8.6 Tests for carbonate, halide and sulfate ions
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What colour precipitate does silver nitrate give with bromide ions?
Choose one answer.
Correct Not quite: the answer is Cream
Mark scheme
B
Model answer
B: silver bromide is a cream precipitate.
Describe the test for sulfate ions and give the result.
Mark scheme
- add dilute hydrochloric acid (1)
- then barium chloride solution (1)
- white precipitate (1)
Model answer
Add dilute hydrochloric acid, then barium chloride solution; a white precipitate shows sulfate ions.
Explain why dilute nitric acid is added before silver nitrate in the test for halide ions.
Mark scheme
- to remove carbonate ions (1)
- which would also form a precipitate / give a false positive (1)
Model answer
The nitric acid reacts with any carbonate ions, which would otherwise form a precipitate with silver nitrate and give a false result.
Explain why hydrochloric acid must NOT be used to acidify the solution in the halide test.
Mark scheme
- it contains chloride ions, which would give a white precipitate (1)
Model answer
Hydrochloric acid contains chloride ions, so it would always give a white precipitate of silver chloride.
A white solid fizzes with dilute acid, and the gas turns limewater milky. Identify the negative ion.
Mark scheme
- carbonate (1)
Model answer
Carbonate ion.
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