CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 17(b)

(a) Amphoteric oxides are metal oxides that react with both acids and bases to form a salt and water; they show both basic and acidic behaviour. Two examples: aluminium oxide (${\text{Al}}_2{\text{O}}_3$) and zinc oxide ($\text{ZnO}$). For example, ${\text{Al}}_2{\text{O}}_3+6\text{HCl}\to 2{\text{AlCl}}_3+3{\text{H}}_2\text{O}$ and ${\text{Al}}_2{\text{O}}_3+2\text{NaOH}\to 2{\text{NaAlO}}_2+{\text{H}}_2\text{O}$ (sodium aluminate). (b) Cinnabar is the ore of mercury: mercury(II) sulphide, $\text{HgS}$. Extraction: mercury is low in the reactivity series, so it is obtained by heating alone. Cinnabar is first heated strongly in air (roasting), which changes it into mercuric oxide: $2\text{HgS(s)}+3{\text{O}}_2\text{(g)}\stackrel{\text{heat}}{\to }2\text{HgO(s)}+2{\text{SO}}_2\text{(g)}$ On further heating, the mercuric oxide is reduced to mercury: $2\text{HgO(s)}\stackrel{\text{heat}}{\to }2\text{Hg(l)}+{\text{O}}_2\text{(g)}$