In which of the following substances will there be :

(a) increase in mass

(b) decrease in mass

(c) no change in mass when they are exposed to air?

1. Sodium chloride
2. Iron
3. Conc. sulphuric acid
4. Table salt
5. Sodium carbonate crystals

(a) Increase in mass: Iron, conc. sulphuric acid, Table salt

Reason — Increase in mass is due to the absorbed water in case of sulphuric acid, Table salt, whereas, gain in mass 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 mass: 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 mass: 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.