Which substance is reduced in this redox reaction? CuO (s) + H2 (

g. → Cu (s) + H2O (l)

Reduction occurs when a chemical specie gains electrons or its valence number is reduced. In the case of this reaction, The valency of copper before this reaction is + 2, the valency of oxygen is - 2 and valency of hydrogen is 0.

After the reaction, the valency of copper is 0, the valency of hydrogen is + 1 and the valency of oxygen remains - 2.

Therefore, Cu is reduced in this reaction.