Sulfates — AQA GCSE Chemistry
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Sulfates explained
Sulfates covers 2 specification points, including Sulfate ions in solution produce a white… and Use of chemical tests to identify the ions in….
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Open a point below for its explanation and revision guidance.
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. 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.
Required practical 7: use of chemical tests to identify the ions in unknown single ionic compounds covering the ions from sections Flame tests (page 73) to Sulfates (page 74).
This required practical brings together the qualitative tests for positive and negative ions in single ionic compounds. You dissolve or moisten the unknown compound, then select tests that reveal its ions: flame tests for metal cations such as lithium, sodium, potassium, calcium and copper; sodium hydroxide solution for metal cations that form coloured or white precipitates; dilute acid with silver nitrate for halide ions; and dilute hydrochloric acid with barium chloride for sulfate ions. By combining results, you identify the compound. For example, a compound giving a lilac flame and a white precipitate with barium chloride contains potassium and sulfate ions, so it is potassium sulfate. The compound is single, so one cation and one anion are present.
Your focus
- Carry out the sulfate test safely by acidifying the sample and then adding barium chloride solution.
- Record the result as a white precipitate when sulfate ions are present.
- Explain how dilute hydrochloric acid prevents carbonate ions from causing a false positive result.
Show all 6 objectives
- Carry out a sequence of chemical tests to identify the ions in an unknown single ionic compound.
- Record flame colours and precipitate colours accurately in a suitable table.
- Use combined observations to name the unknown compound and justify the identification.
Sulfates exam tips
Marking 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.
- Recognise that the test is qualitative and shows the presence or absence of sulfate ions rather than their concentration.
- Select the correct test for each ion: flame tests for metal cations, sodium hydroxide solution for some metal cations, silver nitrate with dilute nitric acid for halides, and barium chloride with dilute hydrochloric acid for sulfates.
- Carry out tests on a clean nichrome or platinum wire or in clean test tubes to avoid contamination between samples.
- Record observations precisely, including flame colours and precipitate colours, before drawing a conclusion.
- Combine the positive results to name the single ionic compound, for example a lilac flame plus a white barium sulfate precipitate indicates potassium sulfate.
- Recognise that a negative result is also evidence and that several tests may be needed to identify one compound.
Examiner 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.
- 💡Plan a sequence of tests that identifies both the cation and the anion before naming the compound.
- 💡Write observations in a table with separate columns for the test, the observation and the conclusion.
- 💡Use the exact colour names for flames and precipitates, because vague descriptions lose credit.
Common Mistakes
- Adding barium chloride before the acid: 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: the correct observation is a solid white precipitate, not a change of solution colour.
- Assuming any white precipitate proves sulfate ions: the correction is that the acid must be present to rule out carbonate ions, which also give a white precipitate with barium ions.
- Using the same wire or test tube for different samples without cleaning: the correction is to clean the wire in concentrated hydrochloric acid and use fresh test tubes to prevent cross-contamination.
- Recording a conclusion instead of an observation, such as writing that the compound contains copper rather than recording a blue-green flame or blue precipitate.
- Adding silver nitrate without dilute nitric acid when testing for halides: the correction is to acidify first so carbonate ions do not produce a misleading precipitate.