Which statement describes a typical observation when copper oxide is reduced by hydrogen?

• A. Black to brown, because copper oxide is reduced to copper.
• B. Brown to black.
• C. Green to blue.
• D. Red to orange.

Answer: (A)

When a reducing agent like hydrogen donates electrons to a metal oxide, the oxide loses oxygen and becomes the metal. For copper oxide, CuO is black, while copper metal is reddish-brown. So, as CuO is reduced by hydrogen, the solid changes color from black to brownish-red, indicating the oxide has been turned into copper metal. This visible color change is a typical and useful clue that reduction has occurred.