Composition of Air

Summary: Clean air contains ~78% N₂, ~21% O₂, ~0.9% Ar, ~0.04% CO₂. Oxygen percentage can be determined by reaction with copper or phosphorus in a closed system. Tags: igcse chemistry environmental-chem Created: 2026-07-14 Last Updated: 2026-07-14

Content

The Composition of Clean, Dry Air

Air is a mixture of gases. The composition of clean, dry air (at sea level) is remarkably constant:

The percentages given above are for dry air. In practice, air nearly always contains water vapour (H₂O), the concentration of which varies significantly — from less than 1% in cold, dry regions to over 4% in hot, humid tropical conditions. This variability means water vapour is typically excluded when stating the “fixed” composition of air.

Carbon dioxide concentration is not fixed either: it has risen from pre-industrial levels of approximately 0.028% (280 ppm) to over 0.04% (420+ ppm) today due to fossil fuel combustion and deforestation.

Roles of Each Major Gas

Nitrogen (N₂) — The most abundant gas in the atmosphere. Nitrogen is relatively inert under normal conditions due to the strong triple covalent bond (N≡N, bond enthalpy 945 kJ/mol). Its roles include:

• Diluting oxygen in the atmosphere; if O₂ concentration were much higher, combustion would be dangerously rapid and widespread
• Serving as the ultimate source of nitrogen for the nitrogen cycle — nitrogen-fixing bacteria convert atmospheric N₂ into nitrates, which plants use to synthesise proteins and nucleic acids
• Providing an inert atmosphere for industrial processes (e.g. food packaging to prevent oxidation, blanketing in chemical reactors)
• Liquid nitrogen (boiling point −196 °C) is used as a cryogenic coolant

Oxygen (O₂) — The second most abundant gas and the chemically active component of air. Its roles include:

• Respiration: all aerobic organisms use O₂ in cellular respiration to release energy from glucose (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy)
• Combustion: O₂ supports the burning of fuels; any combustion reaction requires oxygen as a reactant
• Oxidation reactions: rusting of iron (4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O), corrosion of metals
• Industrial production of steel (oxygen is blown through molten iron to oxidise carbon and other impurities)

Argon (Ar) — A monatomic noble gas, chemically inert. Its roles include:

• Providing an inert atmosphere for welding (argon shielding gas in TIG and MIG welding prevents oxidation of the hot metal weld)
• Filling incandescent light bulbs (argon prevents the tungsten filament from oxidising at high operating temperatures)
• Used as a carrier gas in gas chromatography and as a plasma gas in ICP spectrometry

Carbon dioxide (CO₂) — Present in trace amounts but of great chemical and biological significance:

• Photosynthesis: plants use CO₂ and water to produce glucose (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of light and chlorophyll)
• Greenhouse gas: CO₂ absorbs infrared radiation, contributing to the natural greenhouse effect that keeps the Earth habitable
• Dissolves in rainwater to form weakly acidic carbonic acid (H₂CO₃), responsible for the natural acidity of rain (pH ~5.6)

Water vapour (H₂O) — Variable concentration:

• The most abundant greenhouse gas by warming contribution (but not directly controlled by human activity)
• Essential to the water cycle (evaporation, condensation, precipitation)
• Influences weather and climate patterns

Air as a Mixture, Not a Compound

Several lines of evidence confirm that air is a mixture of gases rather than a chemical compound:

Experimental Determination of the Percentage of Oxygen in Air

Several classical experiments determine the proportion of oxygen in air by reacting the oxygen with a substance and measuring the volume change in a closed system.

Method 1: Reaction with Copper (Gas Syringe Method)

This is the most common IGCSE experimental setup.

Apparatus: Two gas syringes connected by a silica tube containing copper turnings, with a Bunsen burner heating the turnings.

Method:

1. One gas syringe is filled with a known volume of air (typically 100 cm³). The other syringe is set to zero.
2. The copper turnings in the silica tube are heated strongly.
3. The air is passed back and forth over the heated copper by pushing the syringe plungers alternately.
4. As the air passes over the hot copper, oxygen in the air reacts with the copper to form black copper(II) oxide:

[ 2\text{Cu (s)} + \text{O}_2\text{ (g)} \rightarrow 2\text{CuO (s)} ]

5. The plungers are pushed back and forth repeatedly until the volume of gas in the syringes stops decreasing (i.e. all oxygen has been consumed).
6. The apparatus is allowed to cool to room temperature before taking the final volume reading. Cooling is essential because gases expand when hot; a reading taken at elevated temperature would give a falsely high volume and thus an underestimate of the oxygen percentage.

Observations:

• The copper turnings turn from reddish-brown to black (copper(II) oxide forms).
• The total gas volume in the syringes decreases.

Calculation:

• Initial volume of air = 100 cm³
• Final volume after all oxygen consumed = 79 cm³ (approximately)
• Volume of oxygen reacted = 100 − 79 = 21 cm³
• Percentage of oxygen in air = (21 ÷ 100) × 100 = 21%

Why the method works: Oxygen gas is converted to solid copper(II) oxide. The solid does not occupy significant volume in the gas phase, so the reduction in gas volume directly corresponds to the volume of oxygen that was present. Nitrogen and argon do not react with copper and remain as gases.

Sources of error:

• Not allowing the apparatus to cool to room temperature before the final reading
• Not passing the air over the copper enough times, leaving unreacted oxygen
• Leaks in the apparatus allowing air to enter or escape
• The silica tube not being filled sufficiently with copper turnings

Method 2: Phosphorus in a Bell Jar (Over Water)

Apparatus: A bell jar or large glass container inverted over a trough of water, with a small evaporating dish of phosphorus floating on the water surface.

Method:

1. The bell jar is placed over water, and the water level inside and outside is equalised (marking the starting water level).
2. The phosphorus is ignited with a hot wire.
3. The phosphorus burns in the oxygen present in the trapped air, producing dense white fumes of phosphorus(V) oxide:

[ 4\text{P (s)} + 5\text{O}_2\text{ (g)} \rightarrow \text{P}4\text{O}{10}\text{ (s)} ]

4. The white fumes dissolve in the water.
5. As oxygen is consumed, the pressure inside the bell jar decreases, and water is drawn up into the bell jar.
6. When the apparatus cools to room temperature, the water level inside the bell jar will have risen to occupy approximately one-fifth (21%) of the original gas volume.

Observations:

• Phosphorus burns with a bright white flame.
• White smoke (P₄O₁₀) fills the jar, then dissolves into the water.
• Water rises approximately 1/5 of the way up the bell jar.

Why the water does not fill completely: Only oxygen (about 21% of the air) reacts with phosphorus. The remaining gases — primarily nitrogen (~78%) and argon (~0.93%) — do not react and remain in the gas phase, occupying the remaining volume.

Method 3: Iron Wool Rusting (Inverted Measuring Cylinder)

Apparatus: A measuring cylinder filled with air and inverted over a trough of water, containing damp iron wool.

Method:

1. Damp iron wool is placed inside a measuring cylinder. The cylinder is inverted over water, trapping a known volume of air.
2. The apparatus is left for several days (typically 3–5 days) while the iron rusts.
3. Rusting consumes oxygen:

[ 4\text{Fe (s)} + 3\text{O}_2\text{ (g)} + 2x\text{H}_2\text{O (l)} \rightarrow 2\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O (s)} ]

4. As oxygen is consumed, water rises into the measuring cylinder.
5. After rusting is complete (the iron wool has fully rusted and no further water rise is observed), the water level inside the cylinder will indicate the volume that was occupied by oxygen.

This method is slower (taking days) but does not require heating and is a simple demonstration of oxygen consumption by a familiar process.

Common Exam Calculations with Gas Volumes

Worked example (included inline): 100 cm³ of air is passed repeatedly over heated copper until no further volume change occurs. After cooling, the final volume is 79 cm³. Calculate the percentage of oxygen.

• Volume of oxygen = 100 − 79 = 21 cm³
• % O₂ = (21 / 100) × 100 = 21%

Given the percentage of oxygen, the volume of oxygen in any volume of air can also be calculated. For example, the volume of O₂ in 250 cm³ of air at 21% oxygen is (21/100) × 250 = 52.5 cm³.

Key Concepts from Past Papers

• carbon dioxide
• mark each for any 2 of: carbon monoxide carbon water
• calcium oxide carbon dioxide
• carbon dioxide water
• mark each for any two of: plastics sewage microbes
• carbon carbon dioxide
• mark each for any two of: catalytic converters low sulfur fuels flue gas desulfurisation
• mark each for any 2 of:

Keywords from Past Papers

carbon, dioxide, monoxide, water, gas, catalytic, climate, change, global, warming, increased, sulfur, fuels, incomplete, combustion

Related Notes

• Air Pollution
• Climate Change and Greenhouse Gases
• Water Treatment

Sources

• OpenStax, Chemistry 2e, Chapter 9: Gases — Section 9.1, gas pressure and composition of the atmosphere. https://openstax.org/books/chemistry-2e/
• BBC Bitesize, GCSE Chemistry (Single Science), “Composition of the Atmosphere” and “Oxygen and Oxides.” https://www.bbc.co.uk/bitesize/topics/z9k3qyc
• Cambridge IGCSE Chemistry 0620 Syllabus, Topic 11: Air and Water — composition of clean air, experimental determination of percentage of oxygen.
• CK-12 Foundation, Chemistry, Chapter: Atmosphere — composition and properties of air. https://www.ck12.org/chemistry/

Past Paper Sources

• 0620/32 Feb/March 2015: Q66(c)(i) (2m), Q11(a)(ii) (1m)
• 0620/32 Feb/March 2017: Q33(b)(iii) (1m), Q22(e)(ii) (0m), Q22(f)(i) (1m)
• 0620/32 Feb/March 2018: Q11(a)(iv) (1m)
• 0620/32 Feb/March 2019: Q77(b)(ii) (2m)
• 0620/32 Feb/March 2022: Q55(b)(ii) (4m)
• 0620/32 May/June 2018: Q11(a)(iv) (1m)
• 0620/32 May/June 2020: Q11(a)(i) (1m)
• 0620/33 May/June 2016: Q88(c)(i) (1m), Q55(b)(ii) (0m)
• 0620/33 May/June 2019: Q33(b)(i) (1m)
• 0620/33 May/June 2022: Q22(d)(ii) (2m), Q44(c)(i) (1m)
• 0620/33 May/June 2024: Q44(a)(ii) (2m), Q66(e)(ii) (1m), Q88(b)(i) (0m)
• 0620/33 October/November 2019: Q11(a)(i) (1m), Q11(a)(iii) (1m)

Common Misconceptions

• The composition of air is exactly 78% N₂, 21% O₂, and 1% other gases. These are approximate values. The exact composition varies slightly with altitude, location, and humidity. Argon alone accounts for nearly 0.93%, meaning “other gases” are closer to 1% than 0%.
• Carbon dioxide makes up about 1% of air. CO₂ is only about 0.04% of the atmosphere — a trace gas. Despite this tiny proportion, it has a disproportionately large effect on climate because of its infrared-absorbing properties.
• The water in the phosphorus experiment rises because oxygen is “used up” and replaced by water. The water rises because the consumption of oxygen reduces the pressure inside the container. The external atmospheric pressure pushes water in to equalise the pressure. It is a pressure-driven phenomenon, not simply replacement.
• All gases in air are elements. Carbon dioxide (CO₂), methane (CH₄), and water vapour (H₂O) are compounds, not elements.
• Oxygen is the most abundant gas in air. Nitrogen is roughly four times as abundant as oxygen by volume.
• Air has a chemical formula. Air is a mixture, not a compound. It has no fixed chemical formula because its components are not chemically bonded in fixed proportions.
• The final volume in the copper experiment can be read immediately after heating stops. The apparatus must cool to room temperature first. Hot gas occupies a larger volume, so reading without cooling would overstate the final volume and understate the percentage of oxygen.