KS3 · Chemistry

Combustion

A fire needs fuel, oxygen and heat all at once. Take away any one and it dies. Every extinguisher works that way. So which side does each one remove?

The fire triangle

Which side does it take away?

Combustion needs fuel, oxygen and heat all at once. These are the three sides of the fire triangle. Each of these six things stops a fire. For each one, tick every side of the triangle it takes away. Watch out: some take away more than one.

  • A Fuel
  • B Oxygen
  • C Heat
  1. Spraying water from a water extinguisher
  2. Turning off the gas supply to a gas flame
  3. Using a carbon dioxide extinguisher
  4. Covering a small flame so no fresh air can reach it
  5. Spraying foam onto a burning liquid
  6. Leaving a flame until all of its fuel is used up

The heat side

?

Reason it through

Why does a fire keep burning once it's lit, even after you take the match away?

Link 1 of 4

First link · your turn

Fuel and oxygen are sitting together, but nothing is happening. What's missing?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Predict, then check

The candles are identical. Only the size of the beaker changes.

Three identical lit candles are covered at the same moment: one with a small beaker, one with a medium beaker and one with a large beaker. Which candle keeps burning the longest?

Inside the flame

Methane is the gas burned in a gas boiler. Tap each molecule to see what it's made of and where its atoms go. The formulas show what each molecule contains; this word equation doesn't show how many of each molecule react.

Tap a molecule to see what it does in the reaction.

What do you really think?

Five ideas about combustion

Five students each write one sentence about combustion.

Which one is closest to what you think right now?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Fuel, oxygen and heat: take one away and the fire goes out.

What you need to know

  • Combustion is a reaction in which a fuel reacts with oxygen and releases energy. A fuel is a substance that releases its stored energy when it undergoes combustion.
  • The fire triangle: combustion needs fuel, oxygen and an initial source of energy (usually heat) all at once. Take away any one side and combustion stops.
  • Combustion is an oxidation reaction: oxygen bonds to the elements in the fuel, so an oxide is always produced.
  • Hydrocarbon fuels contain only hydrogen and carbon atoms. When one burns: hydrocarbon fuel + oxygen → carbon dioxide + water.

The big picture

Combustion is a reaction in which a fuel reacts with oxygen and releases energy. It needs fuel, oxygen and heat all at once (the fire triangle), and taking away any one side stops it. Combustion is an oxidation reaction, so an oxide always forms: a hydrocarbon fuel burns to make carbon dioxide and water, even when you can't see them.

Key points

1The heat needed to start combustion varies: friction lights a safety match, while magnesium needs a Bunsen burner or blow torch. Once started, combustion carries on without further heating until the fuel or the oxygen is used up.
2Extinguishers take away sides of the triangle. Water removes heat. Carbon dioxide removes oxygen and heat. Foam on a burning liquid removes oxygen, fuel (flammable vapour) and heat.
3Methane, propane and petrol (mainly octane) are hydrocarbon fuels. In methane + oxygen → carbon dioxide + water, the carbon atoms end up in carbon dioxide and the hydrogen atoms end up in water.
4Carbon dioxide is one carbon atom bonded to two oxygen atoms; water is two hydrogen atoms bonded to one oxygen atom. The products of combustion form even though you usually can't see them.
5Burning is combustion that produces flames, but combustion can also happen without flames, as when charcoal is oxidised.
6The oxygen can come from oxidising chemicals instead of the air (as in rocket fuel). Hazard symbols warn of flammable, oxidising and explosive substances.
7A substance that has already combined with oxygen, such as copper oxide, cannot burn.

Worked example

Problem

Propane is a hydrocarbon fuel. Write the word equation for propane burning in oxygen, and say where its carbon atoms and its hydrogen atoms end up.

⚠ Watch out

Thinking that a fuel which burns away has been destroyed because nothing seems to be left. Its atoms have been rearranged into new substances (carbon dioxide and water for a hydrocarbon), which you usually can't see.

🧠

Memory hook

Think of the fire triangle as a three-legged stool. It only stands with all three legs (fuel, oxygen and heat) in place. Kick away any one leg and the whole thing falls over. That's what every fire extinguisher is doing.

✓

Check yourself

Close the lesson and try this: name the three sides of the fire triangle, then pick one side and describe a way of taking it away that stops a fire.

Flashcards

(15)
What is combustion?
A reaction in which a fuel reacts with oxygen and releases energy.
What is a fuel?
A substance that releases its stored energy when it undergoes combustion.
What happens to combustion if one side of the fire triangle is taken away?
It stops. Combustion needs fuel, oxygen and heat all at the same time.
Which side of the fire triangle does a water extinguisher take away?
Heat. The water cools the fuel.
How does a carbon dioxide extinguisher stop a fire?
It removes oxygen and it cools the fuel, so it takes away two sides.
How does foam stop a burning liquid?
It keeps oxygen out, stops flammable vapour (fuel) escaping and cools the liquid: all three sides.
Does it always take the same amount of heat to start combustion?
No. Friction is enough to light a safety match, but magnesium needs a Bunsen burner or blow torch.
Why does combustion keep going after the starting heat is removed?
The energy released by the burning fuel keeps the reaction going, until the fuel or the oxygen is used up.
Why is combustion called an oxidation reaction?
Oxygen bonds to the elements in the reactants, so an oxide is always produced.
Is burning the same as combustion?
Burning is combustion with visible flames. Combustion can also happen without flames, e.g. charcoal being oxidised.
What is a hydrocarbon?
A substance whose molecules contain only hydrogen and carbon atoms, e.g. methane, propane, octane.
What atoms make up carbon dioxide and water?
Carbon dioxide: one carbon and two oxygen atoms. Water: two hydrogen and one oxygen atom.
Why can't copper oxide burn?
Its copper has already combined with oxygen. It's already been oxidised.
Where can the oxygen for combustion come from, apart from the air?
Oxidising chemicals, e.g. in rocket fuel.
What do the flammable, oxidising and explosive hazard symbols warn you about?
Flammable: catches fire easily. Oxidising: can provide oxygen. Explosive: risk of a very rapid combustion reaction.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

More KS3 Chemistry topics

See the full KS3 Chemistry curriculum →

How this lesson was checked. This KS3 Chemistrylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.