Carbon DioxideOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is the test for carbon dioxide gas?
The test for carbon dioxide gas is to bubble the gas through limewater (a solution of calcium hydroxide). If carbon dioxide is present, the limewater turns from colourless to cloudy white.
This is one of the standard gas tests used in chemical analysis to identify an unknown gas produced in a reaction.
Section 2
How is the test carried out?
- Prepare a test tube of limewater (calcium hydroxide solution), which is colourless and clear.
- Pass or bubble the gas to be tested through the limewater, using a delivery tube if the gas is produced in a separate reaction.
- Observe the limewater: if it turns cloudy white, carbon dioxide is confirmed present.
State both the method (bubble the gas through limewater) and the observation (turns cloudy/milky white) to gain full marks.
Section 3
Why does limewater turn cloudy?
Carbon dioxide reacts with the dissolved calcium hydroxide to form calcium carbonate, which is insoluble in water. This insoluble solid forms as a fine white precipitate suspended in the solution, which is seen as the limewater turning cloudy (milky) white.
Do not describe the result as 'the limewater changes colour' — it does not change colour, it becomes cloudy/milky because a solid precipitate forms.
Must Know
- The test for carbon dioxide gas: bubble the gas through limewater
- A positive result is that the limewater turns cloudy white
- Limewater is a solution of calcium hydroxide
- The cloudiness is caused by insoluble calcium carbonate forming as a precipitate
- This is a standard identification test used in chemical analysis
That's the notes covered.
Carry on to the next subtopic.