Describe an experiment to show that water contains hydrogen and oxygen only

Electrolysis

Explanation:

The electrolysis of water is one such experiment that shows that water is made up of hydrogen and oxygen atoms only in the ratio of 2 to 1.

In the electrolysis of water, electricity is passed through acidified water to cause it to decompose.

The electrolysis of water is also known as the electrolysis of dilute tetraoxosulphate  (VI) acid.

At the cathode, H⁺ ions are discharged and hydrogen gas is liberated:

               2H⁺       +     2e⁻     →        H₂

At the anode, both the sulfate ion and hydroxyl ions migrate to this electrode. Only the OH⁻ is selected for preferential discharge due to its lower position in that activity series.

         4OH⁻  →   2H₂O    +    O₂     +     4e⁻

Oxygen gas is produced at the anode.

This electrolysis demonstrates the volumetric composition of water that is, 2 volumes of hydrogen at the cathode and 1 volume of oxygen at the anode.