Chemistry — UPSC Prelims Science & Environment previous year questions

Three of the four elements usually make up battery cathodes: cobalt, lithium and nickel.
Lithium-ion cathodes use metal oxides such as lithium cobalt oxide and lithium nickel-manganese-cobalt oxide.
Graphite is not a cathode material. It forms the anode, where lithium ions are stored during charging.
So the answer is only three.
Remember: In a lithium-ion cell, the cathode is a lithium metal oxide and the anode is graphite.

Source: NCERT Class XII Chemistry, Electrochemistry (batteries); Ministry of Mines on critical minerals for batteries

Two of the three substances can be produced using coal gasification.
Gasification turns coal into syngas, a mix of carbon monoxide and hydrogen. Syngas is a building block for chemicals.
Ethanol can be made from syngas. Urea can be made from syngas, because its hydrogen is used to make ammonia and then urea.
Nitroglycerine is made by nitrating glycerol with nitric acid. Syngas is not its usual route, so it is not counted.
So the answer is only two.
Remember: Syngas can also give methanol, ammonia and synthetic natural gas.

Source: Ministry of Coal, National Coal Gasification Mission document

Statement I is correct. Activated carbon is widely used to remove pollutants from effluents.
Statement II is correct. It has a very large surface area. So it adsorbs heavy metals strongly.
Statement III is correct. It can be made cheaply from carbon-rich wastes such as coconut shells and farm residue.
Statements II and III both explain Statement I. The large surface area makes it effective. The cheap sources make it attractive. Option (a) fits.
Remember: Adsorption means molecules stick to a surface. Absorption means they enter the bulk.

Source: NCERT Class XII Chemistry, Surface Chemistry (adsorption); Central Pollution Control Board material on effluent treatment

Statement I is correct. Some rare earth elements, such as europium and yttrium, are used in making flat television screens and computer monitors.
Statement II is correct. Some rare earth elements are phosphorescent. They give out light after being excited.
Statement II explains Statement I. Their light-giving property is the reason they are used in screens.
So option (a) is right. Option (b) fails because the property does explain the use.
Remember: The 17 rare earth elements are the 15 lanthanides plus scandium and yttrium.

Source: U.S. Geological Survey, Rare Earth Elements fact sheet; Ministry of Mines on critical minerals

A hydrogen fuel cell combines hydrogen with oxygen from air. The reaction produces electricity, heat and water. The only product leaving the exhaust pipe is water vapour.
Hydrogen peroxide and hydronium are not products of this reaction. Oxygen is a reactant taken from air, so it is not the emission.
Remember: fuel cell vehicles are called zero tailpipe emission vehicles.

Source: Ministry of New and Renewable Energy, National Green Hydrogen Mission documents

Statement 1 is correct. Polyester fibre from PET is blended with cotton and wool to add strength and crease resistance.
Statement 2 is incorrect. PET is not suitable for every alcoholic drink, as strong spirits can affect the plastic.
Statement 3 is correct. PET bottles are widely recycled into fibres, sheets and new containers.
Statement 4 is incorrect. Burning PET releases carbon dioxide and other pollutants.
Remember: PET is a thermoplastic polyester and a common recyclable plastic (code 1).

Source: NCERT Class XII Chemistry, Chapter on Polymers; MoEFCC Plastic Waste Management Rules

BPA is the building block of polycarbonate plastics. It is also used in epoxy resins.
BPA acts like a hormone and can disturb the endocrine system. This is the cause of concern.
LDPE and PVC are made from ethylene and vinyl chloride, not BPA.
PET is made from terephthalic acid and ethylene glycol.
Remember: Many countries have banned BPA in baby bottles.

Source: Standard polymer chemistry references; NCERT Class XII Chemistry, Polymers

A water molecule is dipolar. The oxygen end is slightly negative and the hydrogen ends are slightly positive.
This lets water pull apart ionic compounds and dissolve many polar substances.
Option (b) is wrong. Heat conduction does not decide solvent power.
Option (c) is wrong. High specific heat helps in climate regulation, not dissolving.
Option (d) is true but does not explain the dissolving power.
Remember: Water is called the universal solvent.

Source: UPSC CSE Prelims 2021 question and key (a)

A flame has three zones. The outermost zone gets plenty of oxygen, so fuel burns fully there. This makes it the hottest zone.
Goldsmiths need high heat to melt gold and silver, so they use this zone.
Option (b) is wrong. The outer zone is non-luminous, not luminous.
Option (c) is wrong. It is the hottest, not the least hot.
Option (d) is not a reason given for the choice.
Remember: The innermost zone is the coolest, as it has unburnt gas.

Source: NCERT Class VIII Science, Combustion and Flame

Statement 1 is correct. Graphene is a single layer of carbon atoms and conducts electricity well.
Statement 2 is correct. It is extremely thin and very strong.
Statement 3 is incorrect. It is made of carbon, not silicon.
Statement 4 is correct. It is transparent and conductive, so it suits touch-screen electrodes.
Remember: Geim and Novoselov won the 2010 Nobel Prize in Physics for graphene.

Source: Nobel Prize in Physics 2010 scientific background; standard chemistry references

Statement 1 is correct. UV light damages the DNA of microbes, so they cannot multiply.
Statement 2 is incorrect. UV does not remove odours or chemicals.
Statement 3 is incorrect. UV does not settle solids or reduce turbidity. Filters and coagulants do that.
Remember: UV leaves no residual disinfectant in the water.

Source: US EPA / WHO guidance on UV disinfection of drinking water

Statement 1 is incorrect. In osmosis the solvent moves from the dilute solution to the concentrated solution through a semipermeable membrane.
Statement 2 is incorrect. In reverse osmosis, pressure larger than the osmotic pressure is applied on the concentrated solution side.
This forces the solvent to flow from the concentrated side to the dilute side.
Remember: Reverse osmosis is used for desalination of seawater.

Source: NCERT Class XII Chemistry, Solutions (osmosis and reverse osmosis)

Assertion (A) is false. LPG is mainly propane and butane.
Methane is the main component of natural gas and CNG.
Reason (R) is true. Methane is a clean fuel and is supplied by pipeline as piped natural gas (PNG).
Since A is false, R cannot explain it.
Remember: LPG is stored as a liquid under moderate pressure; methane needs very low temperature to liquefy.

Source: NCERT Class X Science, Carbon and its Compounds; Ministry of Petroleum and Natural Gas

Assertion (A) is true. Unsaturated fats have one or more carbon-carbon double bonds.
Double bonds are sites where other atoms can add, as in hydrogenation and oxidation.
Reason (R) is false. Unsaturated fats have double bonds. Saturated fats have only single bonds.
This is why unsaturated oils turn rancid more easily.
Remember: Hydrogenation turns unsaturated oils into saturated fats like vanaspati.

Source: NCERT Class XI Chemistry, Organic Chemistry: Some Basic Principles; NCERT Class XII Chemistry, Biomolecules

Assertion (A) is false. Electron affinity generally decreases down a group.
The added electron is farther from the nucleus, so the attraction is weaker.
The word 'always' also makes A wrong, as there are exceptions such as chlorine and fluorine.
Reason (R) is true. Each new period adds a shell, so atomic radius increases down a group.
Remember: Chlorine has a higher electron affinity than fluorine.

Source: NCERT Class XI Chemistry, Classification of Elements and Periodicity in Properties

Cracking breaks large hydrocarbon molecules of petroleum into smaller ones, so A matches 2.
Smelting extracts a metal from its ore by heating; copper is smelted, so B matches 3.
Hydrogenation adds hydrogen to vegetable oils to make edible fats, so C matches 4.
Vulcanisation heats rubber with sulphur to make it stronger, so D matches 1.
The code is 2 3 4 1.
Remember: Vulcanisation was discovered by Charles Goodyear.

Source: NCERT Class X and XII, Chemistry; Class XII, Chemistry, Ch. 14

German silver is an alloy of copper, zinc and nickel, so A matches 2.
Solder is an alloy of tin and lead, so B matches 1.
Bleaching powder is made using chlorine, so C matches 4.
Hypo is sodium thiosulphate, so D matches 3.
The code is 2 1 4 3.
Remember: Hypo is used as a fixer in photography.

Source: NCERT Class XII, Chemistry; Class X, Science, Ch. 3

A matches 3. Atropine dilates the pupil of the eye.
B matches 1. Ether was used as an anaesthetic. The list's official match is local anaesthesia.
C matches 2. Nitroglycerine widens blood vessels and is used in heart trouble such as angina.
D matches 4. Pyrethrin is a natural insecticide used for mosquito control.
So the code is 3 1 2 4.
Remember: Pyrethrin comes from chrysanthemum flowers.

Source: Standard pharmacology references; NCERT Class XII Chemistry, Ch. 16

Neela thotha is the common name of copper sulphate, so A matches 4.
Epsom salt is magnesium sulphate, so B matches 3.
Baking soda is sodium bicarbonate, so C matches 1.
Caustic soda is sodium hydroxide, so D matches 2.
The correct code is 4 3 1 2.
Remember: Copper sulphate crystals are blue and are used as a fungicide.

Source: NCERT Class X Science, Acids, Bases and Salts

Boric acid is a mild antiseptic. Citric acid is used as a food preservative. Magnesium hydroxide is an antacid, as in milk of magnesia. Acetic acid (vinegar) is used in pickles.
So the match is A-1, B-2, C-3, D-4.

Source: NCERT Class X, Science, Chapter 2; NCERT Class XII, Chemistry, Chemistry in Everyday Life