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? 

Explanation

(a) A mixture of ZnO(s) and `CO_(2)` is formed
`ZnCO_(3) overset("heat")rarrZnO(s) + CO_(2)(g)`
(b) A mixture of copper and `SO_(2)` is formed.
`2Cu_(2)O(s) + Cu_(2)S(s) overset("heat")rarr 6Cu(s) + SO_(2)(g)`.