Question

brown coloured copper powder on heating in air turns into black coloured substance. Why?

Answer 1

This is because of the reaction :
Cu + O2 --> CuO

Copper oxide is black in colour... So this is the black colored substance..

Answer 2

When brown coloured copper powder is heated in air it reacts with atmospheric Oxygen and turns into black colour due to the formation of Copper Oxide which is actually black in colour.

The black coloured copper oxide is actually Cupric Oxide (CuO) with an oxidation state of 2+ for copper metal.

The process of heating of Copper in air can have an intermediate colour formation of red colour which is actually Cuprous Oxide $\bold{\left(\mathrm{Cu}_{2} \mathrm{O}\right)}$. Here the copper has an oxidation state of 1+.