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) Zinc oxide and carbon dioxide are produced when `ZnCO_(3)` is heated in the absence of oxygen. This process is called calcination.
`underset("Zinc carbonate")(ZnCO_(3)(s))overset("Calcination")tounderset("Zinc oxide")(ZnO(s))+CO_(2)(g)`
Moisture and volatile impurities are also expelled out during the process cf calcination.
(b) Copper metal and sulphur dioxide gas are produced when a mixture of `Cu_(2)O" and "Cu_(2)S` is heated. 
`underset("Cuprous oxide")(ZnCO_(3)(s))+underset("Courous sulphide")(Cu_(2)(s))underset("reduction")overset("Heat")tounderset("Copper")(6Cu(s))+underset("Sulphur dioxide")(SO_(2)(g))`