NDA (II) 2022 General Ability Test · Question 71 of 150

Correct — B, helium. Divers going deep breathe heliox, oxygen diluted with helium instead of with nitrogen. Helium is chosen for two properties. It is not narcotic at high pressure, so it avoids the nitrogen narcosis that dulls and endangers a diver breathing compressed air at depth. And it is far lighter — about 0·5 g/L against air's 1·25 g/L at standard conditions — so it flows more easily through the airways and cuts the work of breathing when the gas is compressed at depth.

• (a)chlorine — Chlorine is a toxic, corrosive gas that attacks the lungs and was used as a chemical weapon in the First World War. Nothing about a breathing mixture could include it.
• (c)nitrogen — Nitrogen is the gas being replaced, not the one added. It makes up about 78% of ordinary air, but under the high pressures of a deep dive it becomes narcotic, and the nitrogen dissolved in the tissues is what forms bubbles and causes decompression sickness on a fast ascent.
• (d)ozone — Ozone is a highly reactive and toxic form of oxygen that damages lung tissue. It shields the Earth from ultraviolet radiation in the stratosphere; it is never breathed deliberately.

A diver breathes gas at the pressure of the surrounding water, so at depth every gas in the cylinder is being taken in at many times its usual partial pressure. That changes the physiology of gases we normally ignore. Nitrogen turns narcotic, oxygen itself becomes toxic beyond a limit, and the denser the mixture the harder the lungs must work to move it. Deep-diving mixtures therefore swap nitrogen for helium — heliox — and adjust the oxygen fraction with depth.

Every distractor here is a gas that would harm the diver, so the question rewards knowing why an inert, light diluent is needed at all. Helium's virtues are precisely that it is chemically inert, physiologically non-narcotic under pressure and very low in density. As of the September 2022 exam heliox was already long-established in commercial and saturation diving; that remains true today, alongside trimix, a three-gas mixture that adds a controlled amount of nitrogen back to the helium and oxygen. Helium's low density also gives the familiar squeaky voice, and the same property makes heliox useful in hospitals to ease breathing through an obstructed airway.

• Heliox is a helium–oxygen breathing mixture used in deep and saturation diving.
• Helium is not narcotic at high pressure, so it avoids the nitrogen narcosis that limits diving on compressed air.
• Helium's density is about 0·5 g/L against 1·25 g/L for air at standard conditions, which lowers airway resistance and the work of breathing.
• Ordinary air is roughly 78% nitrogen and 21% oxygen; at depth both components create problems, which is why the mixture is changed.
• Heliox is also used medically to help patients breathe through a severely obstructed airway.

• Choosing nitrogen because it is the main gas in the air we normally breathe.
• Assuming any inert-sounding gas would do — chlorine and ozone are both toxic.
• Thinking the helium is there to supply extra oxygen; it is a diluent, not a fuel.

As a straight recall item like this, or through the reason behind it — why nitrogen is removed, or what causes the bends.

No directly related past PYQ was found.

• practice — not a real PYQ

  Deep-sea divers breathe oxygen diluted mainly with

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

  Answer(b) helium — the helium–oxygen mixture is called heliox.

• practice — not a real PYQ

  Helium is preferred to nitrogen in deep-diving gas mixtures because helium

  • (a)supplies additional oxygen to the blood
  • (b)is not narcotic at high pressure and is much less dense
  • (c)is heavier and therefore easier to compress
  • (d)reacts with carbon dioxide in the lungs

  Answer(b) is not narcotic at high pressure and is much less dense — it avoids nitrogen narcosis and lowers the work of breathing.