# When I do water electrolysis, which water molecules are split apart?

If I do water electrolysis I will get hydrogen at one electrode and oxygen at the other.

Is it because a molecule of water somewhere in the middle splits into H and O and then the H and O travel to the appropriate electrodes? Or is it something more complicated?

• All explained here. – Farcher May 27 at 5:44

Even in "de-ionized" water, some fraction of the water molecules in the bulk will dissociate into $$\rm H^+$$ and $$\rm OH^-$$ ions.
In electrolysis, the $$\rm H^+$$ ions migrate towards the cathode, where they find each other, steal electrons from their surroundings (including the cathode) and form $$\rm H_2$$ gas. Likewise the $$\rm OH^-$$ ions migrate to the anode, further dissociate, and form $$\rm O_2$$ gas. But these migrations are mostly of ions that are already near the electrical terminals. The bulk migration of ions in the fluid, like the "drift velocity" of electrons in a metallic circuit, is surprisingly slow.
• I presume that they turn into $\rm H^+$, which is stable in the bulk, and their migration away from the anode is part of the charge-exchange process. But I don't know the details of that process. – rob May 27 at 5:51