Answer:
On passing Hydrogen gas over the heated black material coating on the surface of copper sheets :
It becomes Brown due to copper formation
Step-by-step explanation:
Copper reacts with atmospheric Oxygen (Moisture) and gets black because of the formation of copper oxide (CuO). This reaction is also called oxidation reaction .
On passing Hydrogen gas over the heated black material coating on the surface of copper sheets this black coating become brown. This is due to formation of Copper again. This is redox- reaction.
Here copper oxide is reduced to copper and Hydrogen is oxidised water.