Testing for carbonates (CO3^2-)
4 marks · NormalA student is testing three white powders (A, B and C) for carbonate ions using dilute hydrochloric acid and limewater. Their results are shown:
| Powder | Acid added | Fizzing observed? | Limewater turns cloudy? |
|---|---|---|---|
| A | Yes | Yes | Yes |
| B | Yes | No | No |
| C | Yes | Yes | No |
Which powder(s) definitely contain carbonate ions? Explain your answer.
Suggest a reason why powder C showed fizzing but did not turn limewater cloudy.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Powder A definitely contains carbonate ions as it both produces fizzing and turns limewater cloudy, showing CO2 is produced from a carbonate.
(b) Possible reasons: the gas from powder C was not carbon dioxide, or not enough CO2 was produced to turn limewater cloudy, or there was a problem in transferring/collecting the gas.
Must show: linking observations to carbonate ions and CO2, alternative explanations
Common mistakes: saying fizzing alone is enough; ignoring limewater result
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Links two positive results (fizzing and cloudy limewater) for powder A to confirmation of carbonate ions.
A1 (a) · 1 mark · Correctly identifies powder A as the only definite carbonate.
M1 (b) · 1 mark · Gives a plausible explanation such as gas not being CO2, not enough CO2 produced, or experimental error.
A1 (b) · 1 mark · Explanation shows understanding of two-step nature of test or sources of error.
Worked solution
a) Powder A definitely contains carbonate as both gas is produced and limewater goes cloudy, confirming CO2. Powders B and C are inconclusive.
b) Powder C may release a gas that is not CO2 (such as hydrogen or another gas from acid reaction), or not enough CO2 is produced to affect limewater, or the gas was not bubbled through limewater properly.