Primary General Studies – Air and Rusting
The composition of air
We cannot touch nor see the presence of air.
Air has weight.
Air is colourless, tasteless and odourless.
Air can move.
Air occupies space and exists everywhere.
The components of air are nitrogen, oxygen and other gases.
Composition of air
Nitrogen 78% (4/5)
Oxygen 21% ( 1/5)
Oxygen is used in burning. Carbon dioxide is produced in burning. Burning can also give out light and
We can put out fire by cutting the oxygen supply, such as using a fire extinguisher or a fire blanket.
If our clothes catch fire, we should lie on the floor immediately and roll ourselves.
Rust affects the appearance and functions of iron objects. Rust is formed when iron objects contact with
water and air.
How can we prevent rusting?
We can put some paint or oil on the surface of the iron products. This can prevent the surfaces from
contacting with oxygen.
Why do we need oxygen?
Oxygen is needed in the burning of food in our body. This generates energy and maintains our lives.
When we have a barbecue, why do we need to fan the fire?
It is because there is more oxygen in the moving air. This can facilitate the burning.
Read the diagram of the following experiment and answer the questions.
Fix a candle in a tray and pour in some coloured water.
Light the candle and cover it with a jar.
What happens to the coloured water? (What can you observe?)
The coloured water rises inside the jar.
Why does the water rise in the jar?
The burning uses up oxygen.
What happens at the end? (What can you observe for a longer time?)
The candle goes out when all oxygen is used up.
Cover a lit candle with a jar. After the candle goes out, cover the jar quickly with a
Lime water is added to the jar on the right, what can you observe?
Lime water turns milky because carbon dioxide is produced after burning.