Potassium is an alkali metal. When a small piece of potassium is placed on the surface of water, a reaction occurs. State the name of the gas produced and the name of the solution formed. Then explain why potassium reacts more vigorously with water than lithium does.

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).

Alkali metals are the elements in Group 1 of the periodic table. They are typically soft and have relatively low melting points. They react with water to produce an alkaline solution.

Model answer (4 marks)

Hydrogen gas is produced.
Potassium hydroxide solution is formed.
Potassium has more electron shells, so its outer electron is further from the nucleus than lithium's.
Therefore the outer electron is lost more easily in potassium than in lithium.

Examiner tips

  • Use the exact terms ‘hydrogen gas’ and ‘potassium hydroxide solution’.
  • Explain the electron‑shell difference to justify the greater reactivity.
  • Keep the answer concise – one sentence per point.

Common mistakes

  • Writing ‘hydrogen’ instead of ‘hydrogen gas’.
  • Using ‘potassium oxide’ instead of ‘potassium hydroxide’.
  • Failing to mention the electron‑shell reason for the increased reactivity.

Mark scheme (4 marks)

  1. Hydrogen (gas) is produced
  2. Potassium hydroxide solution is formed
  3. Potassium has more electron shells / its outer electron is further from the nucleus than lithium's
  4. Therefore the outer electron is lost more easily (in potassium than in lithium)

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