Which of the following gasses is/are lighter than air ?
1. Hydrogen chloride
2. Sulfur dioxide
3. Helium
Select the correct answer from the code given below :

Question

UnlockIAS · Accepted Answer

Correct — C, Only 3. A gas rises in air only when it is less dense than air, and at the same temperature and pressure the density of a gas is set by its molar mass. Dry air is a mixture of about 78 per cent nitrogen, 21 per cent oxygen and 1 per cent argon, giving an average molar mass of roughly 29 g/mol, so the whole question reduces to one comparison: is this gas below 29? Helium is 4 g/mol — about one-seventh as dense as air — so it rises, which is why it fills weather balloons and airships. Hydrogen chloride is 36.5 g/mol and sulphur dioxide is 64 g/mol, both comfortably heavier than air, which is why fumes of either tend to hang low and pool near the ground when released. Only statement 3 qualifies, so the code is (c).

Why the other options are wrong — (a) These are precisely the two heavy gases. Hydrogen chloride at 36.5 g/mol and sulphur dioxide at 64 g/mol both exceed air's roughly 29 g/mol — sulphur dioxide is more than twice as dense as air, which is part of why it lingers near the ground as a pollutant.  (b) Helium is right, sulphur dioxide is not. At 64 g/mol, SO₂ is one of the denser common gases, well above air; a candidate who marks this one has the correct instinct about helium but has not tested the second name against the 29 g/mol benchmark.  (d) This is the trap the question is built around. Hydrogen chloride is not hydrogen. Hydrogen (H₂, 2 g/mol) is the lightest gas there is, but hydrogen chloride (HCl, 36.5 g/mol) is a different substance and is heavier than air; the word 'hydrogen' inside the name does the misleading.

Concept: By Avogadro's law, equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, so the density of a gas is proportional to its molar mass. Dry air is a mixture whose average molar mass is about 29 g/mol. Any gas whose molar mass falls below that figure is buoyant in air and will rise; anything above it sinks. That single comparison settles every 'lighter than air' question without further chemistry, and it also explains the practical behaviour of gases — why helium balloons float, why carbon dioxide smothers a fire from above the fuel, and why heavy pollutant gases accumulate at street level.

State PCS 2025 Q20 — Science & Technology PYQ with Answer

State PCS 2025 Q20 — Science & Technology PYQ with Answer