Testing for carbonates (CO3^2-)
2 marks · EasyA powder is suspected to contain carbonate ions (CO3^2-). Describe the two steps you would take to confirm the presence of carbonate ions in the powder and explain what results would show that carbonate ions are present.
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Also accept: Add dilute acid; observe bubbling; test the gas with limewater; limewater turns cloudy
Common mistakes: Not mentioning gas test or turning limewater cloudy
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Marking points
M1 · 1 mark · Describes adding dilute acid and observing fizzing or bubbling, indicating a gas is given off.
A1 · 1 mark · Explains that the gas turns limewater cloudy, confirming the presence of CO2 and carbonate ions.
Worked solution
Step 1: Add dilute hydrochloric acid to the powder. If bubbles of gas are produced, this suggests a carbonate may be present.
Step 2: Pass the gas through limewater. If the limewater turns cloudy, this shows the gas is carbon dioxide, confirming the presence of carbonate ions.