3 · Types of chemical change

Combustion

A substance (the fuel) reacts quickly with oxygen, releasing heat and usually light. Wood, candle wax, petrol and natural gas all burn this way. Combustion is fast and obvious — flames, heat you can feel from across a room.

fuel + oxygen → oxide(s) + energy

Thermal decomposition

Heat alone breaks a single compound apart into two or more simpler substances. Heat green copper carbonate strongly, and it breaks down into black copper oxide and carbon dioxide gas — a colour change and a gas given off, both signs of a chemical change.

copper carbonate --heat--> copper oxide + carbon dioxide

Oxidation

A substance combines with oxygen — slowly this time, without flames. Two everyday examples matter here:

Rusting: iron combines with oxygen and water to form rust (hydrated iron oxide) — the reason this needs its own experiment is coming up below.
Cellular respiration: the slow "burning" of glucose with oxygen inside your cells, releasing the energy your body runs on. Same core idea as combustion, but controlled and gradual.

A change you'll also meet Plants use light energy to drive a chemical change too — photosynthesis, turning carbon dioxide and water into glucose and oxygen. It's triggered by light rather than heat, but it's still a genuine chemical change: new substances form.

Try it: what does rusting actually need?

Rust doesn't just need air, and it doesn't just need water — it needs both at the same time. Toggle the two conditions below and drag the day slider to test it for yourself.

Quick check

Q An iron tool is kept in a sealed bag with a drying agent, in normal air. Will it rust? Why or why not?

No — the drying agent removes the water vapour from the air, so even though oxygen is present, water isn't. Rusting needs both at once.