What is combustion?
Combustion is another word for burning. A fuel reacts quickly with oxygen and gives out heat and light.
It is a chemical change: new substances form and you cannot get the fuel back. Wax, wood, petrol and cooking gas all burn.
A substance that burns is combustible (paper, wood, kerosene). One that does not burn is non-combustible (stone, glass, iron nail in air).
Conditions for combustion: the fire triangle
Fire needs all three at the same time:
- Fuel: something that can burn.
- Oxygen: usually from air (air is about 21% oxygen).
- Heat: enough to reach the ignition temperature, the lowest temperature at which a substance catches fire.
Fuels with a low ignition temperature (petrol, matchstick head) catch fire easily. Wet wood will not light fast because water keeps it below its ignition temperature.
Fair test
To test one condition, change only that one and keep the rest the same. Example: two equal candles, one covered by a glass and one open. Only the oxygen supply is different.
Products: carbon dioxide, water and the limewater test
Most fuels contain carbon (C) and hydrogen (H). When they burn completely:
fuel + oxygen → carbon dioxide + water + energy
Example, methane (natural gas): CH₄ + 2O₂ → CO₂ + 2H₂O (+ heat).
Test for CO₂: bubble the gas through limewater (calcium hydroxide solution). It turns milky. Test for water: it turns blue cobalt chloride paper pink (or white anhydrous copper sulfate blue).
The energy was stored in the chemical bonds of the fuel and oxygen. Burning changes chemical energy into heat and light.
Complete and incomplete combustion
| Complete | Incomplete | |
|---|---|---|
| Air | plenty | too little |
| Flame | blue, hot, clean | yellow, sooty |
| Products | CO₂ + H₂O | CO, soot (C), H₂O |
| Energy | more | less (fuel wasted) |
Carbon monoxide (CO) has no colour or smell. It stops blood carrying oxygen and can kill. Never burn charcoal, coal or gas in a closed room without air flow.
Fuels and calorific value
A good fuel: easy to get, cheap, gives lots of heat, burns at a steady rate, leaves little smoke or ash, and is safe to store.
Calorific value = energy released when 1 g (or 1 kg) of fuel burns completely. Unit: kJ/g or kJ/kg.
- Wood ≈ 16 kJ/g
- Coal ≈ 25–33 kJ/g
- LPG / petrol ≈ 45–50 kJ/g
- Hydrogen ≈ 142 kJ/g (and makes only water)
Energy released = mass × calorific value.
Putting out fires and preventing explosions
Every fire-fighting method removes one side of the triangle:
- Remove heat: water cools the fuel below its ignition temperature.
- Remove oxygen: a fire blanket, sand, a lid on a pan, or a CO₂ extinguisher.
- Remove fuel: turn off the gas, cut a fire-break in a forest.
Never use water on burning oil (it splashes the fire) or on electrical fires (shock risk): switch off power and use CO₂ or powder.
Explosions
When a burnable gas or fine dust (flour, coal dust) mixes with air in a closed space and is lit, it burns all at once and the hot gas expands suddenly: an explosion. Prevent it: no flames near gas leaks, open windows, do not switch electric switches if you smell gas, keep dust low in mills and mines.
Key formulas and definitions
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Worked examples
1. A candle covered by a 250 mL glass goes out after 10 s. Under a 500 mL glass, what do you expect, and why?
About twice as long (roughly 20 s). The bigger glass holds about twice as much air, so twice as much oxygen for the flame to use. Only the glass size changed, so it is a fair test.
2. How much heat comes from burning 2 kg of LPG? (calorific value 50 kJ/g)
2 kg = 2000 g. E = 2000 × 50 = 100 000 kJ = 100 MJ.
3. Balance the burning of propane: C₃H₈ + O₂ → CO₂ + H₂O.
Carbon: 3 C → 3CO₂. Hydrogen: 8 H → 4H₂O. Oxygen on the right: 3×2 + 4×1 = 10 O → 5O₂. So C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
4. A gas stove flame is yellow and the pan has black marks. What is wrong and what is the fix?
Not enough air reaches the gas, so combustion is incomplete; the black marks are soot (carbon). Clean the burner air holes so more air mixes with the gas; the flame turns blue.
5. Why does a CO₂ extinguisher work on an electrical fire?
CO₂ is heavier than air and does not burn, so it covers the fire and pushes oxygen away. It does not conduct electricity and leaves no water, so there is no shock risk.
6. Which gives more heat: 3 g of hydrogen (142 kJ/g) or 10 g of coal (30 kJ/g)?
Hydrogen: 3 × 142 = 426 kJ. Coal: 10 × 30 = 300 kJ. The 3 g of hydrogen gives more, and only water as product.
Common mistakes
- Thinking fire needs only fuel and heat. Without oxygen nothing burns, which is why covering a flame works.
- Throwing water on an oil or electrical fire. Water spreads burning oil and conducts electricity. Use a lid, a blanket or a CO₂ extinguisher.
- Calling burning a physical change. New substances (CO₂, water) form, so it is a chemical change.
- Mixing up CO and CO₂. CO₂ turns limewater milky; CO is the poisonous gas from incomplete burning.