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

• A. Black to brown, because copper oxide is reduced to copper.
• B. Blue to green.
• C. White to gray.
• D. Red to pink.

Answer: (A)

Reduction of a metal oxide to the metal changes its color. When hydrogen reduces copper oxide (CuO), it forms copper metal and water. CuO is black; metallic copper is reddish-brown, so the observed color change is black to brown. The other options describe color changes that don’t occur for this reaction.