Explain why silicon dioxide does not dissolve in water, whilst sulfur dioxide dissolves readily and forms an acidic solution.

AQA A-Level Chemistry (7405) — 3.2.4 Properties of Period 3 elements and their oxides (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Silicon dioxide has a giant covalent (macromolecular) structure; many strong covalent bonds must be broken for it to dissolve, requiring energy that water cannot supply. In contrast, sulfur dioxide consists of discrete SO₂ molecules; it reacts with water to give H₂SO₃, which dissociates to release H⁺ ions, producing an acidic solution.

Examiner tips

  • Mention the giant covalent network for SiO₂ and the energy requirement for breaking bonds
  • State that SO₂ is molecular and reacts with water to form H₂SO₃
  • Show that H₂SO₃ ionises to give H⁺, giving acidity

Common mistakes

  • Confusing SiO₂ as ionic or covalent liquid
  • Failing to mention the reaction of SO₂ with water
  • Not recognising that H₂SO₃ dissociates to produce H⁺

Mark scheme (5 marks)

  1. Silicon dioxide has a giant covalent (macromolecular) structure
  2. Many strong covalent bonds must be broken in silicon dioxide, requiring too much energy for water to overcome
  3. Sulfur dioxide has a simple molecular structure / consists of discrete molecules
  4. Sulfur dioxide reacts with water to form sulfurous acid (H₂SO₃)
  5. Sulfurous acid dissociates / ionises in water to release H⁺ ions, making the solution acidic

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