GCSE Chemistry revision

Sulfates

Identification of ions by chemical and spectroscopic means (chemistry only)

AQA 4.8.3.5 Foundation & Higher

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Before testing for sulfates, dilute hydrochloric acid is added to the sample to remove carbonate and sulfite ions, which are impurities that could interfere with the results.
This step is essential because carbonate and sulfite ions could also react with barium ions to form a white precipitate, giving a false positive result.

1. Testing for Sulfate Ions — Step 1: Removing Impurities AQA

Before testing for sulfates, dilute hydrochloric acid is added to the sample to remove carbonate and sulfite ions, which are impurities that could interfere with the results.
This step is essential because carbonate and sulfite ions could also react with barium ions to form a white precipitate, giving a false positive result.

Barium chloride solution is added to the acidified sample, providing barium ions that react with any sulfate ions present.
The formation of a white precipitate of barium sulfate is a positive result, confirming that sulfate ions are present in the sample.

2. Testing for Sulfate Ions — Step 2: Adding Barium Chloride AQA

Barium chloride solution is added to the acidified sample, providing barium ions that react with any sulfate ions present.
The formation of a white precipitate of barium sulfate is a positive result, confirming that sulfate ions are present in the sample.

The carbonate test uses dilute hydrochloric acid and limewater, with cloudy limewater as the positive result.
The sulfate test uses dilute hydrochloric acid followed by barium chloride solution, with a white precipitate as the positive result.
The halide test uses dilute nitric acid followed by silver nitrate solution, with the colour of the precipitate identifying the specific halide ion present.

3. Summary / Key Terms AQA

The carbonate test uses dilute hydrochloric acid and limewater, with cloudy limewater as the positive result.
The sulfate test uses dilute hydrochloric acid followed by barium chloride solution, with a white precipitate as the positive result.
The halide test uses dilute nitric acid followed by silver nitrate solution, with the colour of the precipitate identifying the specific halide ion present.