CDS (II) 2024 General Knowledge · Question 16 of 120

Correct — B, Helium is a commonly used gas in electric bulbs. The gas inside an ordinary filament bulb is argon, usually with about seven per cent nitrogen; better lamps use krypton, and the most expensive use krypton with xenon. Helium is not used. The reason lies in what the fill gas is for. A tungsten filament at white heat slowly evaporates, thinning until it breaks, and an inert gas at pressure suppresses that evaporation so the lamp can run hotter and brighter for longer. The gas must therefore be chemically inert, cheap and — importantly — a poor conductor of heat, so that it does not carry warmth away from the filament and waste power. Argon meets all three; helium fails the third badly, since it is one of the best heat-conducting gases known, and it is far more expensive and so light that it leaks through seals. The other three statements are sound: hydrogen has three isotopes, liquid oxygen is the standard oxidiser burned with rocket fuel, and packaged food is routinely flushed with nitrogen to keep air away from it.

• (a)Hydrogen has 3 isotopes. — This statement is correct, so it cannot be the answer. Hydrogen's three isotopes are protium with no neutron, deuterium with one and tritium with two, and only tritium is radioactive.
• (c)Oxygen is used to burn fuel in rocket engines in spacecraft. — Correct as stated. A rocket must carry its own oxidiser because there is no air in space, and liquid oxygen is the commonest one, burned with kerosene or liquid hydrogen.
• (d)Nitrogen can be used in gaseous form to replace air in food packaging. — Correct as stated. Packets of chips and similar foods are flushed with nitrogen, which is unreactive, so that the oxygen that would turn the fats rancid is displaced. The puffed-up packet is the visible sign of it.

Each of the four gases named here is chosen for a job because of one property. Argon is inert and conducts heat poorly, which suits it to filling lamps. Oxygen supports combustion, which is why rockets carry it. Nitrogen is unreactive at ordinary temperatures, which is why it displaces air around food. Hydrogen is the lightest element and the one with the simplest set of isotopes. Helium is inert too, but it is scarce, expensive, exceptionally good at conducting heat and small enough to escape through most seals, so its own uses are different — filling balloons and airships, cooling superconducting magnets, and breathing mixtures for deep divers.

Statement items reward testing each line separately instead of hunting for the answer, and here three lines are settled facts that a candidate should recognise at once. The line to examine is the one about bulbs, and the useful check is to ask what property the job actually needs. Filling a lamp needs a gas that is inert AND a poor conductor of heat; helium is inert but is among the best heat conductors, so it would cool the filament and waste energy. That single reasoning step also explains why the premium lamps use krypton and xenon — heavier atoms, still poorer conductors of heat, and better at holding the tungsten in place — and why fluorescent tubes, which work differently, are filled with argon and mercury vapour instead.

• A common incandescent bulb is filled with about 93 per cent argon and 7 per cent nitrogen; krypton and krypton-xenon are used in better lamps.
• The fill gas keeps the tungsten below its vapour pressure, letting the filament run hotter without shortening its life.
• Helium is inert but conducts heat exceptionally well, is costly and leaks easily, so it is not used to fill lamps.
• Hydrogen's three isotopes are protium, deuterium and tritium; only tritium is radioactive.
• Rockets carry their own oxidiser because there is no air in space, and liquid oxygen is the standard choice.
• Nitrogen flushing displaces oxygen from food packets and prevents the fats in them from turning rancid.

• Treating all inert gases as interchangeable; thermal conductivity, cost and atomic weight decide which one a job gets.
• Assuming helium's inertness alone qualifies it for lamps, when the same property in argon comes far cheaper.
• Reading a statement item as a single-fact recall; each line has to be tested on its own.

Usually as a noble-gas-to-use match or as a not-correct statement item; the argon-in-bulbs and neon-in-signs pairings recur across UPSC, CDS and NDA.

• practice — not a real PYQ

  The gas commonly filled in an incandescent electric bulb is

  • (a)helium
  • (b)argon
  • (c)oxygen
  • (d)carbon dioxide

  Answer(b) argon — it is inert and conducts heat poorly, so it slows the evaporation of the tungsten filament without draining heat from it.

• practice — not a real PYQ

  Packets of potato chips are usually flushed with which gas, and why?

  • (a)Oxygen, to keep the chips crisp
  • (b)Nitrogen, to keep oxygen away so the fat does not turn rancid
  • (c)Carbon dioxide, to kill bacteria
  • (d)Hydrogen, because it is the lightest gas

  Answer(b) Nitrogen, to keep oxygen away so the fat does not turn rancid — nitrogen is unreactive, and displacing the air is what preserves the food.