Air is a mixture, not a compound
A mixture is two or more substances mixed but not joined by a chemical reaction. Air is a mixture. How do we know?
- The amount of each gas can change. Air in a crowded room has more carbon dioxide. Air over the sea has more water vapour. A compound always has a fixed make-up.
- Each gas keeps its own properties. Oxygen in air still helps burning. Nitrogen in air still does not.
- The gases can be separated by physical means. When air is cooled to about −200 °C it turns liquid. Warming it slowly lets each gas boil off at its own temperature (fractional distillation). Nitrogen boils at −196 °C, argon at −186 °C, oxygen at −183 °C.
- Mixing gases to make air gives out no heat or light, unlike making a compound.
What gases are in air?
In dry, clean air (by volume):
| Gas | Percentage | Main job |
|---|---|---|
| Nitrogen, N₂ | about 78% | Dilutes oxygen; plants get it via soil bacteria; used to make fertilisers |
| Oxygen, O₂ | about 21% | Breathing (respiration) and burning (combustion) |
| Argon, Ar | about 0.93% | Unreactive noble gas |
| Carbon dioxide, CO₂ | about 0.04% | Plants use it in photosynthesis; traps heat |
| Neon, helium, krypton, xenon | tiny traces | Noble gases |
Water vapour is not in the table because it changes: from almost 0% in a desert to about 4% in a humid rainforest. Air also carries dust, pollen and microbes.
Noble gases: properties and uses
Helium, neon, argon, krypton and xenon are the noble gases (Group 18). Their atoms have full outer shells, so they hardly ever react. They have no colour and no smell.
- Argon: fills filament bulbs and shields hot metal during welding, so the metal does not burn.
- Helium: very light and does not burn, so it is used in weather balloons and party balloons.
- Neon: glows red-orange in advertising signs when electricity passes through it.
- Krypton and xenon: bright lamps, such as car headlights and lighthouse lamps.
Measuring the oxygen in air
Idea: trap a known volume of air. Use up only the oxygen. See how much the volume shrinks.
Method 1: rusting iron
Push damp iron wool to the top of a measuring tube. Stand the tube upside down in water. Over a few days the iron rusts. Rusting uses oxygen, so water rises into the tube to take its place. It stops at about 1/5 (21%) of the air volume.
Method 2: burning phosphorus (bell jar)
Red phosphorus is burned in a bell jar standing in water. It reacts with oxygen to make a white solid oxide, which dissolves. When the jar cools, water rises about 1/5 of the way up.
Method 3: heated copper and two syringes
100 cm³ of air is pushed back and forth over hot copper between two gas syringes. Copper turns black (copper oxide) as it takes the oxygen. After cooling, about 79 cm³ is left, so about 21 cm³ was oxygen.
Percentage of oxygen = (volume used up ÷ starting volume) × 100. The gas left does not support burning: a burning splint goes out in it. That shows it is mostly nitrogen.
Air pollution: sources, effects and protection
A pollutant is a substance added to air that harms living things or buildings.
| Pollutant | Main source | Effect |
|---|---|---|
| Particulates (PM2.5, PM10, soot) | Diesel engines, burning crop waste, dust | Enter deep into lungs; asthma, heart disease; smog |
| Carbon monoxide (CO) | Fuel burning with too little air; car exhausts | Poisonous: stops blood carrying oxygen |
| Sulfur dioxide (SO₂) | Burning coal and oil that contain sulfur | Acid rain; breathing problems; damages marble |
| Nitrogen oxides (NO, NO₂) | Hot car engines; power stations | Acid rain; smog |
| Extra CO₂, methane | Burning fossil fuels; farms | Trap heat: global warming |
Protecting the air
- Use public transport, cycle or walk; switch to electric and CNG vehicles.
- Catalytic converters in cars change CO and NO into CO₂ and N₂.
- Scrubbers remove SO₂ from power-station smoke; use low-sulfur fuel.
- Do not burn rubbish or crop stubble; plant trees; check the Air Quality Index (AQI) and wear a mask on bad days.
Try it at home
Push a small ball of damp steel wool (the kind used for scrubbing pans) into the bottom of a tall glass. Turn the glass upside down in a plate of water. Mark the water level with tape. Check after 3 days. The water climbs about one fifth of the way up the air space. You just measured the oxygen in air! (Ask an adult to help; no flames needed.)
Key formulas and definitions
- Air by volume: N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.04%
- Percentage of oxygen = (volume of oxygen used ÷ starting volume of air) × 100
- Volume of a gas in air = total volume × (percentage ÷ 100)
- Mixture: parts not chemically joined; proportions can change; separated by physical methods
- Noble gases (Group 18): He, Ne, Ar, Kr, Xe – full outer shells, very unreactive
Worked examples
1. A classroom holds 200 m³ of air. About how much of it is oxygen?
Oxygen ≈ 21% of 200 m³ = 0.21 × 200 = 42 m³.
2. In the copper experiment, 100 cm³ of air shrinks to 79 cm³. Find the percentage of oxygen.
Oxygen used = 100 − 79 = 21 cm³. Percentage = 21 ÷ 100 × 100 = 21%.
3. A tube holds 50 cm³ of air. After the iron wool rusts, 40 cm³ of gas is left. What percentage of the air was oxygen? Is this close to the real value?
Used = 50 − 40 = 10 cm³. Percentage = 10 ÷ 50 × 100 = 20%. Yes, close to 21% (small errors come from reading the scale or the iron not using all the oxygen).
4. Why is argon used in light bulbs and not oxygen?
Oxygen would make the hot filament burn and break. Argon is a noble gas: it does not react, so the filament lasts longer.
Common mistakes
- Saying air is mostly oxygen. Air is mostly nitrogen (78%); oxygen is only about 21%.
- Calling air a compound. It is a mixture: its make-up can change and its gases can be separated physically.
- Thinking carbon dioxide is a big part of air. It is only about 0.04%, even though it matters a lot for climate.
- Using the final volume instead of the volume used up: % oxygen = (start − end) ÷ start × 100.