Testing for carbonates (CO3^2-)
2 marks · HardA solution of potassium carbonate is added to excess dilute nitric acid in a test tube. The student's teacher tells them to test any gas produced with limewater. Explain why limewater is used in this test and state the result that shows carbonate ions were present.
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Must show: limewater is used to test for carbon dioxide, if carbonate present, limewater turns cloudy/milky
Also accept: limewater tests for CO2; goes cloudy/milky if present
Common mistakes: Saying limewater detects 'gas' but not specifying CO2; Not stating the result (e.g., turns cloudy/milky)
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Marking points
B1 · 1 mark · States that limewater is used to test for (identify) carbon dioxide.
B2 · 1 mark · States that limewater turns cloudy/milky if carbonate ions are present (i.e., CO2 is produced).
Worked solution
Limewater is used because it is a test for carbon dioxide gas. If the solution contains carbonate ions, carbon dioxide is produced and the limewater turns cloudy (or milky).