AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The relative solubilities of the sulfates of the elements Mg–Ba in water.
Full specification wording
Group 2, the alkaline earth metals: The relative solubilities of the sulfates of the elements Mg–Ba in water.
Group 2 sulfates become less soluble down the group, the opposite way round from the hydroxides.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Group 2 sulfates become less soluble down the group, the opposite way round from the hydroxides. Magnesium sulfate dissolves freely, calcium sulfate is slightly soluble, and strontium and barium sulfates are effectively insoluble. This is why barium chloride is the reagent for detecting sulfate ions: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) gives a dense white precipitate. The solution must be acidified first with dilute hydrochloric acid to destroy carbonate ions, which would also give white precipitates. Nitric acid must not be used because it oxidises sulfite ions to sulfate ions, causing a false positive. Sulfuric acid must never be used, because it supplies the very ion being tested for. This insolubility makes barium sulfate safe as a barium meal, since it is opaque to X-rays and the toxic Ba²⁺ is not absorbed from the gut.
What earns the marks
- Stating that sulfate solubility decreases from magnesium to barium.
- The ionic equation Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
- The observation of a white precipitate.
- Acidifying with dilute hydrochloric acid first, to remove carbonate ions.
- Explaining that a barium meal is safe because barium sulfate is insoluble, so Ba²⁺ is not absorbed.
Common mistakes and how to put them right
Assuming hydroxide and sulfate solubilities change the same way down the group, when they run in opposite directions.
Acidifying with sulfuric acid, which adds the very ion being tested for.
Acidifying with nitric acid, which oxidises sulfite to sulfate and gives a false positive.
Describing the barium sulfate precipitate as cream or yellow instead of white.
Treating barium sulfate as soluble and so predicting no precipitate.
Exam tips
- State the acid by name; 'acidify' on its own does not identify a reagent and loses the mark.
- Keep the two Group 2 solubility trends on one line of revision notes, since questions often test them against each other.
Try the lesson
The start of the Group 2, the alkaline earth metals: The relative solubilities of the sulfates of the elements Mg–Ba in water. lesson, made from checked teaching notes for this specification point.
Have a go first
Which Group 2 sulfate is the most soluble in water?
Listen
Group 2 sulfates become less soluble down the group, the opposite way round from the hydroxides. Magnesium sulfate dissolves freely, calcium sulfate is slightly soluble, and strontium and barium sulfates are effectively insoluble. This is why barium chloride is the reagent for detecting sulfate ions: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) gives a dense white precipitate. The solution must be acidified first with dilute hydrochloric acid to destroy carbonate ions, which would also give white precipitates. Nitric acid must not be used because it oxidises sulfite ions to sulfate ions, causing a false positive. Sulfuric acid must never be used, because it supplies the very ion being tested for. This insolubility makes barium sulfate safe as a barium meal, since it is opaque to X-rays and the toxic Ba²⁺ is not absorbed from the gut. A strong answer earns marks for these points. stating that sulfate solubility decreases from magnesium to barium. the ionic equation Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). the observation of a white precipitate. acidifying with dilute hydrochloric acid first, to remove carbonate ions. Exam tip: State the acid by name; 'acidify' on its own does not identify a reagent and loses the mark.
See it
Key words
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Group 2 sulfate solubility
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test for sulfate ions
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barium sulfate precipitate
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See every definition
- Group 2 sulfate solubility
- The sulfates of the elements Mg–Ba become less soluble in water down the group, the opposite way round from the hydroxides.
- test for sulfate ions
- Barium chloride is the reagent, giving a dense white precipitate of barium sulfate, and the solution must be acidified first with dilute hydrochloric acid.
- barium sulfate precipitate
- Formed as a dense white precipitate by Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) when barium chloride is added to a solution containing sulfate ions.
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