Testing for carbonates (CO3^2-)
2 marks · HardA student is given a solution that may contain sodium carbonate. Describe two distinct observations the student would expect to make when dilute hydrochloric acid is added to this solution if carbonate ions are present.
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Must show: bubbles/fizzing, gas produced/seen, limewater test—turns cloudy/milky, gas is carbon dioxide
Also accept: effervescence/fizzing; formation of bubbles and/or turns limewater cloudy
Common mistakes: Stating only one observation; Not mentioning limewater or the result if both are expected for full marks
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Marking points
B1 · 1 mark · Observation of bubbles/fizzing/effervescence or gas given off when acid added (accept synonyms)
B2 · 1 mark · Gas turns limewater cloudy/milky (candidate can state test or outcome)
Worked solution
When dilute hydrochloric acid is added to a solution containing sodium carbonate:
(1) Bubbles (fizzing/effervescence) are seen as a gas is produced.
(2) The gas can be bubbled through limewater, which turns cloudy, showing the gas is carbon dioxide.