Question

State the change in colour observed in following case mentioning the reason:

Copper powder is strongly heated in air.

Answer 1

Copper powder turns from brown to black when heated vigorously in the presence of oxygen because oxygen reacts with copper powder to form copper oxide, which has a dark colour. Thus, the appearance of copper oxide is what causes the colour change.

 \[\ce{2Cu(brown) +O2 ->2CuO(black)}\]