← Chemical Changes

Lesson 3 of 8 · Textbook 11.2

Combustion, thermal decomposition, oxidation

Three of the four types of chemical change you need to recognise. The fourth — neutralisation — gets the next two lessons to itself.

Combustion

Combustion is a chemical reaction in which a substance is heated in the presence of oxygen to form one or more new substances. Heat and light are usually given out.

carbon + oxygen → carbon dioxide

methane + oxygen → carbon dioxide + water

magnesium + oxygen → magnesium oxide

Sparklers release a lot of heat and light as they burn. Petrol undergoes combustion inside a car engine, turning chemical potential energy into kinetic energy. In Singapore, burning natural gas in power plants is the main way electricity is generated.

Magnesium burning Magnesium burns in oxygen with a brilliant white flame, producing a white powder — magnesium oxide. Because it is so bright, magnesium was used in early flash photography, and it is still used today to make white fireworks. Magnesium oxide is also used as an antacid to relieve acid indigestion.

The fire triangle Extension

Combustion needs three things at once: fuel, an oxidant (usually oxygen) and heat. Remove any one and the fire goes out — which is exactly how every fire extinguisher works. Water removes heat; carbon dioxide pushes oxygen away from the fire.

Never use a water extinguisher on an electrical fire — water conducts electricity.

Thermal decomposition

Thermal decomposition is the breaking down of a substance into two or more simpler substances by the use of heat. Notice this is the "breaking down" group from Lesson 1 — one reactant in, two or more products out.

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

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

Do not confuse this with melting Melting is a physical change — the substance simply changes from solid to liquid and it is still the same substance. Thermal decomposition is a chemical change, because the products formed have different properties from the original substance. The word "thermal" means heat; "decomposition" means breaking down into simpler substances.

Watch a thermal decomposition happen

Green copper(II) carbonate is heated strongly, and the gas produced is passed through limewater. Watch both the solid and the limewater.

Heating copper(II) carbonate

copper(II) carbonate heat limewater
Solid in tubegreen solid
Limewaterclear and colourless

Oxidation

When a substance gains oxygen, it undergoes oxidation. Combustion is itself a fast, dramatic kind of oxidation, but two slower examples matter just as much:

iron + water + oxygen → rust

glucose + oxygen → carbon dioxide + water

A cut apple browning When fruit such as an apple is cut, it turns brown. Chemical substances on the cut surfaces react with oxygen. The change can be slowed by soaking the cut fruit in salt water or squeezing lemon juice over it — this reduces the amount of oxygen reaching the surfaces.

What does rusting actually need?

Rust needs water and oxygen — not one or the other. Three reference tubes are set up for you; tube D is yours to control.

The rusting investigation

Tube A

Water + air

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Tube B

Boiled water + oil — no air

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Tube C

Drying agent + air — no water

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Tube D

Your experiment

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Day 8

Check yourself