because it omits the formation of toxic phosgene. Option b describes only evaporation, which does occur but is not the main danger described in textbooks. Option d suggests decomposition to chloropicrin, which is not the standard reaction of chloroform under these conditions. Option e, polymerisation to a harmless solid, has no basis in the usual chemistry of chloroform. Thus, these options do not match the known oxidation to phosgene. Common Pitfalls: Students may wrongly assume that all volatile organic solvents behave similarly and only worry about flammability or evaporation. In this case, the main risk is chemical transformation into a very toxic gas. Another pitfall is confusing different chlorinated compounds and their hazards. To avoid these mistakes, it is helpful to memorise that chloroform can produce phosgene on exposure to air and light, which is why dark, tightly stoppered bottles and stabilisers are used. Final Answer: If chloroform is not stored properly, it can get oxidised to toxic phosgene (COCl2) in the presence of air and light, which is why it is preserved in dark, tightly closed bottles.

Question 19

In the industrial process of vulcanisation, which element is mainly used to cross link and strengthen natural rubber?

Accepted Answer

Introduction / Context: Natural rubber by itself is soft, sticky and loses its useful properties over a wide temperature range. Charles Goodyear discovered that heating rubber with certain chemicals can greatly improve its strength, elasticity and resistance to heat and wear. This process is called vulcanisation. A common general knowledge and chemistry question asks which element is primarily responsible for this transformation. Given Data / Assumptions: The process mentioned is vulcanisation of rubber. The options include nickel, copper, sulphur, graphite and chlorine. We assume ordinary natural rubber, which is mainly polyisoprene. The question focuses on the main element used, not minor additives. Concept / Approach: Vulcanisation involves forming cross links between individual polyisoprene chains in natural rubber. These cross links limit the movement of the chains relative to each other and greatly improve mechanical properties. Sulphur atoms form bridges, often called sulphur cross links, between the double bonds or adjacent positions on the polymer chains. Metals like nickel and copper, carbon in the form of graphite and halogens like chlorine do not play this primary cross linking role in classical vulcanisation of rubber. Step-by-Step Solution: Step 1: Recall that natural rubber is mainly cis 1,4 polyisoprene, which has carbon carbon double bonds in the polymer chain.Step 2: In vulcanisation, rubber is heated with sulphur or sulphur containing compounds.Step 3: Sulphur atoms form cross links between polymer chains, turning a soft and sticky material into a tougher, more elastic network.Step 4: This cross linked rubber has improved resistance to heat, abrasion and deformation, which is why it is used in tyres.Step 5: Compare this role with that of nickel, copper, graphite and chlorine, none of which are standard cross linking agents in the basic vulcanisation process.Step 6: Conclude that sulphur is the main element used in vulcanisation of rubber. Verification / Alternative check: Chemistry textbooks often define vulcanisation as heating natural rubber with about 3 to 5 percent sulphur. Advanced formulations may include accelerators and activators, but sulphur is always highlighted as the key element that forms cross links. Trade descriptions of tyres and rubber goods sometimes mention sulphur vulcanisation specifically, reinforcing this association. No similar usage is described for nickel, copper, graphite or chlorine in this context. Why Other Options Are Wrong: Option a, nickel, is a metal used in alloys and catalysts but not as the main vulcanising agent for rubber. Option b, copper, has many electrical and structural uses but is not the key element in vulcanisation. Option d, graphite, is a form of carbon used as a lubricant and in electrodes, not as a cross linking agent for rubber. Option e, chlorine, is used in making some synthetic rubbers and plastics but is not the principal element in classical vulcanisation of natural rubber. Common Pitfalls: Sometimes learners over generalise and think any additive in rubber processing could be the main vulcanising agent. Others may confuse the role of carbon black, which is added as a reinforcing filler, with the cross linking agent. To avoid this, remember that carbon black improves strength and wear resistance through reinforcement, while sulphur changes the chemical structure by forming cross links between chains. Final Answer: In the vulcanisation of rubber, the main element used to cross link and strengthen the polymer chains is sulphur.

Question 20

In nuclear and particle physics, which particle is essentially the same as an electron in mass and charge and is actually emitted from the nucleus during certain radioactive decays?

Accepted Answer

Introduction / Context: Several elementary and subatomic particles appear in discussions of radioactivity and nuclear physics. The electron is well known from atomic structure. There is also the beta particle, which arises in nuclear beta decay. Many exam questions ask which particle is similar to an electron or effectively is an electron originating from the nucleus rather than from the electron cloud. Understanding this helps you correctly interpret radioactive decay equations and the nature of emitted radiation. Given Data / Assumptions: The question asks for a particle similar to the electron. Options include positron, beta particle, photon, meson and neutrino. We assume similarity means same mass and same negative charge. We consider common radioactive emissions: alpha, beta and gamma. Concept / Approach: A beta minus particle, written as β−, is essentially an electron that is emitted from the nucleus when a neutron converts into a proton, electron and antineutrino. It has the same mass and negative charge as an ordinary electron. A positron is an antiparticle of the electron, with the same mass but positive charge, so it is similar in mass but opposite in charge. A photon is a quantum of electromagnetic radiation with no rest mass and no charge. Mesons are heavier particles composed of quarks, and neutrinos have extremely small mass and no charge. Thus, the particle most similar to an electron in both mass and charge is the beta minus particle. Step-by-Step Solution: Step 1: Identify the properties of an electron: negative charge and very small mass relative to protons and neutrons.Step 2: Consider a beta minus particle. It is defined as an electron emitted from the nucleus during beta decay and has the same charge and mass as an ordinary electron.Step 3: Compare this with a positron, which has the same mass but carries a positive charge and is therefore not identical to an electron.Step 4: Note that photons have no rest mass and no charge, so they are quite different from electrons.Step 5: Mesons are much more massive and participate in strong nuclear interactions.Step 6: Neutrinos have almost zero mass and no electrical charge, and they interact only weakly.Step 7: Conclude that the beta minus particle is the one that is essentially the same as an electron. Verification / Alternative check: Beta decay equations in nuclear chemistry are often written with a symbol e− or β− for the emitted particle, and tables of radiation types list beta radiation as a stream of high speed electrons (or positrons in beta plus decay). This confirms that in typical beta minus decay, the emitted particle is indeed an electron. The similarity in name and notation between electron and beta minus particle is deliberate because they are the same type of particle. Why Other Options Are Wrong: Option a, positron, has the same mass as an electron but positive charge, so it is the antimatter counterpart of the electron rather than being identical. Option c, photon, is a massless, neutral particle associated with electromagnetic waves. Option d, meson, refers to a group of heavier particles that do not resemble electrons in mass or charge. Option e, neutrino, has extremely small mass and no charge and interacts very weakly, again quite unlike an electron. Common Pitfalls: Students sometimes confuse positron and beta particle, or think that all beta radiation must involve positrons. In standard notation, beta minus radiation refers to electrons, while beta plus radiation refers to positrons. Another pitfall is to think that a particle emitted from the nucleus must be totally different from an electron, but in beta minus decay it is simply an electron generated inside the nucleus during the conversion of a neutron to a proton. Final Answer: The particle that is essentially the same as an electron in mass and charge and is emitted in beta minus decay is the beta particle (beta minus, β−).

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