Air – JSS2 Intermediate Science Lesson Note (First Term)

Air is the mixture of gases that surrounds the Earth and fills every empty space around us.

Behavioural objectives

By the end of the lesson, students should be able to:

1. State that air is a mixture and name its main gases with their percentages.
2. Describe simple experiments that show air has mass, occupies space, exerts pressure and can be compressed.
3. Explain why air is needed for burning.
4. Explain why air is important to humans, animals and plants.

What is air?

Air is a colourless, odourless and invisible mixture of gases that makes up the atmosphere.

• Invisible: we cannot see, touch or smell it, yet we live at the bottom of this “sea” of gases.
• Everywhere: air fills every space that is not already taken up by a solid or a liquid.
• Atmosphere: the layer of air around the Earth.

Is air a mixture?

Yes, air is a mixture of gases that are mixed together but not joined chemically.

• Not joined: each gas keeps its own properties.
• Easy to separate in use: we breathe in oxygen and the body leaves most of the other gases alone.

What are the gases in air?

Air contains nitrogen, oxygen, carbon dioxide, rare gases, water vapour and dust.

What are the properties of air?

There are four main properties of air: it has mass, occupies space, exerts pressure and can be compressed.

1. Air has mass (weight)

1. Weigh an empty football and record the mass.
2. Pump air into the football.
3. Weigh it again. It is heavier, so air has mass.

2. Air occupies space

1. Fill a bucket with water.
2. Push an empty cup upside down into the water.
3. Water does not fill the cup, because the cup is full of air that takes up space.

3. Air exerts pressure

Atmospheric pressure is the force that air exerts on everything around it, in all directions.

• Suction cup: it sticks to a wall because air pressure outside pushes it against the surface.
• Drinking straw: sucking removes air from the straw, and air pressure on the drink pushes it up into the mouth.

4. Air can be compressed

Compression means squeezing air into a smaller space, and air is elastic because it springs back when released.

• Bicycle tyre: a lot of air is pumped into a small space.
• Gas cylinder: gas is stored under high pressure.
• Syringe: close the nozzle with a finger and push the plunger. The air squeezes, then pushes the plunger back.

How does air support combustion?

Combustion is burning, and it needs oxygen from the air.

Candle and jar experiment

1. Light a candle and stand it firmly on a plate.
2. Cover it with a glass jar.
3. The flame flickers and goes out after a few seconds.
4. The candle has used up most of the oxygen in the jar, so the flame cannot continue.

Safety tip

• Burning clothes: wrap the person in a heavy blanket to smother the fire.
• Why it works: the blanket cuts off the air supply. No air, no fire.
• Get help: call an adult or the fire service at once.

Why is air important to living things?

Air supplies the gases that living things use and protects life on Earth.

• Humans and animals: they breathe in oxygen to break down food and release energy (respiration). Without oxygen, the brain and heart stop working within minutes.
• Plants: they take in carbon dioxide and, with sunlight, make food (starch) and give out oxygen (photosynthesis).
• Balance of gases: animals give out carbon dioxide for plants, and plants give out oxygen for animals.
• Protection: the atmosphere shields us from the sun’s strong heat by day and helps keep us warm at night.
• Sound: sound needs a medium such as air to travel to our ears.
• Flight: birds, insects and aeroplanes push against air to fly.

Evaluation

1. Which gas makes up the largest part of air?
2. Mention two experiments or observations that prove air is present, though we cannot see it.
3. Why does a candle go out when it is covered with a jar?
4. How do plants and animals help each other keep the air suitable for life?
5. Why is a blanket used to put out a fire on a person’s clothes?

Assignment

Draw a table of the gases in air with their percentages. Then describe one simple experiment that shows air takes up space.