Matter — Subtopic: Composition of Air

These notes explain what air is made of and why it is important. They are written for Kenyan learners (age 11) and use simple examples and safe classroom experiments.

What is air?

Air is a mixture of different gases that surround the Earth. We cannot see it, but it fills all the spaces around us and we need it to live and for many everyday activities.

Main gases in the air

The air around us is mostly made of these gases (approximate amounts):

• Nitrogen (N₂) — 78%: a gas that does not burn. Plants need nitrogen too, but in a different form (fertilisers or bacteria change it).
• Oxygen (O₂) — 21%: needed by people, animals and fires to burn.
• Argon (Ar) — 0.93%: a harmless, inactive gas found in small amounts.
• Carbon dioxide (CO₂) — about 0.04%: used by green plants for photosynthesis. It is small in amount but important for plants and the climate.
• Water vapour: the amount varies — more near the ocean or on hot days, less on dry days.
• Other trace gases: neon, helium, methane, ozone and tiny particles (dust and smoke).

Why does air matter?

• Oxygen helps us breathe and keeps fires burning.
• Plants use carbon dioxide to make food (photosynthesis).
• Nitrogen is important for soil and plant growth when turned into usable forms.
• Air pressure affects weather — winds and rain.

Classroom experiments (safe and simple)

1) Test for oxygen — the glowing splint

Materials: glass jar, candle, match or lighter, water tray, help from a teacher.

Procedure:

1. Light the candle and place it on a tray of water.
2. Carefully cover the candle with the jar. The flame will go out.
3. Light a small wooden splint, blow so it glows (not large flame), and put the glowing splint into a fresh jar of air — it may relight if oxygen is present in enough amount.

What you see: A glowing splint will flare up (relight) in air because of oxygen.

2) Air takes up space — balloon in a bottle or syringe

Materials: balloon, empty plastic bottle, syringe (no needle) or teacher demonstration kit.

Procedure:

1. Stretch the balloon mouth over the bottle neck (uninflated).
2. Try to push the balloon into the bottle — you cannot because air inside the bottle takes up space.
3. Use a syringe to push air into a balloon and watch it inflate — air fills the balloon.

3) Test for carbon dioxide — limewater test

Materials: small bottle, limewater (calcium hydroxide solution), straw, exhale into the bottle (teacher must supervise).

Procedure & observation: Blow into limewater through a straw; the water becomes cloudy if CO₂ is present (from your breath).

Safety: Always do experiments with a teacher or adult. Do not put flames near flammable materials. Do not swallow chemicals.

Short glossary

• Mixture: two or more substances mixed but not chemically joined (air is a mixture of gases).
• Gas: a state of matter that fills any container and can flow.
• Photosynthesis: how plants make food using CO₂, water and sunlight, producing oxygen.
• Trace gas: a gas present in very small amounts.

Quick review questions

1. What are the two main gases in air and their approximate amounts?
2. Why do humans need oxygen?
3. Give one way to show that air takes up space.
4. What happens to limewater when it meets carbon dioxide?

Note for Kenyan pupils: You meet these ideas in primary Science and Technology. Try the safe experiments in class or at home with a parent or teacher. Observing nature — trees, smoke from a jiko, and breathing — helps you understand how air and its gases affect everyday life.