Which one of the following statements is true when non-metals are dissolved in water?
- (a)They produce basic oxides.
- (b)They produce acidic oxides.
- (c)They produce neutral oxides.
- (d)They provide hydroxides.
Correct — B, They produce acidic oxides. The result the item is reaching for is the standard divide of the periodic table by the character of the oxides. Non-metals form oxides that are acidic, and those oxides dissolve in water to give acids — sulphur dioxide gives sulphurous acid, sulphur trioxide gives sulphuric acid, nitrogen dioxide gives nitric acid, and carbon dioxide gives carbonic acid, which is why even unpolluted rainwater is slightly acidic at around pH 5·6. Metals do the opposite: their oxides are basic, and the soluble ones give alkalis, as sodium oxide gives sodium hydroxide and calcium oxide gives calcium hydroxide. Since option (b) is the only one that puts acidity on the non-metal side, it is the statement that stands.
- (a)They produce basic oxides. — This is the metal side of the divide, exactly reversed. Sodium oxide and calcium oxide dissolve to give alkalis and turn red litmus blue; non-metal oxides turn blue litmus red.
- (c)They produce neutral oxides. — True for a small handful and false as a rule. Carbon monoxide, nitrous oxide and nitric oxide are genuinely neutral oxides of non-metals, and water itself is one, but the general character of a non-metallic oxide is acidic, which is what the question is after.
- (d)They provide hydroxides. — Hydroxides are what soluble metal oxides give with water. A non-metal oxide dissolving in water yields an oxyacid — carbonic, sulphurous, sulphuric, nitric — not a hydroxide.
Oxides sort into four classes by how they behave with water, acids and bases. Basic oxides are formed by metals and give alkalis with water; acidic oxides are formed by non-metals and give acids. Amphoteric oxides such as aluminium oxide and zinc oxide react with both acids and bases, and neutral oxides such as carbon monoxide and nitrous oxide do neither. The pattern is one of the standard tests for telling a metal from a non-metal, alongside lustre, malleability, conduction and behaviour with dilute acid.
The printed stem is loosely worded and it is worth naming the looseness rather than working around it. Taken literally, dissolving a non-metal in water does not produce anything — most non-metals are barely soluble, and the oxide has to be made first by burning the element in oxygen. What the standard textbook result actually says is that the OXIDES of non-metals are acidic and give acids on dissolving in water. Read that way, option (b) is the intended and the only defensible choice, and the other three are the basic, neutral and hydroxide alternatives set against it. The key and the chemistry agree here; only the phrasing of the stem is imprecise, and the item is still answerable on the metal-versus-non-metal divide alone.
- Non-metallic oxides are acidic: SO₂ + H₂O gives H₂SO₃, and CO₂ + H₂O gives H₂CO₃.
- Metallic oxides are basic, and the soluble ones give alkalis — Na₂O + H₂O gives NaOH.
- Aluminium oxide and zinc oxide are amphoteric, reacting with both acids and bases.
- Carbon monoxide and nitrous oxide are neutral oxides, so 'non-metal oxide' does not automatically mean acidic.
- Dissolved carbon dioxide is why unpolluted rainwater has a pH of about 5·6 rather than 7.
The four classes of oxide. The metal and non-metal rows are the two the question sets against each other.
- Reversing the divide and attaching acidic oxides to metals.
- Treating every non-metallic oxide as acidic and forgetting the neutral exceptions.
- Confusing an oxyacid such as H₂CO₃ with a hydroxide such as NaOH.
As a general statement to be judged, as here, or through a named oxide — whether it turns litmus red or blue, or what it gives on dissolving in water.
Consider the following: 1. Oxides of Hydrogen 2. Oxides of Nitrogen 3. Oxides of Sulphur Which of the above causes/cause acid rain?
- (a) 1 and 2 only
- (b) 3 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer(c) 2 and 3 only
This principle doing work on the scale of the atmosphere. Nitrogen and sulphur are non-metals, so their oxides are acidic and turn into nitric and sulphuric acid in cloud water; the oxide of hydrogen is water itself, which is neutral.
CDS_GK_2021_I_Q1012021Which one of the following elements is a non-metal but is lustrous?
- (a) Carbon
- (b) Silicon
- (c) Germanium
- (d) Iodine
Answer(d) Iodine
The other way of testing the same boundary. Lustre is the property that most often misleads, since iodine has it while remaining a non-metal — which is why the character of the oxide is the more dependable test of the two.
- practice — not a real PYQ
Sulphur dioxide is passed into water and the solution is tested with litmus paper. The result will be that
- (a)red litmus turns blue, because a basic solution is formed
- (b)blue litmus turns red, because sulphurous acid is formed
- (c)neither paper changes colour, because the oxide is neutral
- (d)both papers turn colourless
Answer(b) blue litmus turns red, because sulphurous acid is formed — SO₂ is the oxide of a non-metal and so is acidic.
- practice — not a real PYQ
Which one of the following oxides reacts with both an acid and a base?
- (a)Sodium oxide
- (b)Carbon dioxide
- (c)Aluminium oxide
- (d)Carbon monoxide
Answer(c) Aluminium oxide — it is amphoteric, dissolving in acids as a base does and in alkalis as an acid does.