2.3 Gases in the AtmosphereEdexcel IGCSE Chemistry: Revision notes

Dry air is a mixture of gases with approximately fixed percentages by volume:

These four gases make up almost all of dry air, with nitrogen and oxygen together accounting for around 99%.

The percentage of oxygen in air can be found experimentally by reacting a fixed volume of air with excess metal (e.g. iron) or non-metal (e.g. phosphorus), so that all the oxygen is used up, then measuring the decrease in volume.

Method using iron:

1. Trap a known volume of air over water (or in a sealed syringe system) in contact with damp iron filings/wool.
2. Leave for several days as the iron rusts, using up oxygen.
3. Measure the decrease in volume of air.
4. Percentage oxygen = (decrease in volume ÷ original volume) × 100.

Method using phosphorus: burning phosphorus in a sealed volume of air over water produces solid phosphorus pentoxide, which dissolves, and water rises to fill the volume once occupied by the oxygen used up; the volume risen shows the oxygen used.

Both methods give a result of approximately 21%, close to the true atmospheric percentage of oxygen.

Combustion of elements in oxygen produces their oxides:

• Magnesium burns with a bright white flame to form white magnesium oxide (MgO), a solid: 2Mg + O2 → 2MgO
• Hydrogen burns with a squeaky pop to form water (H2O): 2H2 + O2 → 2H2O
• Sulfur burns with a blue flame to form sulfur dioxide (SO2), a choking gas: S + O2 → SO2

These are all oxidation reactions, as oxygen is gained.

Thermal decomposition is the breakdown of a compound using heat. Heating certain metal carbonates decomposes them into a metal oxide and carbon dioxide gas.

For example, heating copper(II) carbonate (green solid):

Copper(II) carbonate → copper(II) oxide + carbon dioxide CuCO3 → CuO + CO2

The green solid turns black as copper(II) oxide forms, and carbon dioxide gas is released, which can be tested with limewater (turns milky/cloudy).

Carbon dioxide is a greenhouse gas: it absorbs infrared radiation reflected from the Earth's surface and re-emits it, trapping heat in the atmosphere. Human activities, especially burning fossil fuels, are increasing the concentration of carbon dioxide in the atmosphere. This enhanced greenhouse effect is thought to contribute to climate change (global warming), including rising average temperatures and changing weather patterns.