When sodium chloride is dissolved in water, it ionises as NaCl⇌Na++Cl−. Water also dissociates as : H2O⇌H++OH− During passing of electric current through this solution using platinum electrode, Na+ and H+ ions move towards cathode. However, only H+ ions are discharged more readily than Na+ ion because of their low discharge potential (In the electromotive series hydrogen is lower than sodium). These H+ ions gain electrons and change into neutral atoms. At cathode H++e−→H,H+H→H2 Cl− and OH− ions move towards anode. Cl− ions lose electrons and change into neutral atom. At anode, Cl−−e−→Cl, Cl+Cl→Cl2 If mercury is used as cathode, H+ ions are not discharged at mercury cathode because mercury has a high hydrogen overvoltage. Na+ ions are discharged at the cathode in perference to H+ ions, yielding sodium, which dissolves in mercury to form sodium amalgam. At cathode : Na++e−=Na