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2.4.3Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Oxides and Hydroxides — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 4 parts Lesson 28 of 53 Unit quiz

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Part 1 of 4Formation and Basic Character

2.4.3 Oxides and Hydroxides

Metal oxides and hydroxides of Groups 1 and 2 are bases, and many of them are alkalis.

Oxides
  • Oxides form when the metals burn in oxygen: 2Ca(s) + O₂(g) → 2CaO(s).
Hydroxides
  • Hydroxides form from reaction with water or from oxide plus water: CaO(s) + H₂O(l) → Ca(OH)₂(s).
Group 1
  • Na₂O(s) + H₂O(l) → 2NaOH(aq) gives a colourless strongly alkaline solution.
Group 2
  • Oxides react with water to give alkaline solutions that become more alkaline down the group.
Beryllium
  • BeO is an exception because it is amphoteric, reacting with both acids and bases.
Solubility contrast
  • Group 1 hydroxides are very soluble, but Mg(OH)₂ is only sparingly soluble, so its solution is weakly alkaline.

Part 2 of 4Reactions with Acids

Every oxide and hydroxide here neutralises an acid, giving a salt and water.

1
Metal oxide + dilute hydrochloric acid → metal chloride + water, for example MgO + 2HCl → MgCl₂ + H₂O.
2
Metal oxide + dilute sulfuric acid → metal sulfate + water, for example MgO + H₂SO₄ → MgSO₄ + H₂O.
3
Metal hydroxide + dilute hydrochloric acid → metal chloride + water, for example Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O.
4
Metal hydroxide + dilute sulfuric acid → metal sulfate + water, for example 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.
5
Barium and strontium sulfates are insoluble and form a coating on the solid, so reactions with sulfuric acid slow down.

Part 3 of 4Uses of Group 2 Bases

Uses of Group 2 Bases
Calcium hydroxide
  • Known as slaked lime, it neutralises acidic soil and its solution is limewater, used to test for CO₂.
Calcium oxide
  • Quicklime reacts vigorously with water, releasing a lot of heat, and is used in making calcium hydroxide.
Magnesium hydroxide
  • It is only slightly soluble, so it makes a mild antacid for indigestion without being a strong alkali.
Magnesium oxide
  • It has a very high melting point, so it is used to line furnaces.
Barium hydroxide
  • Barium hydroxide is the most soluble Group 2 hydroxide, so its solution is strongly alkaline.

Part 4 of 4Exam Checklist & Tips

2.4.3 – Key Points for the Exam

  • Base A substance that neutralises an acid, accepting H⁺ ions.
  • Alkali A base that dissolves in water to give hydroxide ions.
  • Amphoteric Able to react as both an acid and a base.
  • Limewater A solution of calcium hydroxide in water.

Common Mistakes

  • Writing the product of a Group 1 oxide with water as an oxide again, instead of a hydroxide.
  • Forgetting to balance acid reactions, such as the 2 in front of HCl with Ca(OH)₂.
  • Saying all Group 2 hydroxides are equally soluble; solubility increases down the group.
  • Describing magnesium hydroxide as strongly alkaline when it is only sparingly soluble.

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