Question

on heating copper powder in air in a China dish the surface of copper powder turns black

Answer 1

the surface of copper turns black because the reaction of hot  copper metal  and and oxygen which is the reactive part of the air leads to formation of copper oxide

2CU  {Solid}+O2{gas}=2CUO{solid]

Answer 2

Copper, when heated, turns black because on heating it combines with oxygen and forms the copper oxide which is black in color as a result of which the surface of copper powder turns black.