How it works?

The electrolysis of water is a simple process where a liquid undergoes a chemical change while conducting an electric current. The electrolysis of water breaks the water molecule apart into is hydrogen and oxygen ions. Liquids that conduct electricity must contain freely moving ions in order to complete a circuit.

Water is a great conductor of electricity. When a water molecule is broken up we get two H+ ions but only one 02- ion. The electrolysis of water produces twice as much hydrogen gas as oxygen.

A simple experiment to demonstrate this principle:
If you place two wires connected to a battery the positive ions (the H+ ions) will be attracted to the negatively charged electrons on the wire. All ions strive to be neutral. FOr it to become neutral, it will need to collect an electron from the negatively charged end of the wire, creating a neutral hydrogen atom.

When the two are joined together it forms hydrogen gas. This is why you see bubbles rising from the negative end of the wire.

So which side is producing the hydrogen?
Remember, every water molecule is broken up with two H+ ions and only one 02- ion. The electrolysis of water produces twice as much hydrogen gas as oxgen, so the side with the most bubbles is producing the hydrogen gas.

Oxygen is also much heavier than hydrogen, being sixteen times heavier. So, the height of oxygen (half the quantity) is eight times more than hydrogen.