Giving chemical equations,answer the following:
$(a)$ What happens when copper is heated in air?
$(b)$ What happens when the product obtained in the above reaction is heated in hydrogen?

(N/A) When copper is heated in air,it reacts with oxygen to form copper$(II)$ oxide,which is a black-colored substance.
Equation: $2Cu(s) + O_2(g) \xrightarrow{\text{Heat}} 2CuO(s)$
$(b)$ When the product obtained (copper$(II)$ oxide) is heated in hydrogen gas,it undergoes a reduction reaction to form copper metal and water.
Equation: $CuO(s) + H_2(g) \xrightarrow{\text{Heat}} Cu(s) + H_2O(l)$
The black coating turns brown as copper metal is formed.