Compare Group 2 sulfate solubilities
State and use the decrease in sulfate solubility down Group 2, including precipitation evidence.
Section 10 · Group 2
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Group trends are qualitative unless data are supplied. Show physical states and balance equations; distinguish reaction from dissolution and precipitation. This sequence does not replace supervised practical experience. Written practice is self-checked
Before you begin
- Use Group 2 formulas, ionic charges, state symbols, balanced equations and acid reaction patterns.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How does Group 2 sulfate solubility change from MgSO₄ to BaSO₄?
What you will learn
- Order sulfates by qualitative solubility
- Use sulfate precipitation as evidence
- Explain why barium sulfate can coat a reacting solid
Sulfates become less soluble down the group
The qualitative trend is decreasing solubility from MgSO₄ towards BaSO₄. Magnesium sulfate is soluble; calcium sulfate is sparingly soluble; strontium sulfate and barium sulfate are very poorly soluble, with BaSO₄ effectively insoluble in the usual aqueous tests.
A precipitate forms when the local concentrations exceed what remains dissolved. A white solid observation is evidence of low solubility, but colour alone does not identify an unknown compound.
Apply the trend to acid reactions carefully
When a Group 2 metal, oxide or carbonate reacts with sulfuric acid, its sulfate may precipitate. BaSO₄ can form a coating on the metal or carbonate surface and reduce contact with the acid.
Do not say that all sulfates precipitate: magnesium sulfate is soluble and calcium sulfate is only sparingly soluble. Use the metal identity and the stated conditions.
Pause and explain
Which sulfate is effectively insoluble in the usual aqueous comparison?
Worked example
Compare MgSO₄ and BaSO₄ in water and predict which one forms a persistent precipitate.
Show the worked solution
- Sulfate solubility decreases down Group 2.
- MgSO₄ is soluble while BaSO₄ is effectively insoluble in the usual test.
- BaSO₄ is therefore the persistent precipitate.
Answer MgSO₄ dissolves; BaSO₄ is effectively insoluble and forms a precipitate.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Order MgSO₄, CaSO₄ and BaSO₄ from most to least soluble.
Give me a hint
Sulfate solubility decreases down Group 2.
Compare my reasoning
- Mg is highest in the listed sequence.
- Ca lies between Mg and Ba.
- BaSO₄ is the least soluble.
MgSO₄ > CaSO₄ > BaSO₄ in solubility.
Look for these in your work
- All salts included
- Correct trend direction
- Ca placed between Mg and Ba
Predict what is observed when aqueous magnesium sulfate is mixed with aqueous barium chloride.
Give me a hint
Exchange ions and check the sulfate product's solubility.
Compare my reasoning
- Ba²⁺ combines with SO₄²⁻ to form BaSO₄.
- BaSO₄ is insoluble and appears as a white precipitate.
- Magnesium chloride remains in solution.
BaSO₄(s) forms as a white precipitate; MgCl₂ remains aqueous.
Look for these in your work
- Precipitate formula correct
- State justified
- Other product identified
Explain why dilute sulfuric acid may stop reacting visibly with barium carbonate after initial fizzing.
Give me a hint
Connect carbonate acid reaction products with the low solubility trend.
Compare my reasoning
- The reaction produces CO₂, so initial fizzing occurs.
- BaSO₄ is formed and is very poorly soluble.
- The solid coating can reduce further acid access to BaCO₃.
BaCO₃ + H₂SO₄ → BaSO₄(s) + CO₂ + H₂O; insoluble BaSO₄ can coat the carbonate and slow continued reaction.
Look for these in your work
- Gas formation explained
- Sulfate product identified
- Surface effect linked to solubility
Common mistakes
- Saying sulfate solubility increases down Group 2
- Calling CaSO₄ just as soluble as MgSO₄
- Assuming all products with sulfate are soluble
- Treating a white precipitate as unique proof of barium
What can you explain now?
How does Group 2 sulfate solubility change from MgSO₄ to BaSO₄?
Compare with the explanation
It decreases
The sulfates become less soluble down the group.
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Make it stick
How do sulfate solubilities change down Group 2, and what does that predict in a barium precipitation test?
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