NDA (II) 2016 General Ability Test · Question 141 of 150

Correct — B, carbon dioxide, which the booklet prints as one word. Carbon dioxide is the oxide of a non-metal, and non-metal oxides are acidic. Dissolved in water it reacts to give carbonic acid, H₂CO₃, which then partly ionises to release hydrogen ions and drop the pH below 7. The everyday demonstrations are all around you — a fizzy drink is mildly acidic for exactly this reason, blowing exhaled breath through lime water turns it milky, and the same reaction makes ordinary rainwater slightly acidic even where the air is clean, which is what slowly dissolves limestone into caves and karst.

• (a)Hydrogen — Hydrogen is a neutral, almost insoluble gas. It carries no oxygen and forms no acid with water; the element only produces an acid when it is combined with a more electronegative element, as in hydrogen chloride.
• (c)Nitrogen — Nitrogen gas is famously unreactive because of the very strong triple bond between its two atoms, and it is only slightly soluble in water. Its oxides do give acids, but nitrogen itself does not.
• (d)Oxygen — Oxygen dissolves in water — which is what fish breathe — but it does not react to form an acid. A solution of dissolved oxygen stays neutral.

Oxides fall into groups by the nature of the element they contain. Oxides of non-metals such as carbon, sulphur, nitrogen and phosphorus are acidic — dissolved in water they give acids and they neutralise bases. Oxides of metals such as sodium, calcium and magnesium are basic and give alkalis with water. A few, including aluminium and zinc oxide, are amphoteric and react with both acids and bases, and a small group including carbon monoxide and nitrous oxide is neutral. Only carbon dioxide in this list is an oxide at all, and it is a non-metal oxide, so it is the only candidate.

The elimination here is faster than the chemistry. Three of the four options are elemental gases with no oxygen attached, and an element by itself cannot give an acid with water — only the oxide can. That leaves carbon dioxide before any equation is written. It is worth knowing where the boundary of this fact lies: dissolved carbon dioxide makes clean rain naturally acidic, at about pH 5.6, but that is not what is meant by acid rain. Acid rain, with a pH well below that, is caused by sulphur dioxide and oxides of nitrogen from burning coal and fuel oil, which give the far stronger sulphuric and nitric acids.

• Carbon dioxide dissolves in water to give carbonic acid, H₂CO₃, which turns blue litmus red.
• Non-metal oxides are acidic, metal oxides are basic, and oxides such as aluminium oxide and zinc oxide are amphoteric.
• Clean rainwater is naturally slightly acidic, around pH 5.6, because of dissolved carbon dioxide.
• Acid rain proper is caused by sulphur dioxide and oxides of nitrogen, which give sulphuric and nitric acids, not by carbon dioxide.
• Carbon dioxide turns lime water milky by forming insoluble calcium carbonate, the standard laboratory test for the gas.

• Confusing the natural mild acidity of rainwater with acid rain, which comes from sulphur and nitrogen oxides.
• Assuming any gas that dissolves in water changes its pH — oxygen dissolves readily and leaves the water neutral.
• Forgetting that an element and its oxide behave quite differently; nitrogen is inert while its oxides are strongly acidic.

This is a recurring one-line NDA chemistry item, and its variants ask for the acidic gas, the basic oxide or the amphoteric oxide, so learn the classification of oxides as a block rather than one example at a time.

• practice — not a real PYQ

  Which one of the following oxides is amphoteric in nature?

  • (a)Sodium oxide
  • (b)Aluminium oxide
  • (c)Sulphur dioxide
  • (d)Calcium oxide

  Answer(b) Aluminium oxide — it reacts with both acids and bases, unlike the clearly basic sodium and calcium oxides and the clearly acidic sulphur dioxide.

• practice — not a real PYQ

  When carbon dioxide is passed through lime water, the milkiness observed is due to the formation of

  • (a)calcium hydroxide
  • (b)calcium carbonate
  • (c)calcium bicarbonate
  • (d)calcium oxide

  Answer(b) calcium carbonate — it is insoluble, so it appears as a white suspension; passing more carbon dioxide dissolves it again as the soluble bicarbonate.