Edexcel IGCSE Chemistry · Spec 2.9-2.12
Air Composition, Combustion in Oxygen and Carbonate Decomposition
Covers recall the main gases in dry air, Describe three elements burning in oxygen and Explain the thermal decomposition of copper carbonate.
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Air Composition, Combustion in Oxygen and Carbonate Decomposition
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Explained
Air, burning in oxygen, and heating a carbonate
Dry air is about 78 per cent nitrogen and 21 per cent oxygen. The remaining one per cent is mostly argon, at about 0.9 per cent, with roughly 0.04 per cent carbon dioxide. Those four figures are worth knowing exactly, because questions often ask you to identify a gas from its percentage alone.
Three elements burning in oxygen
Burning in pure oxygen is more vigorous than burning in air, because air is only about a fifth oxygen. The products follow the pattern of the element.
- Magnesium burns with a bright white flame and forms magnesium oxide, a white solid.
- Hydrogen burns with a pale blue flame and forms water, which is hydrogen oxide.
- Sulfur burns with a blue flame and forms sulfur dioxide, a colourless gas with a sharp smell.
Note the spelling. The specification uses sulfur, not sulphur, and marks the modern spelling throughout.
Metal oxides and non metal oxides behave differently
Magnesium oxide dissolves slightly in water to give an alkaline solution. Sulfur dioxide dissolves to give an acidic one. That split is general: metal oxides tend to be basic, non metal oxides tend to be acidic, and a question giving you a pH after burning is usually testing that idea rather than the burning itself.
Thermal decomposition is not burning
Heating copper carbonate breaks it down into copper oxide and carbon dioxide. The green powder turns black, and the gas turns limewater milky. Nothing has burned. No oxygen was involved and no fuel was consumed. The compound was broken apart by heat alone, and that is what thermal decomposition means.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Chemistry record exactly this confusion. Many learners knew that thermal decomposition could produce carbon dioxide from copper carbonate, but some suggested burning it in oxygen instead, which the report describes as a misconception about the difference between thermal decomposition and combustion.
Two further points from the reports are about wording rather than chemistry. Candidates who wrote calcium hydroxide instead of limewater often failed to say it must be a solution, and a number wrote "lime water" as two words, which was marked incorrect. The test is the limewater test, and limewater is one word.
The same reports note that few learners recognised copper carbonate as insoluble, which is worth knowing before you meet a question that depends on it.
Testing for carbon dioxide
Bubble the gas through limewater. A positive result turns it milky, sometimes described as cloudy or as forming a white precipitate. All three descriptions have been credited, but the reagent must be named correctly and identified as a solution.
Spec 2.9-2.12
What you need to know
- Recall the main gases in dry air
- Describe three elements burning in oxygen
- Explain the thermal decomposition of copper carbonate
Active recall
Quick check
Answer each question before opening the answer.
What percentage of dry air is argon?
About 0.9%
What forms when sulfur burns in oxygen?
Sulfur dioxide
What colour change occurs when copper carbonate is heated?
Green to black
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