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.

The surface of copper powder becomes coated with black copper(II) oxide, resulting in a black coating on the copper surface.

The reaction is a double displacement reaction, where ions from the reactants exchange to form a precipitate (BaSO₄) and a soluble product (sodium chloride).

The chemical equation for the displacement reaction between iron and copper sulfate is: Fe + CuSO₄ → FeSO₄ + Cu. Iron reacts with copper sulfate to form iron sulfate and elemental copper.

Reactions that produce a precipitate can be classified as precipitation reactions. These reactions involve the formation of an insoluble solid (the precipitate) from the mixing of soluble reactants in solution.