Test for carbon dioxide — AQA GCSE Chemistry
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Test for carbon dioxide explained
Test for carbon dioxide covers 2 specification points, including The test for carbon dioxide uses an aqueous… and When carbon dioxide is shaken with or bubbled….
Read the full explanation
Open a point below for its explanation and revision guidance.
The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water).
Carbon dioxide is identified using limewater, which is an aqueous solution of calcium hydroxide. The gas is either bubbled through the limewater or shaken with it in a stoppered container. If carbon dioxide is present, the limewater turns milky or cloudy. The active reagent is calcium hydroxide dissolved in water, so the test uses the solution rather than the solid. The milky appearance comes from tiny particles of calcium carbonate formed when carbon dioxide reacts with calcium hydroxide. Limewater must be fresh and clear before use, because stale limewater may already be cloudy and give a false positive.
When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).
This statement describes the positive result of the carbon dioxide test. Carbon dioxide is either shaken with limewater in a sealed container or bubbled through it. The limewater, which is aqueous calcium hydroxide, turns milky or cloudy. The cloudiness is caused by solid calcium carbonate forming as tiny particles suspended in the liquid. The word milky is accepted as equivalent to cloudy. The change is from a clear solution to a cloudy one, so the starting limewater must be clear. A negative result leaves the limewater clear. This observation confirms carbon dioxide, for example from a reaction or from a gas sample.
Your focus
- Identify limewater as an aqueous solution of calcium hydroxide used to test for carbon dioxide.
- Describe how carbon dioxide is brought into contact with limewater.
- State the positive result as limewater turning milky or cloudy.
Show all 6 objectives
- Describe the observation when carbon dioxide is shaken with or bubbled through limewater.
- Explain that the cloudiness is caused by calcium carbonate forming.
- Distinguish a positive result from a negative result by the appearance of the limewater.
Test for carbon dioxide exam tips
Marking Points
- Identify limewater as an aqueous solution of calcium hydroxide.
- State that carbon dioxide is tested by bubbling it through limewater or shaking it with limewater.
- Describe the positive result as the limewater turning milky or cloudy.
- Explain that the cloudiness is caused by calcium carbonate forming as a precipitate.
- Note that limewater should be clear before the test so that any cloudiness is a valid observation.
- State that carbon dioxide is shaken with limewater or bubbled through limewater.
- Describe the positive observation as the limewater turning milky or cloudy.
- Explain that the cloudiness is due to calcium carbonate forming as a solid precipitate.
- Note that the limewater starts clear, so the change to cloudy is the key evidence.
- Recognise that milky and cloudy describe the same observation.
Examiner Tips
- 💡Name the reagent as limewater and identify it as aqueous calcium hydroxide.
- 💡Give the observation as milky or cloudy, not just 'a reaction happens'.
- 💡Mention both possible methods: bubbling the gas through limewater or shaking the gas with limewater.
- 💡Use the phrase 'turns milky' or 'turns cloudy' exactly, as both are accepted observations.
- 💡Link the cloudiness to calcium carbonate forming, showing understanding rather than just recall.
- 💡State the method briefly: bubble the gas through limewater or shake the gas with limewater.
Common Mistakes
- Using solid calcium hydroxide instead of limewater; the correct reagent is an aqueous solution of calcium hydroxide.
- Saying the limewater turns clear; the correct observation is that clear limewater turns milky or cloudy.
- Ignoring the starting appearance of the limewater; the correct approach checks that it is clear before testing so cloudiness can be seen.
- Saying the limewater turns white or solid; the correct observation is that it turns milky or cloudy as fine particles form.
- Claiming the limewater stays clear when carbon dioxide is present; the correct result is cloudiness, while clear limewater indicates no carbon dioxide.
- Describing a colour change to a different colour; the correct change is from clear to milky or cloudy, not to a coloured solution.