Lithium, sodium and potassium are all alkali metals. When sodium is added to water, it reacts vigorously to produce a gas and a solution with a pH greater than 7. State the name of the gas produced and the name of the solution formed when sodium reacts with water. Explain why the solution formed is alkaline.

Pearson Edexcel International GCSE Chemistry (4CH1) — 2.1 Group 1 (alkali metals) · State · 4 marks · View as Markdown

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

The alkali metals in Group 1 all react with water. Sodium reacts more vigorously than lithium but less vigorously than potassium.

Model answer (4 marks)

The gas produced is hydrogen.
The solution formed is sodium hydroxide.
Sodium hydroxide dissociates in water to give hydroxide ions (OH⁻).
The presence of OH⁻ ions makes the solution alkaline (pH > 7).

Examiner tips

  • Use the exact terms ‘hydrogen’ and ‘sodium hydroxide’.
  • Show the dissociation step to link OH⁻ to alkaline pH.
  • Keep the answer concise – one sentence per point.

Common mistakes

  • Writing ‘water’ instead of ‘hydrogen’ as the gas.
  • Calling the solution ‘sodium oxide’ or ‘sodium carbonate’ instead of ‘sodium hydroxide’.
  • Failing to explain that OH⁻ ions raise the pH above 7.

Mark scheme (4 marks)

  1. The gas produced is hydrogen
  2. The solution formed is sodium hydroxide
  3. Sodium hydroxide dissociates / dissolves in water to produce hydroxide ions (OH⁻)
  4. Solutions with a pH greater than 7 are alkaline / hydroxide ions cause the pH to be above 7

Key terms in this question

alkali metals · alkali

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