During electrolysis of aqueoussolution of NaCl in presence of platinum electrodes , hydrogen is evolved at tha cathode , but in presence of mercury as cathoelede , sodium amalgam is produced during electrolysis Why ?

Explanation

2nd part : The cations present in the solution are present in the solution . This is why `H_(2)` gas is evolced when NaCl solution is electrolysed in presence of Pt - electodes . But when Hg is used as the cathode , reduction of `Na^(+)(aq)` to Na(s) , instead of reduction of `H^(+)(aq)` to `H_(2)(g)` , takes place due to hydrogen over Voltage . Na metal so produced forms sodium amalgam with Hg