Natural sciences Year 5: Air: what it is made of and how to keep it clean
Natural sciences, Year 5 (Portugal): Air is matter with mass that takes up space, made mostly of nitrogen and oxygen; human activities can spoil its quality and we can act to protect it.
Air: properties and constituents
Air cannot be seen but it exists: it is colourless, odourless and tasteless, it has mass, it takes up space, it can be compressed and it exerts pressure. Dry air is about 78 % nitrogen, about 21 % oxygen and about 1 % other gases, such as argon and carbon dioxide (only about 0,04 %). It also carries water vapour, in an amount that varies from place to place and day to day.
What each gas does
Each gas has a job. Oxygen is used in the breathing of living things and in burning. Carbon dioxide is used by plants in photosynthesis and, like water vapour, traps heat: a natural greenhouse effect that helps keep Earth mild. Nitrogen reacts very little and dilutes the oxygen. Water vapour forms clouds and rain. In the atmosphere, the ozone layer filters part of the Sun's ultraviolet radiation.
Air takes up space: the upside-down glass
Crumple a piece of paper into the bottom of a glass, turn the glass upside down and push it straight down into a bowl of water. Step 1: the glass looks empty, but it is full of air. Step 2: air takes up space, so the water cannot fill the glass. Step 3: lift the glass straight up and take out the paper: it is dry, or almost. Conclusion: air is matter, because it occupies space.
Clean-looking air can be polluted
A common idea is that the air is polluted only when you see smoke or smell something. But many pollutants are invisible. Carbon monoxide, for example, is colourless and odourless, comes from burning fuel without enough oxygen, and is dangerous to breathe. So the quality of the air is not judged by looking at it: it is measured, and it depends on what we burn and release into it.
Looking after the air
Human activities change the air: burning fuel in transport and industry releases gases and particles that affect health (coughs, breathing problems) and the climate, and fires release smoke. Measures include public transport, walking and cycling, renewable energy, filters in factories and protecting forests. Which ones could you choose in your street or town? Argue for one, with an example from your region.
Test what you learned
Which is the most abundant gas in dry air?
- a) Nitrogen, about 78 %
- b) Oxygen, about 78 %
- c) Carbon dioxide, about 21 %
Correct answer: a) Nitrogen, about 78 % — Right: nitrogen is about 78 % of dry air, oxygen about 21 %.
Nitrogen is about 78 % and oxygen about 21 % of dry air. About what share are all the other gases together?
- a) About 21 %
- b) About 10 %
- c) About 1 %
Correct answer: c) About 1 % — Right: 78 + 21 = 99, and 100 − 99 = 1. Argon and CO₂ are in that 1 %.
Which statement about air pollution is correct?
- a) If you see no smoke and smell nothing, the air is clean
- b) Some pollutants, such as carbon monoxide, are colourless and odourless
- c) Only factories pollute the air, never vehicles or fires
Correct answer: b) Some pollutants, such as carbon monoxide, are colourless and odourless — Right: that is why air quality is measured, not judged by eye.
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