Test for CarbonatesOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is the test for carbonate ions?
Carbonate ions can be detected using a simple reaction: carbonates react with dilute acid to produce carbon dioxide gas. This gas is then identified using the standard test for carbon dioxide, which confirms the presence of a carbonate in the original sample.
Section 2
How does the carbon dioxide confirm a carbonate is present?
The gas released is bubbled through (or passed into) limewater, which is a solution of calcium hydroxide. If carbon dioxide is present, the limewater turns from clear to cloudy white, forming a fine white precipitate of calcium carbonate. This white precipitate is the positive result that confirms both that carbon dioxide gas was produced, and therefore that a carbonate was present in the original solid or solution.
Always describe the full two-step logic in exam answers: acid + carbonate produces carbon dioxide, AND carbon dioxide turns limewater cloudy - both steps carry marks.
Section 3
How is the test carried out in practice?
- Add a small volume of dilute acid (e.g. dilute hydrochloric acid) to the sample being tested.
- Immediately connect the test tube to a delivery tube leading into a second test tube containing limewater.
- Observe any bubbling in the first tube (evidence a gas is being produced).
- Observe the limewater: if it turns cloudy white, the gas is confirmed as carbon dioxide, and the original sample must have contained a carbonate.
Simply observing 'bubbles' when acid is added is not enough evidence of a carbonate on its own - many reactions with acids produce bubbles of different gases, so the limewater step is essential to confirm the gas is specifically carbon dioxide.
Must Know
- Carbonates react with dilute acid to produce carbon dioxide gas.
- The gas is tested by bubbling it through limewater.
- A positive result is that the limewater turns cloudy white.
- This cloudiness is a fine white precipitate of calcium carbonate forming in the limewater.
- Both steps (acid reaction producing gas, then limewater test) are needed to confirm a carbonate is present.
That's the notes covered.
Carry on to the next subtopic.