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Question

Explain giving equation, what happens when:
(a) ZnCO3 is heated in the absence of air?
(b) a mixture of Cu2O and Cu2S is heated?

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Solution

(a) When zinc carbonate is heated in the absence of air, it is converted to zinc oxide with the evolution of carbon dioxide gas. This is one of the methods used to convert concentrated metal carbonates to metal oxides during extraction of metals. This method is called calcination. The given zinc carbonate is nothing but calamine ore of zinc, which gets converted to zinc oxide on calcination.

ZnCO3 Calcination ZnO + CO2Zinc carbonate Zinc oxide carbon dioxide(Calamine ore)

(b) When a mixture of Cu2O and Cu2S is heated, they react together to give copper metal and sulphur dioxide gas. This is one of the methods used to reduce sulphide ore of less reactive copper to copper metal. Copper glance,
Cu2S, is roasted in the presence of air to get converted to Cu2O. Cu2O then reacts with non-oxidised Cu2S ore in the absence of air to form copper metal.

2Cu2O + Cu2S Heated 6Cu + SO2 Copper (I) Copper (I) Copper Sulphur oxide sulphide metal dioxide

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