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2.2.2Physical Chemistry

Solubility of Group 2 Compounds: Hydroxides and Sulfates — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 57 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Two Opposite Trends

Hydroxides
  • Hydroxides Solubility increases down the group, so the solution becomes more alkaline as more OH⁻ ions dissolve.
Sulfates
  • Sulfates Solubility decreases down the group, and barium sulfate is the least soluble of the Group 2 sulfates.
Magnesium hydroxide
  • Magnesium hydroxide Mg(OH)₂ is only slightly soluble, which is why a white precipitate forms with aqueous sodium hydroxide.
Calcium hydroxide
  • Calcium hydroxide Ca(OH)₂ is sparingly soluble, and its solution is known as limewater.
CompoundSolubility down the groupExamples
Hydroxides M(OH)₂IncreasesMg(OH)₂ almost insoluble; Ba(OH)₂ fairly soluble
Sulfates MSO₄DecreasesMgSO₄ soluble; BaSO₄ insoluble

Part 2 of 4Test for Sulfate Ions

Barium sulfate is so insoluble that barium ions give a clear precipitate test for sulfate.

1
Add dilute hydrochloric acid to the unknown solution first, to remove carbonate ions that would also form a precipitate with barium.
2
Add aqueous barium chloride (or barium nitrate) solution to the acidified sample.
3
A white precipitate of barium sulfate confirms the presence of sulfate ions in the original solution.
4
The ionic equation is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
5
Do not use sulfuric acid to acidify the sample, because it would supply sulfate ions and give a false positive.

Part 3 of 4Applications of the Trends

Applications of the Trends
Barium meal
  • Barium meal Insoluble BaSO₄ is swallowed before X-rays because barium absorbs X-rays and is safe, as it does not dissolve in the body.
Antacid
  • Antacid Magnesium hydroxide in milk of magnesia neutralises excess stomach acid, and its low solubility keeps the pH from becoming too high.
Soil treatment
  • Soil treatment Calcium hydroxide is used to neutralise acidic soils because it is cheap and moderately alkaline.
Toxic barium
  • Toxic barium Soluble barium compounds are poisonous, so only the insoluble sulfate is used medically.

Part 4 of 4Exam Checklist & Tips

2.2.2 – Key Points for the Exam

  • Precipitate An insoluble solid formed when two solutions are mixed.
  • Limewater A saturated solution of calcium hydroxide used to test for carbon dioxide.
  • Sparingly soluble Dissolving only to a very small extent.
  • Barium meal A suspension of barium sulfate used to make the digestive tract visible on X-rays.

Common Mistakes

  • Using sulfuric acid to acidify before the sulfate test, which adds sulfate ions.
  • Reversing the two solubility trends, so that hydroxides decrease and sulfates increase down the group.
  • Omitting state symbols such as (s) for the BaSO₄ precipitate.
  • Describing barium sulfate as safe because barium is non-toxic, when it is safe only because it is insoluble.

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