الأسبوع 2 · الفصل 1: 1-3 الهواء بعد الاحتراق، «لنتوسع» (أدوات الكشف عن الغازات)، مقال الاحتراق وطاقة المستقبل · 3 دروس
1
How the air changes after burning
Science P6: Term 1, Chapter 1, pp. 12–17
GoalDescribe how burning changes the air: oxygen is used and carbon dioxide is made.
New words
carbon dioxide · a gas made when fuels burn; it does not help burningwater vapour · water as an invisible gasproduct · a new substance made in a change
Explanation
When a fuel burns, it joins with oxygen from the air. This makes new substances: carbon dioxide, water vapour and heat and light. So after burning, the air near the fire has less oxygen than before and more carbon dioxide. Some oxygen is used, not all of it. Scientists can measure this. If you compare air from a place where a fire burned with fresh air, the burnt air has a lower amount of oxygen and a higher amount of carbon dioxide. This is why a fire in a small closed room gets weaker: the air is changing, and fresh air must come in. The words matter: burning does not make oxygen disappear into nothing. The oxygen is used in the change, and it ends up joined in new substances.
fuel + oxygen→burning→carbon dioxide + water vapour + heat and light
Worked examples
A gas cooker in Mariam’s kitchen burns gas. The kitchen window stays open a little. The burning uses oxygen and makes carbon dioxide and water vapour, so fresh air is needed.
Gas
Before burning
After burning
oxygen
more
less
carbon dioxide
very little
more
Compare air before and after burning in a small closed box (a measuring monitor shows it; children do not light fires).
A car engine burns petrol. It uses oxygen from the air and puts carbon dioxide into the air from the exhaust pipe. Its exhaust also holds a poisonous gas, so a car must never run in a closed garage.
Class activity
Before and after. Draw two boxes of air. In the first, draw 5 dots for oxygen and 1 dot for carbon dioxide. In the second box, show air after burning: use fewer oxygen dots and more carbon dioxide dots. Do not make the oxygen dots all disappear. Explain the change to a grown-up.
Practice
1
Name two substances made when a fuel burns.
Carbon dioxide and water vapour (and heat).
2
After burning, is there more or less oxygen in the air?
Less oxygen (some of it is used).
3
After burning, is there more or less carbon dioxide?
More carbon dioxide.
4
Why must a gas cooker be used with a window or vent open?
Burning uses oxygen and makes carbon dioxide, so fresh air is needed.
