Rusting and combustion

Rusting makes a new brown substance, rust (iron oxide), so it is a chemical change.

Burning magnesium makes a new white powder, magnesium oxide, and gives out heat and light:

Magnesium + Oxygen → Magnesium oxide + Heat + Light

A chemical reaction in which a substance reacts with oxygen and gives out heat and/or light is called combustion. Things that can burn are combustible: wood, paper, cotton, kerosene.

Is oxygen needed? Activity 5.5: light two candles and cover one with a glass tumbler. The open candle keeps burning. The covered one goes out after a while, because it gets no fresh air. The part of air that keeps a fire going is oxygen. Put a little lime water in the dish under the tumbler and it turns milky: the wax's carbon joined with oxygen to make carbon dioxide.

If someone's clothes catch fire, wrap a blanket or thick cloth round them. It cuts off the air, and the fire goes out. Never use a synthetic blanket: it can melt and stick to the skin.

Light without heat. On summer evenings you may see fireflies glowing. Their light comes from a chemical change in their bodies, with no heat. Light made by living things is called bioluminescence.