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 :
- (a)1 and 2
- (b)2 and 3
- (c)Only 3
- (d)Only 1
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).
- (a)1 and 2 — 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)2 and 3 — 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)Only 1 — 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.
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.
The efficient way to prepare for this is to learn the boundary rather than individual cases. Lighter than air, with molar mass below 29: hydrogen 2, helium 4, methane 16, ammonia 17, water vapour 18, neon 20, acetylene 26, hydrogen cyanide 27, carbon monoxide 28 and nitrogen 28 — the last two only marginally. Heavier than air: oxygen 32, hydrogen sulphide 34, hydrogen chloride 36.5, argon 40, carbon dioxide 44, nitrogen dioxide 46, sulphur dioxide 64, chlorine 71. The examiner's device here is the name 'hydrogen chloride', which reads like hydrogen but weighs eighteen times as much per molecule. (The commission's booklet prints 'gasses'; the ordinary English plural is 'gases' — a printing variant, not a clue.)
- Dry air has an average molar mass of about 29 g/mol — roughly 78 per cent nitrogen, 21 per cent oxygen and 1 per cent argon — and that is the benchmark for 'lighter than air'
- Helium is 4 g/mol, about one-seventh as dense as air, and non-flammable, which makes it the standard lifting gas for weather balloons and airships
- Hydrogen chloride is 36.5 g/mol and sulphur dioxide is 64 g/mol; both are heavier than air, so both settle rather than rise
- Hydrogen (2 g/mol) is lighter still than helium and lifts better, but it is flammable — the reason helium replaced it in airships after the Hindenburg disaster of 1937
- Carbon dioxide, at 44 g/mol, is also heavier than air, which is why it blankets a fire from a CO₂ extinguisher and why it can collect in wells, cellars and disused mines
Only helium falls below air's 29 g/mol, so the answer is 'Only 3'. The name 'hydrogen chloride' is the trap — it is HCl, not H₂.
- Reading 'hydrogen chloride' as 'hydrogen' — the first is 36.5 g/mol and sinks, the second is 2 g/mol and rises
- Assuming any colourless, odourless gas must be light: carbon dioxide is colourless and about one and a half times denser than air
- Forgetting that nitrogen and carbon monoxide, both 28 g/mol, sit only just below air's 29 — 'lighter than air' does not mean 'very light'
UPPSC asks the single-property static — which gas is lighter than air, which is a noble gas, which is laughing gas — in a one-line stem or a short numbered list. UPSC prefers to embed the same chemistry in a situation: a hydrogen-filled balloon rising through the atmosphere, or a match-list of fuel gases and their constituents.
A hydrogen-inflated polythene balloon is released from the surface of the earth. As the balloon rises to an altitude up in the atmosphere, it will
- (a) Decrease in size
- (b) Flatten into a disc-like shape
- (c) Increase in size
- (d) Maintain the same size and shape
Answer(c) Increase in size
The same physical setting, one step further on. A hydrogen-filled balloon rises only because hydrogen is lighter than air — the fact this question tests — and UPSC then asks what happens to it as the outside pressure falls: it expands. Learn the density rule here and the 2003 item becomes a gas-law question rather than a chemistry one.
Which one of the following is the correct sequence in increasing order of molecular weights of the hydrocarbons?
- (a) Methane, ethane, propane and butane
- (b) Propane, butane, ethane and methane
- (c) Butane, ethane, propane and methane
- (d) Butane, propane, ethane and methane
Answer(a) Methane, ethane, propane and butane
The same comparison skill applied to a homologous series. Ordering methane, ethane, propane and butane by molecular weight is precisely the ranking exercise used here against air's 29 g/mol; note also that methane at 16 g/mol is the only one of the four that is lighter than air.
- practice — not a real PYQ
Which one of the following gases is heavier than air?
- (a)Methane
- (b)Ammonia
- (c)Carbon dioxide
- (d)Helium
Answer(c) Carbon dioxide — 44 g/mol against air's roughly 29 g/mol; methane (16), ammonia (17) and helium (4) are all lighter than air.
- practice — not a real PYQ
Helium rather than hydrogen is used to fill airships and weather balloons mainly because
- (a)helium is lighter than hydrogen
- (b)helium is non-flammable
- (c)helium is cheaper than hydrogen
- (d)helium diffuses more slowly than air
Answer(b) helium is non-flammable — hydrogen is both lighter and cheaper, but it burns, as the Hindenburg disaster of 1937 demonstrated.