In the electrolysis of brine (concentrated sodium chloride solution), the gas liberated at the anode is

In the chlor-alkali process, chloride ions are oxidised at the anode to give chlorine gas, hydrogen gas is liberated at the cathode and sodium hydroxide is left in the solution. Sodium is not liberated as the metal because water is reduced in preference to sodium ions. Oxygen is not formed in the concentrated solution because chloride is discharged first.