Combustion — Gases in the Atmosphere | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Air composition, greenhouse effect and climate implications appear in most series.

What combustion is

Combustion is the reaction of a substance with oxygen, producing one or more oxides
Every combustion reaction is exothermic — heat (and usually light) is released
A combustion reaction is also classed as an oxidation reaction in the "addition of oxygen" sense

Three required combustion reactions

Element burnt	Observations	Balanced equation
Magnesium	Intense white flame; fine white powder collects	2 Mg(s) + O₂(g) → 2 MgO(s)
Hydrogen	Burns quietly with a pale blue flame; a colourless liquid (water) condenses on a cool surface	2 H₂(g) + O₂(g) → 2 H₂O(l)
Sulfur	Burns with a pale blue flame; produces a colourless, choking gas	S(s) + O₂(g) → SO₂(g)

The magnesium product is an alkaline metal oxide
The hydrogen product is water itself, the simplest non-metal oxide
The sulfur product is sulfur dioxide, an acidic non-metal oxide and a major air pollutant linked to acid rain

Exam tip

Observations when magnesium burns in oxygen

What comes up: "State one observation" or "State two observations made when magnesium burns in oxygen/air."

Write (two marks): (1) A bright white flame (or intense white light). (2) A white solid/powder/ash is produced.

Watch out: "white precipitate" is rejected by the mark scheme — precipitate implies a substance forming in solution, which is not what happens here. Write "white solid", "white powder", or "white ash" instead.