NDA (II) 2016 General Ability Test · Question 52 of 150

Correct — B, both statements are individually true but Statement II is not the correct explanation of Statement I. Ozone really is O3, a molecule of three oxygen atoms and an allotrope of the ordinary diatomic oxygen we breathe, so Statement I is true. Roughly nine-tenths of all atmospheric ozone really does sit in the stratosphere, in the band about 15 to 35 km up that we call the ozone layer, so Statement II is true as well. But the two statements answer completely different questions. Statement I is about chemical composition — how many atoms make up one molecule. Statement II is about geographical distribution — where in the atmosphere those molecules pile up. Nothing about living in the stratosphere makes a molecule triatomic; ozone is O3 in a laboratory jar, in city smog and in a photocopier room just as much as it is at 25 km. Two true statements that never touch is exactly the situation option (b) describes.

• (a)Both the statements are individually true and Statement II is the correct explanation of Statement I — This would mean the stratosphere somehow gives ozone its third atom. It does not. The stratosphere explains why ozone accumulates there — ultraviolet light splits O2 into free atoms which then attach to other O2 molecules — but that is an explanation of Statement II itself, not of the molecular formula asserted in Statement I.
• (c)Statement I is true but Statement II is false — Statement II is a textbook fact. The great majority of atmospheric ozone lies in the stratosphere; only a small residue occurs near the ground, and there it counts as a pollutant rather than a shield.
• (d)Statement I is false but Statement II is true — Statement I is plainly true — the very formula O3 says three oxygen atoms. Candidates sometimes hesitate because they remember oxygen as diatomic, forgetting that ozone is oxygen's other allotrope.

Allotropes are different molecular or crystalline forms of the same element. Oxygen has two familiar allotropes: the diatomic O2 that supports respiration, and the triatomic O3, ozone, a pale blue gas with a sharp smell. Ozone is made high in the atmosphere when short-wave ultraviolet light breaks O2 into single oxygen atoms, each of which then joins an intact O2 molecule. Because that ultraviolet light is absorbed before it reaches lower levels, ozone forms and survives mainly in the stratosphere.

The examiner has paired a composition fact with a location fact and is checking whether a candidate notices that they are unrelated. A quick test that works on any statement-pair item is to ask what would change if the second statement were false. If all the ozone were in the troposphere instead, would ozone stop being O3? Obviously not — so Statement II cannot be the explanation of Statement I. Anchoring to the exam, this paper was written weeks before the Kigali Amendment to the Montreal Protocol was adopted in October 2016; that amendment has since come into force, and the successive scientific assessments now report the ozone layer slowly healing, with full recovery projected over the middle decades of this century and latest over Antarctica.

• Ozone is O3, a triatomic allotrope of oxygen; the oxygen gas of the air is the diatomic O2.
• About nine-tenths of atmospheric ozone lies in the stratosphere, concentrated roughly 15 to 35 km above the surface.
• Stratospheric ozone absorbs most incoming ultraviolet-B radiation, which is why temperature rises with height through the stratosphere instead of falling.
• Near the ground, ozone is a secondary pollutant and a principal ingredient of photochemical smog.

• Assuming that two true statements about the same substance must be linked as cause and effect.
• Confusing the protective stratospheric ozone with the harmful ozone formed near the ground.

Ozone appears in NDA GAT both as pure chemistry, on its formula and allotropy, and as environment, on depletion and the Montreal Protocol, so learn the molecule and the treaty together.

• practice — not a real PYQ

  Ozone and the oxygen gas of the atmosphere are best described as

  • (a)isotopes
  • (b)isomers
  • (c)allotropes
  • (d)isobars

  Answer(c) allotropes — different forms of the same element, O3 and O2.

• practice — not a real PYQ

  Temperature increases with height in the stratosphere mainly because

  • (a)it is closer to the Sun
  • (b)ozone there absorbs ultraviolet radiation
  • (c)water vapour there releases latent heat
  • (d)carbon dioxide there traps outgoing heat

  Answer(b) ozone there absorbs ultraviolet radiation — the absorbed energy warms the layer from within.