When hydrogen gas is passed over heated copper(II) oxide, the black coating on the surface turns brown as the reverse reaction occurs, and copper metal is obtained.
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Passing hydrogen gas over heated copper(II) oxide results in a reduction reaction where hydrogen acts as a reducing agent. The reaction can be represented as:
CuO(s) + H₂(g) → Cu(s) + H₂O(g)
During this reaction, copper(II) oxide (CuO) is reduced to elemental copper (Cu), and water vapor (H₂O) is formed. The appearance of copper(II) oxide changes from its original black color to the characteristic reddish-brown color of copper metal. This alteration in color signifies the reduction of copper ions in the oxide back to their elemental form.