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2.4.4Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Group 2 Hydroxides and Sulfates — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 29 of 53 Unit quiz

📚 IAL Edexcel Linear (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 5Solubility Trends

The two sets of compounds show opposite trends down the group.

Hydroxides
  • Hydroxides Solubility increases down the group, so Mg(OH)₂ is almost insoluble but Ba(OH)₂ dissolves much more.
Sulfates
  • Sulfates Solubility decreases down the group: MgSO₄ is very soluble, CaSO₄ slightly soluble, SrSO₄ and BaSO₄ insoluble.
Alkalinity
  • Alkalinity As the hydroxides become more soluble, more OH⁻ forms and the solution made from the oxide becomes more alkaline.
Group 1
  • Group 1 All Group 1 hydroxides and sulfates are soluble, so they give no precipitate in these tests.
CompoundMgCaSrBa
HydroxideAlmost insolubleSlightly solubleSolubleMore soluble
SulfateVery solubleSlightly solubleInsolubleInsoluble

Part 2 of 5Precipitation Tests

Precipitation Tests
Hydroxide test
  • Hydroxide test Adding NaOH(aq) to Mg²⁺(aq) gives a white precipitate: Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s).
Calcium
  • Calcium Ca²⁺(aq) with NaOH(aq) gives only a faint precipitate because Ca(OH)₂ is slightly soluble.
Barium
  • Barium Ba²⁺(aq) gives no visible precipitate with dilute NaOH(aq) because the hydroxide is much more soluble.
Sulfate test
  • Sulfate test Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) gives a dense white precipitate.

Part 3 of 5Uses and Safety of Barium Sulfate

1
Barium chloride solution is added to a sample acidified with dilute hydrochloric acid to test for sulfate ions.
2
The acid first removes carbonate ions, which would otherwise form a white precipitate of barium carbonate.
3
A white precipitate of barium sulfate that stays when acid is present confirms sulfate ions.
4
Barium sulfate is insoluble, so it is swallowed as a barium meal to make the gut visible on X-rays.
5
Soluble barium compounds are toxic, but BaSO₄ is safe because so few Ba²⁺ ions dissolve.

Part 4 of 5Worked Example

Question

  • 25.0 cm³ of 0.100 mol dm⁻³ barium chloride is added to excess sulfate solution.
  • Calculate the mass of barium sulfate precipitated (Mr = 233.4).

  1. EquationBa²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
  2. Moles of Ba²⁺ = 0.100 × 0.0250 = 2.50 × 10⁻³ mol.
  3. The ratio is 1 : 1, so moles of BaSO₄ = 2.50 × 10⁻³ mol.
  4. Mass = 2.50 × 10⁻³ × 233.4.

Answer About 0.584 g of barium sulfate is precipitated.

Part 5 of 5Exam Checklist & Tips

2.4.4 – Key Points for the Exam

  • Precipitate An insoluble solid formed when two solutions react.
  • Solubility The maximum amount of solute that dissolves in a given amount of solvent.
  • Barium meal A suspension of barium sulfate used in X-ray imaging of the digestive system.
  • Acidified barium chloride The reagent used to test for sulfate ions.

Common Mistakes

  • Stating that Group 2 hydroxides become less soluble down the group; they become more soluble.
  • Using barium chloride without acid first, so carbonate ions give a false positive.
  • Mixing up the hydroxide and sulfate trends because they run in opposite directions.
  • Naming the precipitate as barium hydroxide instead of barium sulfate.

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