Reduction involves the gain of electrons or the loss of oxygen by a substance during a chemical reaction. This leads to a decrease in the oxidation state of the substance.

Oxidation refers to the process in which a substance gains oxygen or loses electrons during a chemical reaction. It results in an increase in the oxidation state of the substance.

The chemical equation for the formation of barium sulfate precipitate is: Ba²⁺ + SO₄²⁻ → BaSO₄. Barium ions (Ba²⁺) react with sulfate ions (SO₄²⁻) to produce insoluble barium sulfate (BaSO₄), which precipitates out of the solution.

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.