NDA (II) 2017 General Ability Test · Question 117 of 150

Correct — A, Oxygen. Dry air near the surface is 78.08 per cent nitrogen and 20.95 per cent oxygen by volume, so nitrogen is first and oxygen second, with the two together making up more than 99 per cent of the atmosphere. Argon comes third at about 0.93 per cent and carbon dioxide a distant fourth. The whole item is a ranking question with the first place given away in the options, so the answer is simply the runner-up.

• (b)Hydrogen — Hydrogen is the commonest element in the universe but only a trace gas in the Earth's atmosphere. Being so light, it escapes the planet's gravity readily, and it does not appear among the major constituents of air.
• (c)Nitrogen — Nitrogen is the correct answer to a different question. At 78 per cent it is first, not second, and the option is placed here to catch a candidate who reads the stem too quickly.
• (d)Carbon dioxide — Carbon dioxide is fourth at only about 0.04 per cent by volume, behind argon. It matters enormously for climate and for photosynthesis, but importance is not abundance.

Composition figures for air are quoted for dry air, because water vapour varies from almost nothing to about 3 per cent depending on place and weather and would otherwise make the numbers unstable. On that basis the order is nitrogen, oxygen, argon, carbon dioxide, then neon, helium, methane, krypton and the rest in traces. Nitrogen and oxygen are both diatomic molecules, and their overwhelming share is why they are called the permanent gases of the atmosphere.

The abundance ranking should be held separately from three other rankings students often merge with it. The most abundant greenhouse gas by concentration among the classic four is carbon dioxide, ahead of methane, nitrous oxide and the chlorofluorocarbons. The most abundant element in the Earth's crust is oxygen, followed by silicon. And the most abundant element in the universe is hydrogen. Each of these is a favourite one-line question, and each has a different answer from the one here. Note also that carbon dioxide's share has kept rising since this paper was set, from about 0.040 per cent then to a higher figure now, though it remains far below argon.

• Dry air by volume is 78.08 per cent nitrogen, 20.95 per cent oxygen, 0.93 per cent argon and about 0.04 per cent carbon dioxide.
• Nitrogen and oxygen together make up more than 99 per cent of the atmosphere.
• Water vapour is excluded from these figures because it varies from near zero to roughly 3 per cent.
• Argon, an inert gas, is the third most abundant constituent, ahead of carbon dioxide.
• Oxygen is also the most abundant element in the Earth's crust, while hydrogen is the most abundant element in the universe.

• Answering nitrogen because it is the gas most associated with air, when the stem asks for the second place.
• Ranking carbon dioxide above argon; carbon dioxide is important but scarcer.
• Confusing abundance in the atmosphere with abundance in the crust or in the universe.

NDA asks for the share of a named gas, or for the gas that ranks first, second or third, or which layer holds a given phenomenon.

• practice — not a real PYQ

  Which one of the following is the third most abundant gas in dry air ?

  • (a)Carbon dioxide
  • (b)Argon
  • (c)Neon
  • (d)Methane

  Answer(b) Argon — about 0.93 per cent by volume, ahead of carbon dioxide.

• practice — not a real PYQ

  Figures for the composition of air are normally quoted for dry air because

  • (a)water vapour is not a gas
  • (b)the proportion of water vapour varies widely from place to place and time to time
  • (c)water vapour is heavier than air
  • (d)water vapour has no effect on weather

  Answer(b) the proportion of water vapour varies widely from place to place and time to time — it can range from almost nothing to about 3 per cent.