Testing for carbonates (CO3^2-)
4 marks · NormalA student is given a white powder thought to contain carbonate ions.
The student adds dilute hydrochloric acid to the powder.
State what the student should observe if carbonate ions are present.
The student collects the gas given off and bubbles it through limewater. What result would confirm the gas is carbon dioxide?
Write the ionic equation for the reaction between carbonate ions and hydrochloric acid.
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(a) Fizzing or bubbles of gas are produced.
(b) The limewater turns cloudy.
(c) \ce{CO3^{2-} (s) + 2H^+ (aq) -> CO2 (g) + H2O (l)}
Common mistakes: Incorrect formula or omission of states; Not mentioning 'limewater turns cloudy'
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Marking points
B1 (a) · 1 mark · States 'fizzing', 'bubbling', or 'effervescence' is observed.
B2 (b) · 1 mark · States limewater turns cloudy/milky.
M1 (c) · 1 mark · Writes correct ionic equation with correct ions and formulae.
A1 (c) · 1 mark · Includes correct state symbols for all species.
Worked solution
(a) The student should observe fizzing or bubbling as the acid reacts with the powder.
(b) If the gas turns limewater cloudy, this confirms the gas is carbon dioxide.
(c) The ionic equation for the reaction is: