What happens when
$(a)$ $ZnCO_{3}$ is heated in the absence of oxygen?
$(b)$ $A$ mixture of $Cu_{2}O$ and $Cu_{2}S$ is heated?

(N/A) It undergoes calcination. The chemical reaction is:
$ZnCO_{3} \xrightarrow{\text{Heat}} ZnO + CO_{2}$
$(b)$ It undergoes auto-reduction,forming copper metal and sulphur dioxide gas:
$2Cu_{2}O + Cu_{2}S \xrightarrow{\text{Heat}} 6Cu + SO_{2}$