4CH1
Gases in the Atmosphere
Inorganic Chemistry · 1 question type
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.