The Composition of Water (H2O) A molecule of water is composed of two atoms of hydrogen each individually bonded to oxygen by a single covalent bond. Electrolysis is using an electric current to bring about a chemical change in the liquid conducting the electric current. When an electric current passes through water some of the water is decomposed to its constituent elements, hydrogen and oxygen.

Pure water is a bad conductor of electricity so to improve its ability to conduct a little sulphuric acid is often added. The water is then often titled ‘acidulated water’. After electrolysis the same amount of sulphuric acid is present at the end as added at the start – so sulphuric acid is like a catalyst but not one (dept of ed June 2006).

An electrode is a conductor delivering an electric current to or taking electric current from an electrolyte.

Cathode: the negative electrode at which the electric current leaves the electrolyte. Anode: the positive electrode at which the electric current enters the electrolyte. (A for anode, a for addition, addition sign is +)

Decomposition of Water by Electrolysis

1. Two graphite carbon electrodes are in the acidulated water.

2. Each electrode is covered by a wide test tube filled with water.

3. The electrodes are connected to a battery and the switch closed.

4. An electric current flows in the circuit through the water.

5. Colourless gas bubbles form at and rise from each electrode when the electric current flows.

6. The gas from each electrode is collected in its test tube and its volume measured.

7. The gas collected from the anode relights a glowing splint — it is oxygen.

8. The gas collected from the cathode burns with a ‘pop’ — it is hydrogen.

9. Therefore water is composed of hydrogen and oxygen.

10. Twice as much hydrogen is produced than oxygen. The volume of water in the apparatus decreased.

Hydrogen is collected at the cathode: H is the symbol for hydrogen and there is a H in catHode. A special item of equipment used to demonstrate electrolysis of water is Hofmann Voltameter.