Mammoth Memory

Rule 1: Electrolysis and hydrogen

If the liquid contains hydrogen, then hydrogen will always form at the cathode and be given off in bubbles, unless the solution contains a metal that is less reactive than hydrogen.


Rule 1 - Hydrogen rich liquids will always give off hydrogen gas from the cathode in electrolysis
Imagine a high dragon (hydrogen) clinging to the roof of a cathedral (cathode).


You can see which metals are less reactive than hydrogen by looking at the reactivity series: 

Hydrogen is quite low on the reactivity series but there are metals that are less reactive.

So the electrolysis of a copper chloride solution (CuCl2 + H2O) produces copper at the cathode, because copper is less reactive than hydrogen; whereas the electrolysis of a sodium chloride solution (NaCl + H2O) produces hydrogen at the cathode, because sodium is more reactive than hydrogen.

 Hydrogen > then according to the reactivity series

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