Section 1 of 4What Combustion Involves
Combustion is the scientific name for burning, and it always needs oxygen.
Combustion
- A reaction in which a substance reacts with oxygen and releases energy, usually as heat and often as light.
Oxide products
- Burning an element in oxygen produces an oxide, such as magnesium oxide from burning magnesium.
Exothermic
- Combustion gives out heat to the surroundings, so the temperature of the surroundings rises.
Oxidation
- Combustion is also oxidation, because the burning substance gains oxygen.
Fuel and oxygen
- A fuel needs a supply of oxygen to keep burning, so cutting off the air stops combustion.
Section 2 of 4Three Combustion Reactions
Each element reacts with oxygen in a characteristic way.
1
Magnesium burns with an intense white flame, forming a white powder of magnesium oxide.
2
Hydrogen burns with a pale blue flame, and the only product is water.
3
Sulfur burns with a blue flame to produce sulfur dioxide, a colourless and poisonous gas.
4
All three reactions are exothermic, so each releases heat energy to the surroundings.
| Element | Observation | Equation |
|---|---|---|
| Magnesium | Intense white flame; white powder of magnesium oxide | 2Mg(s) + O₂(g) → 2MgO(s) |
| Hydrogen | Burns with a pale blue flame and forms water | 2H₂(g) + O₂(g) → 2H₂O(l) |
| Sulfur | Blue flame; colourless, poisonous gas formed | S(s) + O₂(g) → SO₂(g) |
Section 3 of 4Uses and Hazards
Uses and Hazards
Magnesium
- Burning magnesium releases a large amount of heat and light, which is why it was used in flares.
Hydrogen
- Hydrogen burns cleanly because its product is water, so it is considered for use as a fuel.
Sulfur dioxide
- Sulfur dioxide is poisonous and contributes to acid rain, so burning sulfur must be done in a fume cupboard.
Ventilation
- Good ventilation is needed when fuels burn, to prevent the build-up of poisonous gases.
Section 4 of 4Exam Checklist & Tips
2.3.3 – Key Points for the Exam
- Combustion A reaction in which a substance burns in oxygen, releasing energy.
- Exothermic A reaction that releases energy to the surroundings, usually as heat.
- Oxide A compound of an element with oxygen.
- Fuel A substance that releases energy when it burns.
- Oxidation A reaction in which a substance gains oxygen or loses electrons.
Common Mistakes
- Writing an unbalanced equation such as 2Mg + O → MgO instead of using the O₂ molecule.
- Describing combustion as endothermic; combustion releases energy.
- Leaving out state symbols, for example writing MgO as gas instead of MgO(s).
- Assuming sulfur dioxide is harmless; it is poisonous and causes acid rain.