Week 2 · Chapter 1: 1-3 Air After Burning; Let’s Expand (tools that check gases); Column: Combustion and Future Energy · 3 lessons
2
Limewater, a milky test for carbon dioxide
Science P6: Term 1, Chapter 1, pp. 12–17
GoalExplain how limewater shows carbon dioxide, and how detector tubes and monitors measure gases.
New words
limewater · a clear liquid that turns milky when carbon dioxide mixes with itgas detector tube · a tube whose colour changes along a scale to show how much of a gas is presentgas monitor · a device that shows the amount of a gas as a number
Explanation
How do we know that burning makes carbon dioxide? We use a test. Limewater is a clear liquid. When carbon dioxide mixes with it, it turns milky white. The result is a clue that carbon dioxide is present. Limewater can irritate skin and eyes, so only teachers or adults use it, and children never make it at home. A gas detector tube works in another way: air passes through the tube, a colour changes along it, and we read the amount on the scale. A gas monitor is a digital device. It shows the percentage of oxygen or carbon dioxide on a screen. Detector tubes and monitors are used in laboratories, mines and factories to keep people safe.
Tool
How it shows the gas
Limewater
Turns from clear to milky when carbon dioxide is present
Gas detector tube
The colour change stops at a mark on a scale
Gas monitor
A number on a screen
Worked examples
Test
Limewater after the test
fresh air
stays clear (almost)
air after burning
turns milky
A teacher shows a flask of limewater. Air from a burning candle (in a closed jar) is mixed with it and the limewater turns milky. Clear stays clear if there is little carbon dioxide.
Seif reads a gas detector tube. The colour stops at the mark for 2 on the scale. So the gas amount is 2 on that scale, and a higher mark would mean more gas.
A cave guide carries a gas monitor. If it shows too little oxygen or too much carbon dioxide, the group leaves the cave quickly. The tool keeps people safe.
Class activity
Which tool? Cut three cards: limewater, detector tube, monitor. Write one place where each could be used by an adult or a scientist (for example a laboratory or a cave). Do not mix or taste any liquid at home.
Practice
1
What colour does limewater turn with carbon dioxide?
Milky white.
2
Why do children not make limewater at home?
It can irritate skin and eyes, so only adults or teachers use it.
3
Which tool gives a number on a screen?
A gas monitor.
4
Limewater stays clear after a test. What does it suggest?
