Testing for carbonates (CO3^2-)
3 marks · Easy| Sample | Dilute acid added | Fizzing observed | Gas turns limewater cloudy |
|---|---|---|---|
| X | Yes | Yes | Yes |
| Y | Yes | No | - |
| Z | Yes | Yes | No |
Which sample(s) most likely contain carbonate ions? Explain your answer.
Suggest a possible identity for the gas produced from sample Z.
Explain why sample Y does not give the same results as X.
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) Sample X; fizzing and clouding of limewater show presence of carbonate ions.
(b) Sample Z could produce a gas like hydrogen or another acid-reactive gas not CO2.
(c) Sample Y does not fizz, so no gas was produced; it likely does not contain carbonate ions.
Common mistakes: Assuming any fizzing means carbonate; Not distinguishing gas types
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies sample X as containing carbonate ions with explanation based on both fizzing and cloudy limewater.
B1 (b) · 1 mark · Suggests sample Z may release hydrogen or another gas that does not turn limewater cloudy.
B1 (c) · 1 mark · Explains that Y does not fizz—no carbonate ions present.
Worked solution
a) Sample X shows fizzing and the gas turns limewater cloudy, confirming carbonate ions are present.
b) Sample Z produces a gas that does not turn limewater cloudy; this could be hydrogen from a metal reacting with acid.
c) Sample Y does not fizz, so no gas was produced and no carbonate ions are present.