AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Sulfate ions in solution produce a white precipitate with barium chloride…
Full specification wording
Sulfates: Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid.
This test identifies sulfate ions in an aqueous sample.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This test identifies sulfate ions in an aqueous sample. Acidify the solution with dilute hydrochloric acid, then add barium chloride solution dropwise. Any sulfate ions present react with barium ions to form barium sulfate, which is insoluble and appears as a fine white precipitate. The acid prevents false positives from carbonate ions, which would also give a white precipitate with barium ions; carbonate ions are removed as carbon dioxide. The method is qualitative: a white precipitate shows sulfate ions are present, while no precipitate or only cloudiness from the reagents means they are absent.
For example: Adding the reagents to dilute sodium sulfate solution gives an immediate white solid, whereas pure water stays clear.
What earns the marks
- State that dilute hydrochloric acid is added before barium chloride solution, so the sample is acidic during the test.
- Identify barium chloride solution as the reagent that supplies barium ions, which combine with sulfate ions.
- Describe the positive result as a white precipitate of barium sulfate.
- Explain that the acid removes carbonate ions by reacting them to form carbon dioxide, preventing a false positive white precipitate.
- Recognise that the test is qualitative and shows the presence or absence of sulfate ions rather than their concentration.
Common mistakes and how to put them right
Adding barium chloride before the acid.
Put it right: The correct order is to acidify first, then add barium chloride, so carbonate ions cannot cause a misleading precipitate.
Confusing the white precipitate with a colour change in the solution.
Put it right: The correct observation is a solid white precipitate, not a change of solution colour.
Assuming any white precipitate proves sulfate ions.
Put it right: The acid must be present to rule out carbonate ions, which also give a white precipitate with barium ions.
Exam tips
- Name both reagents and give the order of addition when describing the test.
- Link the acid to the removal of carbonate ions rather than treating it as an unnecessary extra.
- Use the term precipitate and state its colour, because a colourless solution turning cloudy is not the same observation.
Try the lesson
The start of the Sulfates: Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid. lesson, made from checked teaching notes for this specification point.
Have a go first
A student adds barium chloride solution to a solution containing sulfate ions. What colour is the precipitate formed?
Listen
This test identifies sulfate ions in an aqueous sample. Acidify the solution with dilute hydrochloric acid, then add barium chloride solution dropwise. Any sulfate ions present react with barium ions to form barium sulfate, which is insoluble and appears as a fine white precipitate. The acid prevents false positives from carbonate ions, which would also give a white precipitate with barium ions; carbonate ions are removed as carbon dioxide. The method is qualitative: a white precipitate shows sulfate ions are present, while no precipitate or only cloudiness from the reagents means they are absent. For example, adding the reagents to dilute sodium sulfate solution gives an immediate white solid, whereas pure water stays clear. A strong answer earns marks for these points. State that dilute hydrochloric acid is added before barium chloride solution, so the sample is acidic during the test. Identify barium chloride solution as the reagent that supplies barium ions, which combine with sulfate ions. Describe the positive result as a white precipitate of barium sulfate. Explain that the acid removes carbonate ions by reacting them to form carbon dioxide, preventing a false positive white precipitate. A common slip: Assuming any white precipitate proves sulfate ions. Instead, the acid must be present to rule out carbonate ions, which also give a white precipitate with barium ions. Exam tip: Name both reagents and give the order of addition when describing the test.
See it
Key words
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Barium chloride solution
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White precipitate
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Dilute hydrochloric acid
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Qualitative
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See every definition
- Barium chloride solution
- The reagent added dropwise that supplies barium ions, which combine with sulfate ions.
- White precipitate
- The fine solid of barium sulfate that appears when sulfate ions are present, showing they are present.
- Dilute hydrochloric acid
- Added to acidify the sample before the barium chloride solution, removing carbonate ions by reacting them to form carbon dioxide.
- Qualitative
- Describes a test that shows the presence or absence of sulfate ions rather than their concentration.
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