Physics & chemistry in everyday life

Answer: (d) 1, 2 and 3

PFAS are man-made fluorinated chemicals used for non-stick, stain-proof and grease-proof products. Their carbon–fluorine bonds make them extremely persistent ('forever chemicals'), so they spread through water, food and packaging and build up in the bodies of people and animals. All three statements hold.

• ✓ 1. PFAS turn up in drinking water, in foods such as fish and dairy, and in grease-resistant food packaging like fast-food wrappers and pizza boxes.
• ✓ 2. They are long-lasting chemicals whose components break down very slowly, so they persist in the environment.
• ✓ 3. Because they persist, many PFAS are found in the blood of people and animals worldwide, and some accumulate in the body over time.

Remember · PFAS = 'forever chemicals': very slow to break down, found in water, food and packaging, and accumulate in human and animal bodies.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (d) Statement-I is incorrect, but Statement-II is correct

A star's mass decides how fast it burns. Big, massive stars need very high rates of nuclear fusion in their cores to hold themselves up against their own gravity, so they use up their hydrogen in a few million years, while small dwarf stars burn slowly and can shine for billions or even trillions of years. So the fusion-rate statement is true, and the longer-life statement is the reverse of the truth.

• ✗ Statement-I Massive stars are short-lived: some last only a few million years, whereas low-mass red dwarfs can last trillions of years; the Sun will spend about 10 billion years on the main sequence.
• ✓ Statement-II More massive stars must fuse hydrogen at a much higher rate to generate the outward pressure that balances their stronger gravity, which also makes them hotter and brighter.

Remember · Bigger star → faster fusion → shorter life. Massive stars: millions of years, end as supernovae; red dwarfs: trillions of years. Sun: about 10 billion years on the main sequence.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (a) 1 only

Hydrogels are water-swollen networks of cross-linked polymers, soft and compatible with body tissue. Their best-established use is medical: a drug trapped in the gel is released over time at a controlled rate, and the gels are also used in wound dressings, contact lenses and tissue engineering. UPSC's key treats statements 2 and 3 as incorrect.

• ✓ 1. Hydrogels can hold small drugs, proteins or even cells and release them over time and at a chosen site, and several such delivery systems are in clinical use.
• ✗ 2. UPSC's official key treats this statement as incorrect; we could not confirm the detail from an official source, so we do not explain it here.
• ✗ 3. UPSC's official key treats this statement as incorrect; we could not confirm the detail from an official source, so we do not explain it here.

Remember · Hydrogel = cross-linked, water-absorbing polymer network; soft, biocompatible. Uses: controlled drug release, wound dressings, soft contact lenses, tissue engineering.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink · Report an error

Answer: (d) Water vapour

A hydrogen fuel cell combines hydrogen from the tank with oxygen from the air to produce electricity for the motor. The only chemical product of that reaction is water (H2O), so the tailpipe of a fuel-cell car releases just water vapour and some warm air, with no smog-forming pollutants.

• ✓ (d) At the cathode, the hydrogen ions, the electrons that have passed through the circuit and oxygen join to form water, which leaves as vapour.
• ✗ (c) Oxygen is consumed by the fuel cell, not given out; it is taken in from the air as a reactant.
• ✗ (a) Hydrogen peroxide (H2O2) is not a product of a fuel cell's hydrogen–oxygen reaction; the cell is designed to yield plain water.

Remember · Hydrogen fuel cell: 2H2 + O2 → 2H2O + electricity + heat. FCEV tailpipe emits only water vapour and warm air.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error