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2.2.3Organic Chemistry

Group 2 Hydroxides and Sulfates — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 7 of 42

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Part 1 of 5Hydroxide Solubility

2.2.3 Group 2 Hydroxides and Sulfates
Trend
  • Solubility of Group 2 hydroxides increases down the group.
Magnesium hydroxide
  • Almost insoluble, so it is a white precipitate when sodium hydroxide is added to magnesium ions.
Calcium hydroxide
  • Sparingly soluble; its solution is limewater.
Barium hydroxide
  • Much more soluble and gives a strongly alkaline solution.
Alkalinity
  • The pH of a saturated hydroxide solution increases down the group because more hydroxide ions are released into solution.

Part 2 of 5Sulfate Solubility

Sulfate Solubility
Trend
  • Solubility of Group 2 sulfates decreases down the group.
Magnesium sulfate
  • Very soluble in water, so no precipitate forms when barium ions are added to it.
Calcium sulfate
  • Sparingly soluble, giving a slight cloudiness when barium ions or sulfate ions are added.
Strontium and barium sulfates
  • Essentially insoluble in water, so they appear as dense white precipitates when formed.
Solubility rule
  • If a sulfate is insoluble it provides a quick way to separate barium ions from mixtures by precipitation.
CompoundHydroxideSulfate
MgAlmost insolubleSoluble
CaSparingly solubleSlightly soluble
BaSolubleInsoluble

Part 3 of 5Testing for Sulfate Ions

1
Add dilute hydrochloric acid to the solution to remove carbonate ions that could interfere.
2
Add barium chloride or barium nitrate solution.
3
A white precipitate of barium sulfate forms if sulfate ions are present.
4
The ionic equation is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
5
The precipitate does not dissolve in the added acid, confirming sulfate.
6
Always add dilute acid before barium chloride in the sulfate test, since carbonates also give white precipitates with barium ions.

Part 4 of 5Uses

Magnesium hydroxide
  • Used as an antacid because it neutralises excess stomach acid.
Calcium hydroxide
  • Added to soil to reduce acidity.
Barium sulfate
  • Used in barium meals for X-ray imaging because it is insoluble and not absorbed into the body.
Sulfur dioxide removal
  • Calcium-based compounds are used to remove SO₂ from power station emissions.

Part 5 of 5Exam Checklist & Tips

2.2.3 – Key Points for the Exam

  • Sparingly soluble Only a very small amount dissolves in water.
  • Precipitate An insoluble solid produced in a solution reaction.
  • Antacid A substance that neutralises excess stomach acid.
  • Limewater Aqueous calcium hydroxide.

Common Mistakes

  • Reversing the trends so that sulfate solubility increases down the group.
  • Using sulfuric acid to acidify the sulfate test, which supplies extra sulfate ions.
  • Forgetting to describe the precipitate as white.
  • Saying barium compounds are always safe; soluble barium compounds are toxic.

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