What happens when

(a) When zinc carbonate is heated in the absence of oxygen, zinc oxide and carbon dioxide are produced. The process of reduction carbonate ores to form oxides in absence of oxygen is called calcination. Zinc is found in its ore – zinc carbonate. When this ore is heated, zinc oxide is formed along with carbon dioxide.

ZnCO₃ → ZnO + CO₂

(b) When a mixture of copper oxide and copper sulphide is heated, pure copper is obtained. It undergoes auto reduction. When sulphide ore of copper is heated, a part of this ore is converted into oxide which then reacts with remaining sulphide ore to give pure copper and sulphur dioxide.

2Cu₂O + Cu₂S → 6Cu + S