Chemical Concepts Demonstrated: Processes of oxidation and reduction
The copper metal turns black when it is heated in the burner.
When the metal is placed in the H2 atmosphere inside the funnel, it returns to the original copper color.
When the funnel is removed, the copper turns black again.
Explanations (including important chemical equations):
2 Cu (s) + O2 (g) ---> 2 CuO (s)
CuO (s) + H2 (g) ---> Cu (s) + H2O (g)
Heated copper metal reacts with oxygen to form the black copper oxide. The copper oxide can then react with the hydrogen gas to form the copper metal and water. When the funnel is removed from the hydrogen stream, the copper was still be warm enough to be oxidized by the air again.