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Gases in the air and burning in oxygen
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Air is a mixture of gases. Simple experiments measure how much of it is oxygen, and elements burn brightly in pure oxygen.
What dry air contains
Measuring the oxygen in air
Using a metal: damp iron wool is pushed into the top of a measuring cylinder, which stands upside down in water. As the iron rusts it uses up the oxygen, and the water rises about one fifth of the way up.
Using a non-metal: phosphorus is burned in a bell jar over water; the water rises as the oxygen is used.
With syringes: 100 cm3 of air is pushed back and forth over heated copper. About 79 cm3 is left, so about 21% was oxygen.
% oxygen = (starting volume − final volume) ÷ starting volume × 100
Using a non-metal: phosphorus is burned in a bell jar over water; the water rises as the oxygen is used.
With syringes: 100 cm3 of air is pushed back and forth over heated copper. About 79 cm3 is left, so about 21% was oxygen.
% oxygen = (starting volume − final volume) ÷ starting volume × 100
Burning and decomposing
Magnesium burns with a bright white flame to form white magnesium oxide (a basic oxide).
Hydrogen burns to form water.
Sulfur burns with a blue flame to form sulfur dioxide (an acidic oxide).
Thermal decomposition: metal carbonates break down on heating to give carbon dioxide. Green copper(II) carbonate turns into black copper(II) oxide: CuCO3 → CuO + CO2
Carbon dioxide is a greenhouse gas; increasing amounts may contribute to climate change.
Hydrogen burns to form water.
Sulfur burns with a blue flame to form sulfur dioxide (an acidic oxide).
Thermal decomposition: metal carbonates break down on heating to give carbon dioxide. Green copper(II) carbonate turns into black copper(II) oxide: CuCO3 → CuO + CO2
Carbon dioxide is a greenhouse gas; increasing amounts may contribute to climate change.
Air in a syringe is passed over hot copper. The volume falls from 80 cm3 to 63 cm3. What percentage of the air was oxygen?
- Oxygen used = 80 − 63 = 17 cm3
- 17 ÷ 80 × 100
Answer: 21.25%, about 21%
Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide. Oxygen is measured by reacting it with iron, phosphorus or copper and finding the volume lost. Magnesium, hydrogen and sulfur burn in oxygen. Metal carbonates decompose on heating to give carbon dioxide.
The interactive lesson includes the diagrams for this topic.
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