Pollution & waste

Answer: (c) Only three

Hydrofluorocarbons (HFCs) replaced ozone-depleting CFCs and HCFCs in cooling, aerosol propellants, foam blowing and fire suppression. Lubricants are not an HFC application, so three of the four are correct.

• ✓ 1. HFCs serve as propellants in aerosol cans, one of the uses listed by the US EPA and UNEP.
• ✓ 2. HFCs are used as blowing agents to make insulating and other foams.
• ✓ 3. Some HFCs are used as fire-suppression (fire-extinguishing) agents, a replacement for ozone-depleting halons.
• ✗ 4. Lubricants are oils and greases; HFCs are gases used as refrigerants, propellants, blowing agents and fire suppressants, not as lubricants.

Remember · HFCs: refrigerants, AC, aerosols, foam blowing, fire suppression, solvents. Not ozone-depleting but potent greenhouse gases; phased down under the Kigali Amendment (2016).

Question and answer: UPSC's official GS Paper I (2023, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) Only two

Statements 1 and 2 are counted as correct, and statement 3 is wrong. A biofilter is the waste-treatment unit of a recirculating fish farm: its bacteria turn toxic ammonia from fish waste and feed into less harmful nitrate, and it does not add phosphorus.

• ✓ 1. Waste treatment in a recirculating system includes removing waste solids, both faeces and uneaten feed, and the biofilter is part of that treatment train. Strictly, the mechanical filter removes solids and the biofilter handles dissolved nitrogen; UPSC's key counts this statement as correct.
• ✓ 2. Nitrifying bacteria in the biofilter convert ammonia into nitrite and finally into nitrate, which is far less toxic to fish than ammonia.
• ✗ 3. The biofilter removes toxic ammonia; it does not add phosphorus. Phosphate is an inert dissolved compound that simply passes through the filtration stage.

Remember · In a recirculating aquaculture system, the biofilter's nitrifying bacteria convert toxic ammonia to nitrite and then to less toxic nitrate. It adds no phosphorus.

Question and answer: UPSC's official GS Paper I (2023, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) Only two

Statements 1 and 2 are correct; statement 3 is not. Gold mining and coal-fired power stations are both major sources of mercury releases, but international health bodies have set tolerable intake levels for mercury, so it is wrong to say that no safe level is known.

• ✓ 1. WHO lists mining for mercury, gold and other metals among the main human causes of mercury release, and warns that mercury use in artisanal and small-scale gold mining is particularly hazardous.
• ✓ 2. WHO names coal-fired power stations among the main sources of mercury releases. Coal contains mercury, which escapes when it is burned.
• ✗ 3. The FAO/WHO expert committee JECFA set a tolerable intake of 1.6 micrograms per kg of body weight per week for methylmercury. Exposure is harmful at high levels, but a tolerable level has been defined.

Remember · Mercury comes from coal burning and gold mining, among other sources. WHO and FAO experts have set a tolerable weekly intake for methylmercury, so a safe level is defined.

Question and answer: UPSC's official GS Paper I (2023, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (d) None

None of the statements is correct, because each says that no coal plant in India does something that at least one plant does. The private Mundra plant in Gujarat uses seawater, and many coal plants stand in water-stressed areas.

• ✗ 1. The 4,000 MW Mundra Ultra Mega Power Project, a coastal plant, uses seawater; the Central Electricity Authority lists its water source as seawater, with fresh water made by desalination.
• ✗ 2. A World Resources Institute study, cited in a NITI Aayog-hosted report, found that more than 70% of India's coal power plants are in water-stressed or water-scarce areas.
• ✗ 3. Mundra UMPP is developed and run by Coastal Gujarat Power Limited, a Tata Power company, so privately owned coal plants exist. Coal capacity is spread across central, state and private owners.

Remember · A single counter-example defeats an 'none of them' claim: Mundra (Tata Power) is a private coal plant using seawater, and most coal plants lie in water-stressed areas.

Question and answer: UPSC's official GS Paper I (2023, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (c) All three

All three activities are discussed as ways to capture and lock away carbon dioxide. Spreading crushed basalt (enhanced weathering) and adding lime to the sea (ocean alkalinity enhancement) are carbon-removal options assessed by the IPCC, and abandoned mines are listed among niche sites for underground CO2 storage.

• ✓ 1. Enhanced weathering spreads crushed silicate rock such as basalt on soil so that it reacts with atmospheric CO2. The IPCC cites estimates that spreading basalt on farmland in 12 countries could remove up to 2 GtCO2 a year by 2050.
• ✓ 2. Ocean alkalinity enhancement adds alkaline material such as lime to the sea so that it takes up and stores more CO2. The IPCC notes this idea goes back to a 1995 proposal to dissolve highly reactive lime in surface ocean water.
• ✓ 3. The IPCC's report on CO2 capture and storage names abandoned mines, along with basalts and salt caverns, as niche options for geological storage of captured CO2.

Remember · Enhanced weathering (crushed basalt on land), ocean alkalinity enhancement (lime in the sea) and storing captured CO2 underground, even in abandoned mines, are all discussed for carbon removal.

Question and answer: UPSC's official GS Paper I (2023, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error