When copper oxide is reacted with hydrogen, what is the colour change?

• A. Black to brown
• B. Brown to green
• C. Black to blue
• D. White to gray

Answer: (A)

When copper oxide is heated with hydrogen, the oxide is reduced to copper metal. Copper oxide is black, and the metal copper has a reddish-brown, metallic appearance. So the visible change is from black to brown. The reaction is CuO + H2 → Cu + H2O, with hydrogen acting as the reducing agent to remove oxygen from the oxide.