Two chemical changes driven by something other than heat — one by electricity, one by light.
Electrolysis is the breaking down of substances using electrical energy. It is one of the "breaking down" chemical changes, but the trigger is an electric current rather than heat.
When electricity is passed through water containing some dissolved salt or acid, using platinum electrodes, the water is decomposed into its elements:
water --electricity--> hydrogen + oxygen
Two volumes of hydrogen are produced for every one volume of oxygen. The hydrogen forms at the negative electrode and the oxygen at the positive electrode.
Pick what to put in the cell, then switch the current on. Watch the rate of bubbling at each electrode.
Sodium chloride can also be decomposed into its elements by the electrolysis of molten sodium chloride, with graphite rods as the electrodes:
sodium chloride --electricity--> sodium + chlorine
Sodium metal forms at the negative electrode and chlorine gas at the positive one. In industry, an iron screen keeps the two apart so they cannot meet and react back together.
Electroplating uses electrolysis to coat an object with a thin layer of metal — to protect it, to make it look better, or to improve its electrical conductivity.
The rule to remember: the object being plated is always the negative electrode, and the metal used for coating is the positive electrode. Positively charged metal particles move through the solution and build up on the object layer by layer.
Electrolysis involves both oxidation and reduction happening at the same time. Oxidation occurs at the positive electrode (the anode) while reduction occurs at the negative electrode (the cathode).
| At the anode (+) | At the cathode (−) | |
| Water | OH⁻ lose electrons → O₂ and H₂O | H⁺ gain electrons → H₂ |
| Molten NaCl | Cl⁻ lose electrons → Cl₂ | Na⁺ gain electrons → Na |
Photosynthesis is the process by which plants manufacture food, using carbon dioxide, water and light energy from the Sun. Chlorophyll in the leaves absorbs the light energy needed to drive the reaction.
carbon dioxide + water --light--> sugar (glucose) + oxygen
It is vital for the survival of all living beings, including humans and animals — and plants themselves — because oxygen is created as a by-product.
photosynthesis: carbon dioxide + water → sugar + oxygen
respiration: sugar + oxygen → carbon dioxide + water
They are exact opposites. Respiration, decomposition by bacteria, and the combustion of fossil fuels all release carbon dioxide; photosynthesis absorbs it. That balance is why planting trees is encouraged to help slow climate change — Singapore has its own annual tree-planting day.