CBSE Class 10 Science 2019 · Paper 31/1/3 · Question 15

(a) In solid sodium chloride the ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions are held in fixed positions by strong electrostatic forces of attraction, so they cannot move and the solid does not conduct. On melting, heat overcomes these forces and the ions become free to move; in aqueous solution the ions separate and move freely in the water. These moving ions carry the current, so molten sodium chloride and its solution conduct electricity. (b) Nitric acid is a strong oxidising agent. It oxidises the surface of aluminium to a thin, tough layer of aluminium oxide, ${\text{Al}}_2{\text{O}}_3$, which covers the metal and protects it from further reaction, so its reactivity decreases. (c) Calcium and magnesium are highly reactive metals, near the top of the reactivity series. They react easily with other elements, such as oxygen, so in nature they occur only as compounds (for example carbonates, chlorides and sulphates) in their ores, never in the free state.