Question

1 g of copper powder was taken in a china dish and heated, what changes take place on heating? When hydrogen gas is passed over this heated substance, a visible change is seen in it. Give the chemical equation of reaction, the name, and the color of the products formed in each case.

Answer 1

Hint: When the copper is heated, the heating occurs in the presence of oxygen, so the copper gets oxidized. When hydrogen is passed over oxidized copper, the reduction of copper takes place, because the hydrogen is used as a reducing agent.

Complete step by step answer:
When copper is heated, it is heated in the presence of oxygen. So the copper gets oxidized. Therefore, when the copper powder is heated in a china dish, copper oxide is formed. So, the red-brown color of copper is changed into black colored copper oxide. The reaction is given below:
\[\underset{\text{Red brown}}{\mathop 2Cu}\,+\underset{Air}{\mathop {{O}_{2}}}\,\xrightarrow{Heat}\underset{Black}{\mathop 2CuO}\,\]

Now hydrogen gas is passed from this product. We know that hydrogen is a reducing agent and it is used to reduce the copper oxide. So, when the hydrogen gas is passed over the copper oxide, copper metal is formed. The black-colored copper oxide is changed into brown colored copper metal. The reaction is given below:

$\underset{Black}{\mathop CuO}\,+{{H}_{2}}\to \underset{Brown}{\mathop Cu(s)}\,+{{H}_{2}}O$
So, the powdered copper changes to copper oxide, and then it changes to copper metal.

Note: In this reaction, chemical change and physical change occurs. Chemical change because the reactant is converted into products in both reaction and physical change because of the color changes in both reactions.