Why is limewater part of the test for carbonate ions?

Limewater turns milky only with carbon dioxide, so it confirms which gas the acid has released.

Name the gas that turns limewater milky and state which negative ion its release identifies.

Carbon dioxide, and its release on adding acid identifies the carbonate ion, CO₃²⁻.

Describe the test for sulfate ions in a solution.

Dilute hydrochloric acid is added, then barium chloride solution, and a white precipitate shows sulfate ions are present.

Explain why dilute hydrochloric acid is added before the barium chloride solution.

The acid removes any carbonate ions, which would otherwise form a white precipitate and give a false positive result.

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Chemical Analysis, Organic Chemistry and Materials

Tests for carbonate, sulfate and halide ions

3 things to know
Paper 2

Overview

The acid and limewater test for carbonate ions, the acidified barium chloride test for sulfate ions, the acidified silver nitrate test for chloride, bromide and iodide ions and their precipitate colours, and why each acid is added first.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe the tests for carbonate and sulfate ions, and why limewater and dilute hydrochloric acid are each part of them.

Key term

Describe the test that shows a substance contains carbonate ions.

Key term

What is seen immediately when dilute acid is added to a carbonate?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe the test that shows a substance contains carbonate ions.
Model answer
Dilute acid is added and the gas released is bubbled through limewater, which turns milky if carbon dioxide is given off.
2
What is seen immediately when dilute acid is added to a carbonate?
Model answer
The mixture fizzes as bubbles of carbon dioxide are released.
3
Why is limewater part of the test for carbonate ions?
Model answer
Limewater turns milky only with carbon dioxide, so it confirms which gas the acid has released.
4
Name the gas that turns limewater milky and state which negative ion its release identifies.
Model answer
Carbon dioxide, and its release on adding acid identifies the carbonate ion, CO₃²⁻.
5
Describe the test for sulfate ions in a solution.
Model answer
Dilute hydrochloric acid is added, then barium chloride solution, and a white precipitate shows sulfate ions are present.
6
Explain why dilute hydrochloric acid is added before the barium chloride solution.
Model answer
The acid removes any carbonate ions, which would otherwise form a white precipitate and give a false positive result.

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AChlorine is a pale green gas at room temperature.

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