What’s in Air
Air is a colourless, odourless, tasteless mixture of gases surrounding the Earth — about 78% nitrogen, 21% oxygen, and the remaining 1% made up of carbon dioxide, water vapour, rare gases and dust.
Measuring the Percentage of Oxygen in Air
Floating candle method: a lit candle floating on water is covered with an upturned gas jar. The height of trapped air is measured, and the candle is left to burn until it goes out as the oxygen is used up (replaced by CO₂ and water vapour). The change in air height, compared to the original, gives the percentage of oxygen used.
Heated copper method: copper is heated in a hard glass tube connected to two syringes. As the copper reacts with the oxygen in the trapped air (turning from reddish-brown to black), the syringe readings show a drop in volume — this difference gives the percentage of oxygen present.
Separating Air Into Its Components
Air is separated by fractional distillation. Dust is first filtered out, then the air is compressed and cooled to about −200°C to liquefy it (water vapour freezes out as ice during cooling and is removed). As the liquid air slowly warms up, each gas boils off at its own boiling point and is collected separately: carbon dioxide at −78°C, oxygen at −185°C, and nitrogen at −195°C.
The Active and Inactive Parts of Air
Air’s active part is oxygen — it supports both combustion and respiration, and combines with other substances during burning to form entirely new ones (making burning a chemical change). The inactive part — mainly nitrogen, plus carbon dioxide and the rare gases — is just as important: carbon dioxide is used by plants in photosynthesis and in fire extinguishers (since it doesn’t support combustion and is denser than air), while nitrogen is used to manufacture nitrate fertilisers for soil fertility, and dilutes the oxygen in air enough to prevent overly rapid burning and keep respiration at a manageable rate.
How Oxygen Reacts With Other Substances
The reaction of oxygen with a substance is called combustion, or burning.
- With metals: most metals burn in air to form basic oxides — solutions of these oxides turn red litmus paper blue. For example: 4Na + O₂ → 2Na₂O; 2Mg + O₂ → 2MgO; 2Cu + O₂ → 2CuO; 3Fe + 2O₂ → Fe₃O₄; 2Ca + O₂ → 2CaO.
- With non-metals: most non-metals burn in oxygen to form acidic oxides, which turn moist blue litmus paper red — sulphur burns with a blue flame to form sulphur dioxide, and carbon, nitrogen and phosphorus behave similarly.
- Neutral oxides: some oxides are neither acidic nor basic and have no effect on litmus at all — hydrogen burning in air (with a characteristic “pop”) produces water vapour, a neutral oxide; carbon monoxide and nitrogen monoxide are further examples.
- Amphoteric oxides: a few oxides show both acidic and basic behaviour, e.g. aluminium oxide (Al₂O₃) and zinc oxide (ZnO).
- With hydrocarbons (compounds of carbon and hydrogen, like cooking gas, petrol or kerosene): the products are carbon dioxide and water vapour.
These oxide-forming reactions can be confirmed experimentally: burning a piece of metal (e.g. calcium) in a Bunsen flame, dissolving the resulting powder in water, and testing the solution on litmus paper — a basic oxide’s solution has no effect on blue litmus but turns red litmus blue.
Rusting
In the presence of both moisture and oxygen, iron reacts to form a reddish-brown solid called rust — hydrated iron(III) oxide, Fe₂O₃·xH₂O. Both oxygen and water must be present for rusting to occur (hot, dry conditions won’t cause it), and it can be made worse by dissolved salts, air pollutants, and heat.
Preventing rust works by keeping oxygen and/or water away from the iron surface:
- Painting or varnishing: coats the surface, e.g. on iron doors and windows.
- Plating: covering the iron with another metal such as tin, copper, silver, nickel or chromium (often by dipping in the molten metal); doing this using an electric current is called electroplating.
- Galvanising: plating specifically with zinc.
- Alloying: mixing iron with other elements, e.g. stainless steel (iron and carbon, among others) resists rusting.
- Oiling or greasing: also stops the squeaking of rusted hinges by reducing friction.
- Sacrificial protection: attaching a more reactive metal, like magnesium or zinc, which corrodes preferentially and protects the iron — commonly used on ships.
- Plastic coating: another way of sealing the surface from air and moisture.
