This grade 8 science lesson on combustion notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on combustion and check yourself with the quiz.
Quick answer: Combustion is a fast reaction of a fuel with oxygen that releases heat and light. Fire needs fuel, oxygen and heat, and removing any one puts it out. Complete combustion of carbon fuels makes carbon dioxide and water, while limited oxygen makes poisonous carbon monoxide.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-2
Combustion notes: the key ideas
What is combustion?
A cooking flame, a candle and a car engine all depend on the same chemistry. Combustion is a reaction in which a substance reacts quickly with oxygen and gives out energy as heat and light. It is an exothermic reaction, meaning energy goes out into the surroundings.
Combustion is the reaction of a fuel with oxygen that releases heat and usually light. A fuel is a substance that burns to release useful energy, such as wood, charcoal, methane or ethanol.
What does a fire need?
Fire needs three things at once: fuel, oxygen and enough heat to start the reaction. This is the fire triangle. If any one side is removed, the fire stops.
| Method | Which side it removes | Example |
|---|---|---|
| Water spray | Heat | Camp fire in a park |
| Fire blanket or lid | Oxygen | Burning pan in a kitchen |
| Closing a gas valve | Fuel | Leaking cooker pipe |
| CO₂ extinguisher | Oxygen (and some cooling) | Electrical fire in an office |
What are the products of burning carbon and methane?
When there is plenty of oxygen the burning is complete combustion. A fuel containing carbon and hydrogen then makes carbon dioxide and water vapour. The atoms are only rearranged, so the equation must balance.
Carbon: C + O₂ → CO₂
Methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O
Tunde in Ibadan burns 16 g of methane with 64 g of oxygen and collects 44 g of carbon dioxide. What mass of water forms?
Mass in = 16 + 64 = 80 g. Mass out = 44 + water, so water = 80 − 44 = 36 g.
Isabel in Mexico City uses a spirit burner with ethanol: C₂H₅OH + O₂ → CO₂ + H₂O. Balance it.
Carbon: 2 CO₂. Hydrogen: 6 H, so 3 H₂O. Oxygen on the right: 4 + 3 = 7; the ethanol supplies 1, so 6 more atoms, which is 3 O₂.
C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O. Check: C 2/2, H 6/6, O 7/7.
How do you test for carbon dioxide?
Bubble the gas through limewater, which is a clear solution of calcium hydroxide. Carbon dioxide reacts with it to form tiny white crystals of calcium carbonate that make the liquid look milky.
Keep bubbling and the milkiness can vanish again, because extra CO₂ turns the carbonate into a soluble substance. A burning splint is not a CO₂ test: many gases put out a flame. Only limewater going milky confirms CO₂.
Why is carbon monoxide dangerous?
If the supply of oxygen is limited, incomplete combustion happens. It makes carbon monoxide (CO) and soot (carbon) instead of only CO₂, and it releases less energy. For example, 2 CH₄ + 3 O₂ → 2 CO + 4 H₂O. Carbon monoxide has no colour, no smell and no taste, but it stops the blood carrying oxygen and can be fatal.
| Feature | Complete combustion | Incomplete combustion |
|---|---|---|
| Oxygen supply | Plenty | Limited |
| Carbon products | CO₂ | CO and soot |
| Flame | Blue, clean | Yellow, sooty |
| Energy released | More | Less |
Link each flame colour to a risk: blue means plenty of air, yellow means too little air. A yellow flame on a gas cooker or heater is a signal to get it checked.
- Fire triangle: fuel, oxygen and heat; remove one to stop a fire.
- Complete combustion of methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O.
- Limewater turns milky when carbon dioxide is bubbled through it.
- Incomplete combustion makes carbon monoxide (CO) and soot.
- Carbon monoxide is colourless, odourless and toxic.
Fire, fuels and burning safely: frequently asked questions
What are the three parts of the fire triangle?
They are fuel, oxygen and heat. A fire needs all three together, so removing any one of them puts the fire out.
What are the products of complete combustion of methane?
Carbon dioxide and water, shown by CH₄ + 2 O₂ → CO₂ + 2 H₂O. The atoms are only rearranged, so each side of the equation has one carbon, four hydrogen and four oxygen atoms.
How does limewater test for carbon dioxide?
Carbon dioxide reacts with clear limewater to make white calcium carbonate, so the liquid turns milky. If the water stays clear, the gas is not carbon dioxide.
Why is carbon monoxide so dangerous?
It is made when fuels burn without enough oxygen and has no smell or colour. It stops blood carrying oxygen around the body, so people may not notice the danger until they feel unwell.
What is the difference between complete and incomplete combustion?
Complete combustion has plenty of oxygen and makes CO₂ and water with a blue flame. Incomplete combustion has too little oxygen and makes carbon monoxide and soot with a yellow, sooty flame.
