Question

In the electrolysis of water,
Why is the volume of gas collected at one electrode double than the other?

Answer 1

In electrolysis, water is decomposed in the presence of electricity to its components. The reaction is shown as below:

$2H_{2}O\left(l\right)\to 2H_2\left(g\right)+O_2\left(g\right)$

As you can see that water splits in to $2$ molecules of hydrogen and $1$ molecule of oxygen. Since, number of molecules of hydrogen released is double the number of molecules of oxygen released, Volume occupied by hydrogen gas is double the volume occupied by oxygen gas.

Hence, electrode at which hydrogen gas is collected (Cathode) shows double the volume than the electrode at which oxygen gas is collected (Anode).