You might have noted that when copper powder is heated in a china dish, the reddish brown surface of copper powder becomes coated with a black substance. How can the black coating on the surface be turned reddish brown.

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Solution

Dear Student

1.) The black substance is formed because of the oxidation of copper.

2.) The black substance is copper (II) oxide (CuO).

3.) 2Cu + O₂→ 2CuO

When hydrogen is passed over it, hydrogen displaces oxygen from copper oxide restoring back the brown colour of the copper.
The reverse reaction is

CuO + H₂—–> Cu + H₂O

Regards

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