Chapter 10 — Metals, nonmetals and pollution pathways
Recognise a pattern without treating it as a law
Many familiar metals are lustrous, malleable, ductile and good conductors of heat and electricity. Copper can be drawn into wire; aluminium can be formed into sheets. These examples establish a useful pattern, but a good exam answer must respect exceptions and conditions. Mercury is a metal that is liquid near ordinary room temperature. Some metals are comparatively soft, and graphite, a form of the nonmetal carbon, conducts electricity. “All nonmetals are poor electrical conductors” therefore fails as an absolute statement. A question asking for a typical property is different from one asking for an exception.
Metals tend to lose electrons and form positive ions in many common reactions; nonmetals often gain or share electrons. This is a broad chemical tendency, not a guarantee that every metal always appears as a simple ion in every compound. The solid metal contains mobile electrons in its metallic bonding model, which helps explain its electrical conduction. A salt solution conducts by moving ions, a different mechanism. If a stem asks which charged carriers move, identify the material's state before choosing electrons or ions.
Oxygen reacts with many metals to form oxides. Magnesium oxide is a basic oxide in the school-level classification. Some metal oxides are amphoteric: they can react with both acids and bases under appropriate conditions; aluminium oxide and zinc oxide are common textbook examples. Nonmetal oxides such as carbon dioxide can show acidic behaviour in water or react with a base. Yet “every metal oxide is basic and every nonmetal oxide is acidic” is too strong. Carbon monoxide, for instance, does not fit the familiar acidic-oxide example. Use the named substance and reaction, not the element category alone.
Reactivity and displacement
A reactivity series orders selected metals by their tendency to take part in certain reactions. In a simple salt-solution displacement example, a more reactive metal can displace a less reactive metal from a solution of its salt, subject to the relevant conditions. Zinc can displace copper from copper sulfate solution: Zn + CuSO₄ → ZnSO₄ + Cu. Copper cannot be used to displace zinc from zinc sulfate in the same introductory comparison. The direction follows their positions in the series. The word reactive is comparative here; it does not mean the more reactive metal must react visibly with every substance at every temperature.
Metals above hydrogen in a commonly taught series can often displace hydrogen from suitable dilute acids. This is a guide for exam equations, not a universal procedure: passivation, acid type, concentration and reaction conditions can change observations. Metals such as copper are below hydrogen in the standard introductory list and do not release hydrogen from ordinary dilute hydrochloric acid in that simple comparison. An oxidising acid presents different chemistry, so “copper never reacts with any acid” would be misleading. Select an answer matching the specified acid and conditions.
Extraction methods depend partly on reactivity. Very reactive metals are difficult to obtain by simply heating their common compounds with carbon; electrolysis of suitable molten compounds is a broad route. Less reactive metals may be extracted by chemical reduction under appropriate conditions. This is an organising idea, not a recipe. Mineral, ore and metal also differ: a mineral is a naturally occurring material with a characteristic composition; an ore is a mineral or rock from which a metal can be extracted economically under the stated context. Not every mineral occurrence is a useful ore.
Alloys combine elements, at least one of which is a metal, to obtain useful properties. Steel is based on iron with carbon and may contain other additions depending on grade. Brass is a copper–zinc alloy. These are not fixed single molecular formulas. The properties of an alloy depend on its composition and processing, so an MCQ that treats every steel as one exact composition is poorly phrased. At exam level, connect common alloy names to their main constituents, then remember that exact industrial grades vary.
Corrosion and prevention
Corrosion is a chemical or electrochemical deterioration of a material due to its environment. Rusting of iron typically involves oxygen and water; the rust formed is not a neat protective surface in ordinary exposure. Paint, oil and polymer coatings can act as barriers. Galvanising coats iron or steel with zinc, which has a protective role; it is not a silver coating. The choice of prevention depends on material and use. A scratched barrier coating may lose protection locally, while zinc can offer a different protective mechanism. Avoid treating every coating as identical.
Silver tarnish and copper's green surface products are examples of surface chemistry other than iron rust. A question that asks which metal is rusting should reserve that word for iron or iron-based materials in the usual school sense. Another common distinction is between a corrosion product and an ore. A rust layer formed during exposure does not become an ore just because it contains iron. Ore is a source material evaluated for extraction; corrosion is an environmental change to an object.
Follow a pollutant from source to effect
Pollution questions become easier when the chain is explicit: source → pollutant → environmental pathway → exposure or change. A vehicle exhaust may release gases and particles into air; wastewater can carry dissolved or suspended material into a water body; discarded plastic may accumulate in soil or water. Naming the source does not itself prove a specific measured concentration or health outcome. The severity depends on amount, duration, weather, location and the substances involved. An exam question should ask for a mechanism or well-established category, not a universal prediction from one source label.
Particulate matter is classified by aerodynamic size conventions. PM2.5 denotes particles with aerodynamic diameter at or below 2.5 micrometres in the standard air-quality terminology. PM10 is a broader size category, so PM2.5 is included within it by size definition, though actual monitoring methods and reporting have operational details. A micrometre is one millionth of a metre. Do not confuse PM2.5 with a concentration of “2.5 particles” or with a chemical formula. A measured PM2.5 value needs units such as micrograms per cubic metre and a stated averaging period.
Carbon monoxide can be produced by incomplete combustion of carbon-containing fuels and interferes with oxygen transport in the body. Carbon dioxide is produced in complete combustion of carbon fuels and is a greenhouse gas; its role and exposure profile differ. Sulfur dioxide and nitrogen oxides can contribute to air-pollution chemistry, including acid deposition under suitable atmospheric processes. Ground-level ozone is an air pollutant formed through photochemical reactions involving precursors; the ozone layer higher in the atmosphere has a different environmental role. A stem saying simply “ozone is good” or “ozone is bad” is missing location and context.
Eutrophication describes nutrient enrichment of a water body followed by ecological changes such as excessive growth of algae and depletion of dissolved oxygen during decomposition. This is a pathway, not the claim that every visible green surface is caused by one named fertiliser. Biodegradation means breakdown by living processes under conditions that permit it. A biodegradable item can still cause litter or other problems if discarded carelessly. Recyclability, biodegradability and low toxicity are separate properties; one cannot be inferred from another by the appearance of the material.
Test the units and the time scale
If two air-quality reports give one pollutant as 40 µg/m³ averaged over a day and another as 30 µg/m³ averaged over a year, the values cannot be compared as if their averaging periods and locations were identical. A concentration is mass per air volume, not total mass emitted by a source. Emission is a source-side measure; ambient concentration describes air at a measured place and time. Weather can transport or dilute pollutants, so a roadside concentration cannot automatically be assigned to a distant factory without evidence.
Likewise, a water contaminant described in mg/L is a mass-per-volume concentration. Converting units requires care: 1 mg is one-thousandth of a gram. A 2 mg/L reading means 2 mg of the substance per litre of sample under that reporting convention; it does not say that exactly 2 mg entered the entire river. When an option switches from concentration to total load, it needs a flow or volume to justify the conversion. The physical unit is a useful defence against an attractive but unsupported answer.
Work backward from a displacement observation
Suppose a question reports that metal A displaces metal B from a suitable salt solution, while B displaces C under comparable stated conditions. In the introductory model, the supported reactivity order is A above B above C. The evidence predicts that A can displace C in the corresponding simple comparison. It does not establish A's position relative to an untested metal D. A common multiple-statement item adds an unsupported fourth metal and invites an unjustified extension. Write only the comparisons supplied, then combine the inequalities that actually connect.
For the equation Zn+CuSO₄→ZnSO₄+Cu, zinc enters solution as Zn²⁺ while Cu²⁺ becomes copper metal. Sulfate remains unchanged in the simplified ionic account. A piece of the original zinc can lose mass as zinc dissolves while copper deposits elsewhere; interpreting a measured mass change requires knowing exactly which object was weighed. The balanced equation accounts for all substances, not just the starting metal strip. This links displacement to both redox and conservation of mass rather than treating them as unrelated chapters.
Aluminium illustrates why reactivity and everyday appearance can seem to disagree. Its exposed surface commonly forms a thin oxide layer that limits further attack under many conditions. The underlying metal can be reactive while the surface film slows visible change. The term passivation describes such a protective surface effect. This does not mean aluminium is immune to every chemical environment. If a question specifies a condition that damages the film, the observation may change. Use the stated surface and environment instead of equating “does not visibly corrode quickly” with “chemically unreactive.”
Compare barrier and sacrificial protection
Paint mainly works by separating a metal from its surroundings. If a coating is damaged, moisture and oxygen may reach the exposed area. Zinc coating can also offer sacrificial protection to iron because zinc can oxidise preferentially under suitable electrochemical conditions. That mechanism differs from a purely physical barrier. The exam distinction is the role of the protective material, not a claim that any coating lasts forever. Thickness, contact, environment and wear influence performance in actual use.
Alloys can alter hardness, corrosion resistance and other properties because composition and structure change. Stainless steels rely on sufficient chromium and an appropriate protective surface film; they are a family of materials rather than one fixed formula. Brass mainly combines copper and zinc, while conventional bronze mainly combines copper and tin. A multiple-choice item asking for constituents needs those names; one asking whether every alloy has an exact molecule needs the broader materials concept. An alloy can be a solid solution or contain multiple phases, so it is not automatically a single chemical compound with a fixed atom ratio.
Distinguish emission, concentration and exposure
A factory's emission rate might be expressed in grams per second, an air-monitor concentration in micrograms per cubic metre, and a person's exposure description as a concentration experienced over a duration. These quantities are related but not interchangeable. If a source emits 2 g/s for 10 s, the released mass is 20 g. That does not determine the concentration at a nearby school without information about transport and dilution. Similarly, a concentration measured for one hour cannot be assumed to represent a person's full-day exposure. The missing time and place information matters.
For water, concentration multiplied by volume gives a contained mass under the stated sampling model. A tank with 500 L of water at 4 mg/L contains 2,000 mg, or 2 g, of the measured substance if the concentration is uniform. A flowing river requires flow rate and time to estimate transported load. If a hypothetical steady flow is 100 L/s at 4 mg/L, the corresponding rate is 400 mg/s; over 60 s that is 24,000 mg or 24 g. These calculations illustrate units, not a claim about a real river or a safe concentration.
Follow nutrients through a water body
Nutrient enrichment can stimulate algal growth. When substantial organic matter is decomposed, microorganisms consume dissolved oxygen, potentially stressing aquatic organisms. The oxygen decline is connected to the biological processing of organic matter; it is not explained by saying fertiliser simply “uses up oxygen” as soon as it touches water. The chain of nutrient input, growth, death and decomposition matters. A question may ask for the initiating cause, an intermediate process or the resulting oxygen condition, and those require different answers.
Biochemical oxygen demand, or BOD, describes oxygen consumed by microorganisms under specified test conditions while breaking down suitable organic matter. A higher BOD can indicate a greater oxygen demand from biodegradable material, but it is not a direct count of all pollutants and does not measure every toxic substance. Dissolved oxygen is the oxygen already present in the water; BOD is a demand measured over a defined test period. Confusing the two can reverse an interpretation. A full water-quality judgment requires more measurements than one of these numbers alone.
Retrieval exercise with worked checks
If A displaces B and B displaces C in the stated simple model, which is most reactive among the three? A. Which observation alone proves the metal order relative to D? None, because D was not tested. How much substance is present in 200 L at 5 mg/L? The mass is 1,000 mg, or 1 g. Does a 5 mg/L label specify the total mass discharged to a river? No; volume or flow and time are also required. Finally, explain why a damaged zinc coating and a damaged paint coating may protect iron differently: zinc can provide a sacrificial electrochemical effect in addition to its barrier role.
Worked comparison
An introductory item gives Zn, Cu and Ag in descending reactivity order and asks which metal can displace copper from copper sulfate solution. Zinc is higher in the given order, so Zn is the expected answer. Silver is lower and is not expected to displace copper in that stated comparison. This reasoning requires the provided series; it is more robust than an unqualified memory slogan about “strong metals.” If the item instead asks whether zinc can displace magnesium, magnesium's place must be supplied or known, and the direction is different.
A second item says a sample contains particles described as PM2.5 at 24 µg/m³. The number 2.5 describes a size threshold, while 24 µg/m³ gives the measured mass concentration. Neither number alone identifies the chemical composition or source. If an option says “24 particles per cubic metre,” it has changed both quantity and unit. Write size class next to PM2.5 and mass concentration next to µg/m³ before choosing.
Recall and review
State three common metallic properties and one exception to an absolute version of each claim. Use a supplied reactivity order to predict displacement direction. Explain why an ionic solution and a metal wire conduct by different moving charged particles. Distinguish rust, tarnish and an ore. Trace one air or water pollutant through a source and pathway without inventing an exact effect. Finally, interpret PM2.5 and µg/m³ as different kinds of information. Attempt the chapter practice set, then use the explanations to revisit any uncertain concepts.
Figure 10. Original chapter diagram.
Practice 43 questions
Choose one answer for each item. The answer key and explanations follow this chapter set.
1. In air-quality reporting, PM₂.₅ means particles with an aerodynamic diameter of approximately:
A. 2.5 millimetres or less B. 2.5 micrometres or less C. 2.5 nanometres or less D. Exactly 25 micrometres
2. Which element is widely used to disinfect municipal drinking water and swimming pools?
A. Fluorine B. Chlorine C. Nitrogen D. Argon
3. The property of metals by which they produce a ringing sound when struck is called:
A. Sonority B. Malleability C. Ductility D. Lustre
4. Activated charcoal removes colour, odour and organic impurities from water by the process of:
A. Osmosis B. Coagulation C. Adsorption D. Distillation
5. CFCs destroy stratospheric ozone mainly by releasing which reactive species under UV light?
A. Chlorine free radicals (Cl•) B. Fluoride ions (F⁻) C. Carbon dioxide D. Hydrogen gas
6. Electrostatic precipitators fitted in thermal power plant chimneys are used to remove:
A. Carbon dioxide B. Particulate matter such as fly ash C. Carbon monoxide D. Methane
7. Which chlorine-free oxidising agent can be selected for certain bleaching processes and decomposes to water and oxygen under suitable conditions?
A. Bleaching powder (CaOCl₂) B. Sulphur dioxide (SO₂) C. Hydrogen peroxide (H₂O₂) D. Sodium hypochlorite (NaOCl)
8. Match List-I (pollutant) with List-II (main effect).
List-I: (a) Chlorofluorocarbons (b) Sulphur dioxide (c) Phosphate and nitrate run-off (d) Carbon monoxide
List-II: (i) Eutrophication (ii) Reduced oxygen transport in blood (iii) Ozone layer depletion (iv) Acid rain
A. a-iv, b-iii, c-i, d-ii B. a-iii, b-iv, c-ii, d-i C. a-iii, b-iv, c-i, d-ii D. a-iii, b-i, c-iv, d-ii
9. Match the ultraviolet band with its usual wavelength interval. List I: (a) UVA; (b) UVB; (c) UVC. List II: (i) 280–315 nm; (ii) 100–280 nm; (iii) 315–400 nm.
A. a-i, b-iii, c-ii B. a-ii, b-i, c-iii C. a-iii, b-i, c-ii D. a-iii, b-ii, c-i
10. In which broad layer of the atmosphere is the ozone concentration maximum, forming the familiar ozone layer?
A. Troposphere B. Stratosphere C. Mesosphere D. Thermosphere
11. Itai-itai disease, marked by severe kidney damage and brittle bones, is caused by poisoning with which heavy metal?
A. Mercury B. Lead C. Chromium D. Cadmium
12. Peroxyacetyl nitrate (PAN), a strong eye irritant, is a characteristic component of:
A. Photochemical smog B. Acid rain C. Classical (sulphurous) smog D. The stratospheric ozone layer
13. In the usual school-level convention, rain is termed acid rain when its pH is below approximately:
A. 7.0 B. 6.5 C. 5.6 D. 3.5
14. The Kigali Amendment (2016) to the Montreal Protocol aims to phase down which group of chemicals that replaced CFCs but are potent greenhouse gases?
A. Hydrofluorocarbons (HFCs) B. Carbon dioxide C. Sulphur dioxide D. Nitrous oxide
15. Pewter, an alloy used for decorative items and tableware, consists mainly of:
A. Copper B. Aluminium C. Tin D. Iron
16. The pre-industrial concentration of carbon dioxide in the Earth's atmosphere is generally estimated at about:
A. 180 ppm B. 420 ppm C. 560 ppm D. 280 ppm
17. Which air pollutant, formed when nitric oxide from vehicle engines is oxidised in air, is a reddish-brown toxic gas that contributes to photochemical smog?
A. Carbon monoxide B. Sulphur dioxide C. Nitrogen dioxide D. Methane
18. Which air pollutant, produced by incomplete combustion of fuels, combines with haemoglobin more strongly than oxygen and reduces the blood's oxygen-carrying capacity?
A. Carbon monoxide B. Carbon dioxide C. Sulphur dioxide D. Methane
19. Which pollution-control device passes sulphur-dioxide-rich flue gas through an alkaline liquid or slurry so the SO₂ is absorbed?
A. Electrostatic precipitator B. Cyclone separator C. Flue-gas scrubber D. Bag filter
20. Which greenhouse gas is released in large quantities from flooded paddy fields and the digestive systems of cattle?
A. Sulphur dioxide B. Carbon monoxide C. Chlorofluorocarbon D. Methane
21. Haematite, an important iron ore, has which chemical formula?
A. Fe₃O₄ B. FeS₂ C. Fe₂O₃ D. FeCO₃
22. Excessive growth of algae in a lake due to nutrient-rich run-off, followed by depletion of dissolved oxygen and death of fish, is called:
A. Biomagnification B. Eutrophication C. Desertification D. Salinisation
23. Seawater is desalinated by forcing it through a semipermeable membrane under pressure greater than its osmotic pressure. This technique is called:
A. Reverse osmosis B. Osmosis C. Dialysis D. Electrophoresis
24. The increase in concentration of a persistent pesticide such as DDT at successive trophic levels of a food chain is called:
A. Eutrophication B. Nitrification C. Leaching D. Biomagnification
25. Consider the statements. I: A properly operating three-way catalytic converter can oxidise carbon monoxide and reduce nitrogen oxides in vehicle exhaust. II: A suitable alkaline flue-gas scrubber can remove sulphur dioxide. Which statement(s) is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
26. Consider the statements. I: Bioaccumulation is a build-up of a substance within an organism over time. II: Biomagnification is an increase in concentration across successive trophic levels. Which statement(s) is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
27. Consider the following statements:
Statement I: The Montreal Protocol was framed mainly to cut emissions of carbon dioxide.
Statement II: Rainwater with a pH of 6.5 is classified as acid rain.
Which of the above is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
28. Which of the following gases is NOT a greenhouse gas?
A. Carbon dioxide (CO₂) B. Nitrogen (N₂) C. Methane (CH₄) D. Nitrous oxide (N₂O)
29. Which of the following is NOT an essential requirement for the formation of photochemical smog?
A. Sunlight B. Nitrogen oxides C. Volatile organic compounds (hydrocarbons) D. Sulphur dioxide
30. In which year was the Montreal Protocol, the international treaty to phase out ozone-depleting substances such as CFCs, signed?
A. 1985 B. 1992 C. 1987 D. 1997
31. For a simplified route in which a sulphide ore is first converted to its oxide, arrange the stages: 1. reducing oxide to metal; 2. concentrating ore; 3. refining crude metal; 4. roasting concentrated ore.
A. 4, 2, 1, 3 B. 2, 1, 4, 3 C. 2, 4, 1, 3 D. 2, 4, 3, 1
32. Which process is commonly used to concentrate many sulphide ores by exploiting differences in surface wettability?
A. Simple distillation B. Magnetic separation C. Froth flotation D. Fractional crystallisation
33. Magnetite, an iron ore that can be concentrated by magnetic separation, has the formula:
A. Fe₂O₃ B. Fe₃O₄ C. FeCO₃ D. FeS₂
34. Moist chlorine acts as a bleaching agent because it reacts with water to form:
A. Perchloric acid (HClO₄) B. Chloric acid (HClO₃) C. Chlorine dioxide (ClO₂) D. Hypochlorous acid (HOCl)
35. Which gas may replace some nitrogen in specialised deep-diving breathing mixtures to reduce nitrogen narcosis?
A. Hydrogen B. Helium C. Neon D. Carbon dioxide
36. Packets of chips and other oily snacks are flushed with which gas to provide an inert atmosphere and prevent oxidation (rancidity) of the fats?
A. Oxygen B. Hydrogen C. Chlorine D. Nitrogen
37. Match each material with a representative main composition. List I: (a) brass; (b) bronze; (c) common austenitic stainless steel; (d) duralumin. List II: (i) Fe + Cr + Ni; (ii) Cu + Zn; (iii) Al + Cu + Mg + Mn; (iv) Cu + Sn.
A. a-ii, b-iv, c-i, d-iii B. a-iv, b-ii, c-i, d-iii C. a-ii, b-iv, c-iii, d-i D. a-iv, b-ii, c-iii, d-i
38. Bauxite is the principal commercial ore used to extract which metal?
A. Iron B. Zinc C. Magnesium D. Aluminium
39. Duralumin, used for making aircraft bodies because it is light yet strong, is an alloy chiefly of:
A. Titanium B. Nickel C. Zinc D. Aluminium
40. In the electrolytic refining of copper, insoluble impurities such as gold and silver collect below the anode as:
A. Slag B. Anode mud C. Flux D. Matte
41. Pyrolusite (MnO₂) is an important ore of which metal?
A. Manganese B. Magnesium C. Molybdenum D. Chromium
42. Stainless steel resists rusting mainly because of the presence of which element that forms a protective oxide film?
A. Carbon B. Chromium C. Manganese D. Tungsten
43. What is the name of the process of coating iron or steel with zinc to protect it from corrosion?
A. Galvanisation B. Vulcanisation C. Anodisation D. Roasting
Answers and explanations
1. B. PM₂.₅ is the fraction of particulate matter with an aerodynamic diameter of 2.5 µm or less. Aerodynamic diameter is a standardised measure that accounts for particle behaviour, rather than necessarily its longest physical dimension. Fine particles can reach deep into the respiratory tract; sources include direct emissions and particles formed in air from gases.
2. B. Chlorine (or chlorine compounds such as hypochlorite) kills bacteria in water supplies and swimming pools. Fluorine is too reactive and toxic for this purpose, while nitrogen and argon are unreactive gases with no disinfecting action.
3. A. Metals are sonorous — they ring when struck, which is why bells and school gongs are made of metal. Malleability is being beaten into sheets, ductility is being drawn into wires, and lustre is the shiny surface of metals.
4. C. Activated charcoal has an enormous porous surface area on which impurity molecules accumulate — adsorption, a surface phenomenon. Osmosis is solvent movement through a membrane and coagulation is settling of colloids.
5. A. UV light can break CFC molecules in the stratosphere and release reactive chlorine atoms. A chlorine radical reacts with ozone to form ClO and O₂; subsequent reactions can regenerate chlorine, allowing catalytic ozone loss. The number of ozone molecules affected depends on atmospheric conditions and is not a universal fixed count.
6. B. In an electrostatic precipitator, particles in flue gas are given an electric charge and collected on oppositely charged plates; it removes fly ash and dust very efficiently but not gases. SO₂ is removed separately by scrubbers.
7. C. Hydrogen peroxide, H₂O₂, is a chlorine-free oxidising bleach; its simple decomposition products are water and oxygen. Whether a whole bleaching process is environmentally preferable depends on production, dosage, effluent and energy use. Chlorine-based agents in the other options can form different by-products.
8. C. CFCs deplete stratospheric ozone; SO₂ forms H₂SO₄ causing acid rain; nutrient (N, P) run-off causes eutrophication; CO binds haemoglobin and cuts oxygen transport.
9. C. The usual WHO bands are UVA 315–400 nm, UVB 280–315 nm and UVC 100–280 nm, so a-iii, b-i, c-ii. UVA is not harmless: sunlight UVA contributes to skin damage; all solar UVC and much UVB are filtered by the atmosphere. Avoid ranking a band as simply safe or unsafe from wavelength alone.
10. B. The ozone layer lies mainly in the stratosphere. Ozone is also present in the troposphere, where it acts as an air pollutant. This distinction helps separate stratospheric ozone depletion from ground-level photochemical smog.
11. D. Itai-itai disease (Japan) was caused by cadmium from mining waste in river water used for rice fields; cadmium damages the kidneys. Mercury causes Minamata disease, and lead mainly harms the nervous system.
12. A. Photochemical smog forms when nitrogen oxides and volatile organic compounds react in strong sunlight, producing ozone and oxidants such as PAN and formaldehyde. Classical smog is a mix of smoke, fog and SO₂ in cold, damp conditions; acid rain involves H₂SO₄ and HNO₃.
13. C. The conventional classroom threshold is pH 5.6. Rain is naturally mildly acidic because atmospheric CO₂ dissolves to form carbonic acid; sulphur and nitrogen oxides can make rain more acidic. Natural rain pH varies by location and atmospheric chemistry, so 5.6 is a standard teaching boundary rather than a universal measurement.
14. A. The Kigali Amendment, adopted in Rwanda in 2016, extends the Montreal Protocol to phase down HFCs. HFCs do not deplete ozone but have very high global-warming potential; CO₂ is dealt with under the UNFCCC/Paris framework.
15. C. Pewter is mostly tin (roughly 85–99%) hardened with small amounts of antimony and copper; older pewter also contained lead. Copper is the base of brass and bronze, aluminium of duralumin, and iron of steel.
16. D. Ice-core measurements place pre-industrial atmospheric CO₂ near 280 parts per million. About 180 ppm is characteristic of some glacial periods, and 560 ppm would be twice the pre-industrial reference. The question asks for the historical baseline, so no rapidly changing present-day concentration is needed.
17. C. Nitrogen dioxide (NO₂) is a reddish-brown gas formed by 2NO + O₂ → 2NO₂. Sunlight splits NO₂ to start ozone formation in photochemical smog. CO, SO₂ and methane are all colourless gases.
18. A. Incomplete combustion can release carbon monoxide. CO binds haemoglobin strongly to form carboxyhaemoglobin and impairs oxygen transport. Exact affinity ratios vary by experimental definition and need not be memorised for this item; the important distinction is CO versus CO₂, SO₂ and methane.
19. C. A suitable wet flue-gas scrubber uses an alkaline absorbent, often limestone or lime based, to capture acidic SO₂. An electrostatic precipitator, cyclone and bag filter are principally particle-control devices; they do not perform this alkaline gas absorption.
20. D. Methane is produced by microbes in waterlogged rice soils and by microbial digestion in ruminants such as cattle. It is a greenhouse gas. Comparisons with CO₂ depend on the chosen time horizon and assessment method, so a fixed multiplier is unnecessary to identify the gas.
21. C. Haematite is iron(III) oxide, Fe₂O₃. Magnetite is Fe₃O₄, iron pyrite is FeS₂ and siderite is FeCO₃. Checking the oxidation state helps: two Fe³⁺ ions balance three O²⁻ ions.
22. B. Eutrophication: nitrogen and phosphorus from fertilisers and sewage cause algal blooms; decomposition of dead algae uses up dissolved oxygen, making water anoxic. Biomagnification is the build-up of toxins along food chains.
23. A. In reverse osmosis, applied pressure exceeds osmotic pressure, so pure water moves from the salt solution through the membrane, leaving salts behind. In normal osmosis water moves the other way, into the more concentrated solution.
24. D. Biomagnification is an increase in the concentration of certain persistent pollutants at successively higher trophic levels. DDT is a standard example because it is persistent and can accumulate in tissues. Such substances may be eliminated slowly; claiming that they are never excreted is too absolute. Nitrification is a different nitrogen-cycle process.
25. C. Both statements are correct for the stated devices and operating conditions. A three-way catalyst can oxidise CO to CO₂ and reduce NOₓ to N₂ when the engine and exhaust mixture are controlled appropriately. An alkaline wet scrubber absorbs acidic SO₂. Not every vehicle catalyst or scrubber design performs every function.
26. B. Both statements are correct. Bioaccumulation follows a substance building up in one organism, whereas biomagnification compares average concentrations from one trophic level to the next. A persistent pollutant can show both processes, but the terms answer different questions.
27. D. Neither statement is correct. The Montreal Protocol addresses ozone-depleting substances rather than being framed mainly for CO₂. In the conventional school definition, acid rain is below about pH 5.6, so rain at pH 6.5 is not classed as acid rain. Actual natural-rain pH varies, and the threshold is a teaching convention.
28. B. CO₂, CH₄ and N₂O absorb outgoing infrared radiation and trap heat. Diatomic N₂, though it makes up about 78% of air, does not absorb infrared significantly, so it is not a greenhouse gas; nitrous oxide (N₂O) is.
29. D. Photochemical smog forms when NOₓ and hydrocarbons from vehicles react in strong sunlight, producing ozone, PAN and aldehydes. Sulphur dioxide is the key component of classical (London-type) smog, not photochemical smog.
30. C. The Montreal Protocol was signed in September 1987 and entered into force in 1989. 1985 is the Vienna Convention (ozone framework), 1992 the Rio Earth Summit and 1997 the Kyoto Protocol on greenhouse gases.
31. C. In this specified teaching route the ore is concentrated, roasted to form an oxide, reduced to metal, and the crude metal is refined: 2 → 4 → 1 → 3. Real extraction flows differ by metal and ore; the question tests this stated oxide-route sequence.
32. C. Froth flotation uses differences in how mineral and gangue particles attach to air bubbles or remain wetted by water. It is common for many sulphide ores. Magnetic separation instead exploits magnetic properties; distillation and crystallisation concern different separation settings.
33. B. Magnetite is Fe₃O₄ and is strongly magnetic, so it is separated from gangue magnetically. Fe₂O₃ is haematite, FeCO₃ is siderite and FeS₂ is iron pyrites (fool's gold).
34. D. Chlorine reacts reversibly with water to form hydrochloric acid and hypochlorous acid: Cl₂ + H₂O ⇌ HCl + HOCl. HOCl and related reactive chlorine species oxidise coloured substances, explaining bleaching in moist conditions. “Nascent oxygen” is an obsolete shorthand and is not the mechanism to memorise.
35. B. Helium may be included with oxygen, or with oxygen and nitrogen, in specialised diving mixtures to lower nitrogen partial pressure and reduce nitrogen narcosis. Such mixtures do not eliminate decompression-sickness risk and require trained dive planning. This item only asks for the gas.
36. D. Nitrogen is relatively inert because of its strong N≡N triple bond, so it is used to flush food packets and keep oxygen away from the fats. Oxygen would speed up rancidity, while hydrogen is flammable and chlorine is toxic.
37. A. Representative compositions are brass Cu–Zn, bronze Cu–Sn, common austenitic stainless steel Fe–Cr–Ni, and duralumin Al with Cu, Mg and Mn. Thus a-ii, b-iv, c-i, d-iii. Stainless-steel grades vary; some contain little or no nickel, so the list names the austenitic family.
38. D. Bauxite is the main commercial source of aluminium. It is an ore mixture rich in aluminium hydroxide minerals, rather than one pure compound with a single fixed hydration number. Haematite and magnetite are iron ores; sphalerite is a major zinc ore.
39. D. Duralumin is mostly aluminium (about 95%) with small amounts of copper, magnesium and manganese, giving a light, strong alloy for aircraft. Titanium, nickel and zinc are not its base metal.
40. B. During electrolytic refining the impure metal is the anode and pure metal deposits on the cathode; insoluble impurities fall below the anode as anode mud. Slag forms when flux combines with gangue in a furnace; matte is a Cu₂S–FeS melt.
41. A. Pyrolusite is manganese dioxide, MnO₂, and an important manganese ore. Magnesium is a different element despite the similar name. The ore formula, rather than a potentially changing mining-location list, provides the reliable exam clue.
42. B. Stainless steel contains at least about 10.5% chromium, which forms a thin, non-porous, self-healing Cr₂O₃ layer that stops further oxidation. Carbon is present in all steels and does not give rust resistance.
43. A. Galvanisation applies a protective zinc coating to iron or steel. The coating acts as a barrier and zinc can also give sacrificial protection where exposed. Vulcanisation cross-links rubber; anodisation creates a protective oxide on suitable metals, often aluminium; roasting heats ores in air.