Planet Earth: The Atmosphere
FoundationAir is a mixture of nitrogen, oxygen, argon and carbon dioxide. Industry liquefies air and separates nitrogen and oxygen by fractional distillation — a physical change; oxygen relights a glowing splint.
- Air by volume: 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% CO₂ — a mixture of variable proportions
- Making oxygen: remove CO₂ and water vapour, compress and liquefy, then separate in a fractionating column
- Boiling points: nitrogen −196°C, oxygen −183°C; the lower-boiling nitrogen vaporises first and leaves at the top
文體小知識：科學知識說明文——以數據和實例說明空氣的成分、分餾液態空氣的原理與氧的試驗，是化學科的第一課。
§Text Passage
1The atmosphere is the layer of gases surrounding the Earth and a vital chemical resource. By volume, dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide, with small amounts of water vapour and noble gases. The proportions vary slightly from place to place, so air is a mixture and its components can be separated by physical methods.
2To make nitrogen and oxygen industrially, carbon dioxide and water vapour are first removed; the air is then repeatedly compressed and cooled to about −200°C until it becomes liquid air. The liquid air enters a fractionating column and is warmed slowly. Nitrogen boils at −196°C, lower than oxygen's −183°C, so nitrogen vaporises first and leaves at the top of the column, while liquid oxygen is collected at the bottom.
3Distillation merely separates the components that air already contains; no new substance is formed, so it is a physical change. The oxygen obtained supplies hospitals and helps steelworks; the nitrogen is used to make fertilisers and to package food; the argon fills light bulbs and shields welding. Many everyday materials come from the air.
4Chemists identify gases with tests: place a glowing splint in a jar of gas — if it relights, the gas is oxygen. To test for carbon dioxide, bubble the gas through limewater, which turns milky. Meanwhile air pollutants such as sulfur dioxide and nitrogen dioxide cause acid rain and damage ecosystems; monitoring air quality and using the atmosphere wisely are part of learning chemistry.
1Summary 重點摘要
Air is a mixture of nitrogen, oxygen, argon and carbon dioxide. Industry liquefies air and separates nitrogen and oxygen by fractional distillation — a physical change; oxygen relights a glowing splint.
2Vocabulary cards Vocabulary
A method that separates the components of a liquid mixture using their different boiling points.
Air compressed and cooled to about −200°C into liquid form; the raw material for fractional distillation.
The temperature at which a liquid boils; nitrogen boils at −196°C and oxygen at −183°C.
A change in which no new substance forms, such as the fractional distillation of liquid air.
A colourless, unreactive gas making up about 78% of air, formula N₂, used to make fertilisers.
A noble gas making up about 0.9% of air; very unreactive, used to fill light bulbs.
A glowing splint relights in the gas, proving that it is oxygen.
A splint that has stopped burning but still glows — the standard tool for testing oxygen.
Substances that lower air quality, such as sulfur dioxide, nitrogen dioxide and suspended particulates.
Matter made of two or more substances in variable proportions, like air.
3Step by step Steps
- Step 1: What air is made ofBy volume, dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide, plus a little water vapour and other noble gases. For example, in 100 litres of dry air there are about 78 litres of nitrogen and 21 litres of oxygen, but only about 0.04 litres of carbon dioxide. The proportions vary slightly with place and weather, so air is a mixture, not a compound — the first key judgement of this topic and a common DSE concept question.
- Step 2: Turning air into liquid airCarbon dioxide and water vapour would freeze and block the equipment at very low temperatures, so they are removed first. The air is then repeatedly compressed and cooled to about −200°C, becoming pale-blue liquid air. In a plant, for example, compressors pressurise the air, condensers carry the heat away, and the liquid is stored in insulated vessels ready for the fractionating column. Liquefaction is also a physical change: the particles simply move closer together, and no new substance forms.
- Step 3: A race of boiling pointsFractional distillation works on the difference in boiling points. Nitrogen boils at −196°C and oxygen at −183°C — nitrogen is 13 degrees lower. As the liquid air is warmed slowly in the column, the temperature reaches nitrogen's boiling point first, so nitrogen vaporises first and leaves at the top; oxygen boils only at the higher temperature and stays at the bottom as liquid oxygen. Picture a race: the lower the boiling point, the earlier a runner starts, so nitrogen always leaves the liquid first.
- Step 4: Physical or chemical changeThroughout fractional distillation no new substance forms: nitrogen is still nitrogen and oxygen is still oxygen — the components that air already had are merely separated. So the whole process is a physical change. For example, separating a bowl of red and green beans changes neither bean; distillation likewise takes out old components rather than creating new matter. The only criterion is whether a new substance forms — state this rule explicitly in answers, as it is the key to distinguishing physical from chemical change.
- Step 5: The test for oxygenTo prove a jar of gas is oxygen, use the standard test: insert a glowing splint; if it relights, the gas is oxygen, because oxygen supports combustion. For example, if gas collected from the top of the column cannot relight a glowing splint, it is not oxygen; if the splint bursts into flame again, that is the evidence. In answers, always write 'a glowing splint relights', not 'a burning splint burns brighter' — imprecise wording is a common place to lose marks.
- Step 6: Uses of the productsOxygen from liquid air supplies hospitals, enriches the air in steelworks and fills diving tanks. Nitrogen, being unreactive, is used to make ammonia and fertilisers, and to keep food fresh by keeping oxygen away. Argon fills light bulbs and shields welding. For example, a bag of crisps is actually puffed up with nitrogen, precisely to stop the oil reacting with oxygen and going bad. Knowing the uses helps you answer extension questions linking chemistry to daily life — do not memorise the composition while ignoring applications.
- Step 7: Air pollution and resource awarenessAlthough air is freely available, it can be polluted: sulfur dioxide and nitrogen dioxide dissolve in raindrops to form acid rain, corroding buildings and acidifying lakes and soils, while suspended particulates harm breathing. For example, burning sulfur-containing fuels in factories releases sulfur dioxide, a main cause of acid rain, so power stations install desulfurisation equipment. The curriculum also reminds us that the Earth's resources are finite, that mining and extraction should be regulated, and that protecting the atmosphere is part of a chemist's attitude.
4Diagrams Diagrams
5Worked example Example
Data question on liquid air: a plant feeds liquid air into a fractionating column and warms it slowly. Given that nitrogen boils at −196°C and oxygen at −183°C, which gas distils out first? Explain, and state whether the process is a physical or chemical change.
- Read the question: it asks which gas distils first and why — the key data are the two boiling points, so compare −196°C with −183°C
- Compare boiling points: nitrogen (−196°C) boils lower than oxygen (−183°C), so nitrogen's boiling point is reached first as the temperature rises slowly
- Explain: liquid nitrogen vaporises first and leaves at the top of the column; oxygen boils only at −183°C and stays at the bottom
- Judge the type of change: neither nitrogen nor oxygen becomes a new substance — they are merely separated by boiling point, so it is a physical change
- Answer: Nitrogen distils out first. As the liquid air is warmed slowly, the temperature reaches nitrogen's boiling point of −196°C before oxygen's −183°C, so nitrogen vaporises first and is collected at the top of the column; oxygen boils at −183°C and remains at the bottom as liquid oxygen. The process only separates the components that air already contains using their different boiling points, and no new substance is formed — so the fractional distillation of liquid air is a physical change.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the percentages of the main components of dry air, by volume?
Q2During the fractional distillation of liquid air, which gas vaporises first, and why?
Q3Is the fractional distillation of liquid air a physical or chemical change, and why?
Q4How do you test whether a jar of gas is oxygen?
Q5Roughly how many litres of nitrogen and oxygen are in 100 litres of dry air?
8Recap Recap
- Air by volume: 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% CO₂ — a mixture of variable proportions
- Making oxygen: remove CO₂ and water vapour, compress and liquefy, then separate in a fractionating column
- Boiling points: nitrogen −196°C, oxygen −183°C; the lower-boiling nitrogen vaporises first and leaves at the top
- Fractional distillation of liquid air is a physical change; oxygen relights a glowing splint
- Uses: oxygen for medicine and steel, nitrogen for fertilisers and food packaging, argon for light bulbs