Oxygen in air and its physical properties
Air is a mixture of gases. By volume: about 78% nitrogen, 21% oxygen, 0.9% argon, about 0.04% carbon dioxide, plus water vapour.
Oxygen is element number 8. Two oxygen atoms join to make one molecule, O₂ (molar mass 32 g/mol).
- Colourless, no smell, no taste.
- Slightly heavier than air.
- Only a little dissolves in water (about 9 mg per litre at 20 °C), but this is enough for fish.
- Turns into a pale blue liquid at −183 °C.
How oxygen is prepared
In the laboratory
- From hydrogen peroxide (no heating): 2H₂O₂ → 2H₂O + O₂. Manganese dioxide (MnO₂) is a catalyst: it speeds up the reaction and is left unchanged at the end.
- By heating potassium manganate(VII) (potassium permanganate): 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂.
- By heating potassium chlorate with MnO₂: 2KClO₃ → 2KCl + 3O₂.
The gas is collected by downward displacement of water (it pushes water out of an upside-down jar), because it does not dissolve much. It can also be collected by upward displacement of air, since it is a bit heavier than air.
In industry
Air is cooled until it becomes liquid, then warmed slowly. Nitrogen boils off first (−196 °C), oxygen later (−183 °C). This is fractional distillation of liquid air. Oxygen can also be made by electrolysis of water.
Chemical properties: burning, oxides and the test
Test for oxygen: a glowing splint relights. Oxygen is not flammable, but it supports combustion.
Most elements react with oxygen to form oxides. Gaining oxygen is called oxidation.
- Metals → metal oxides, which are basic: 2Mg + O₂ → 2MgO (white, bright flame). MgO in water gives a solution that turns red litmus blue.
- Non-metals → non-metal oxides, mostly acidic: S + O₂ → SO₂ (blue flame); C + O₂ → CO₂. These turn blue litmus red.
- Some oxides are neutral (CO, H₂O) and some are amphoteric (Al₂O₃, ZnO react with both acids and bases).
Combustion of fuels: CH₄ + 2O₂ → CO₂ + 2H₂O + heat. Respiration in cells: glucose + oxygen → carbon dioxide + water + energy. Rusting is slow oxidation of iron with oxygen and water.
Ozone, the ozone layer and the oxygen cycle
Ozone (O₃) is another form (an allotrope) of oxygen with three atoms per molecule. It is a pale blue gas with a sharp smell and is a strong oxidising agent. UV light or electric sparks change oxygen into ozone: 3O₂ → 2O₃.
About 15–35 km up, the ozone layer absorbs most harmful UV rays. Chlorofluorocarbons (CFCs) from old fridges and sprays broke ozone down; the 1987 Montreal Protocol banned them, and the layer is slowly healing. Near the ground, ozone is a harmful pollutant in smog.
Oxygen cycle: plants release oxygen in photosynthesis; animals, burning and decay use it up. This keeps the level in air nearly steady.
Uses of oxygen
- Breathing support in hospitals, high-altitude climbing, aircraft and space.
- Oxy-acetylene flame (over 3000 °C) for cutting and welding metals.
- Steel making: oxygen blown through molten iron burns away carbon.
- Rocket fuel: liquid oxygen burns liquid hydrogen.
- Water treatment and fish farming.
Try it: candle under a glass
With an adult: stand a small candle in a plate with a little water. Light it and cover it with a glass. Predict: what will happen to the flame? Watch it go out after a few seconds as the oxygen is used up. Then play the last 3D step and slide the oxygen % below 15 to see the same thing.
Key formulas and definitions
- 2H₂O₂ → 2H₂O + O₂ (MnO₂ catalyst)
- 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂ (heat)
- 2KClO₃ → 2KCl + 3O₂ (heat, MnO₂)
- 2Mg + O₂ → 2MgO (basic oxide)
- S + O₂ → SO₂ (acidic oxide)
- CH₄ + 2O₂ → CO₂ + 2H₂O
- 3O₂ → 2O₃ (UV light)
Worked examples
1. A room holds 50 m³ of air. About how much of it is oxygen?
Oxygen is about 21% of air. 21/100 × 50 = 10.5 m³ of oxygen.
2. How many moles of O₂ form when 4 mol of H₂O₂ decompose? What mass is this?
2H₂O₂ → 2H₂O + O₂, so 2 mol H₂O₂ give 1 mol O₂. 4 mol give 2 mol O₂. Mass = 2 × 32 = 64 g.
3. A white ash from a burnt metal ribbon is shaken with water and tested with litmus. Predict the colour and explain.
The ash is MgO, a metal oxide. In water it forms Mg(OH)₂, a base, so red litmus turns blue.
Common mistakes
- Saying oxygen burns. It does not burn itself; it helps other things burn.
- Thinking MnO₂ is a reactant. It is a catalyst and is not used up.
- Mixing the tests: a glowing splint relighting means oxygen; a lighted splint going out with a "pop" means hydrogen.
- Thinking all oxides are acidic. Metal oxides are basic; some oxides are neutral or amphoteric.