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Question

In the reaction where copper(II) oxide reacts with hydrogen gas to form copper and water, which substance undergoes reduction?

The correct answer is

Copper(II) oxide

The reaction is CuO + H2 → Cu + H2O. In terms of oxygen, reduction is the removal of oxygen from a substance (or addition of hydrogen), while oxidation is the addition of oxygen to a substance.

In this reaction, copper(II) oxide loses its oxygen and changes into copper metal. So copper(II) oxide is the substance that is reduced; in terms of electrons, Cu2+ gains two electrons to become Cu.

Hydrogen gains oxygen to form water, so hydrogen is oxidised and acts as the reducing agent. Water and copper are the products formed in the reaction; they are not the substances that undergo reduction.

Hence, Copper(II) oxide.

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