Testing for carbonates (CO3^2-)
4 marks · NormalA student tests a solid substance to find out if it contains carbonate ions. The student adds dilute sulfuric acid and observes bubbles of gas. They collect the gas and pass it through limewater.
Write the ionic equation for the reaction between carbonate ions () and sulfuric acid.
State the identity of the gas formed.
Describe the observed result when the gas is bubbled through limewater.
Explain how these observations confirm that the original solid contained carbonate ions.
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)
(b) Carbon dioxide
(c) Limewater turns milky/cloudy
(d) The fizzing shows a gas (CO2) is produced when acid is added. Passing it through limewater and observing it turns cloudy confirms the gas is carbon dioxide, which is only produced if carbonate ions are present.
Common mistakes: incorrect equation coefficients; mislabeling gas as hydrogen; incorrect limewater result; explaining limewater test only
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correct balanced ionic equation. Allow state symbols absent or incorrect as long as substances are correct.
B1 (b) · 1 mark · Correct identification of gas as carbon dioxide.
B1 (c) · 1 mark · Limewater turns milky (accept 'cloudy', 'white precipitate').
M1 (d) · 1 mark · Linking the gas evolved, the positive limewater test, and carbonate ions.
Worked solution
a) The ionic equation is:
b) The gas formed is carbon dioxide.
c) When the gas is bubbled through limewater, the limewater turns milky or cloudy.
d) These observations confirm carbonate ions because the typical reaction is carbonate + acid → CO2. The fizz proves a gas is formed; limewater turning milky confirms that it is CO2, and therefore that carbonate ions were in the original solid.