In which of the following four substances will there be :

(a) increase in weight

(b) decrease in weight

(c) no change in weight, when exposed to air.

(i) sodium chloride

(ii) sodium carbonate crystals

(iii) conc. H₂SO₄ and

(iv) iron?

(a) Increase in weight : Iron, conc. sulphuric acid

Reason — Increase in weight is due to the absorbed water in case of sulphuric acid. Gain in weight of iron is due to the increased weight of oxygen which has combined with the iron to form iron oxide or rust.

(b) Decrease in weight : Sodium carbonate crystals

Reason — Decrease in mass is because sodium carbonate loses its water of crystallization on exposure to dry air.

(c) No change in weight : Sodium chloride

Reason — Pure sodium chloride is neither deliquescent nor efflorescent i.e., it does not absorb moisture from atmospheric air nor does it lose it, hence there is no change in mass.