Sodium and potassium are both alkali metals in Group 1 of the periodic table. When a small piece of potassium is added to water, it reacts more vigorously than sodium does with water. Explain why potassium reacts more vigorously with water than sodium does, and describe two observations a student would make when potassium is added to water.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alkali metals react with cold water to produce a metal hydroxide and hydrogen gas. Potassium is below sodium in Group 1.
Model answer (5 marks)
Potassium atoms have more electron shells than sodium atoms, so the outermost electron is further from the nucleus and more shielded by inner electrons. This makes the ionisation energy lower, allowing the outer electron to be lost more easily. Consequently, potassium reacts more vigorously with water.
Observations:
1. The potassium piece fizzes and melts into a ball on the water surface.
2. The ball burns with a lilac/purple flame (or ignites).
Observations:
1. The potassium piece fizzes and melts into a ball on the water surface.
2. The ball burns with a lilac/purple flame (or ignites).
Examiner tips
- Use the term ‘ionisation energy’ and explain the effect of shielding and distance from the nucleus.
- Show the two specific observations – fizzing/melting and the lilac flame – to gain full marks.
Common mistakes
- Confusing the number of shells with the number of valence electrons.
- Mentioning only one observation or describing a non‑lilac flame colour.
Mark scheme (5 marks)
- Potassium atoms have more electron shells (energy levels) than sodium atoms
- The outer electron in potassium is further from the nucleus / more shielded from the nucleus
- The outer electron is lost more easily in potassium, so it reacts more vigorously
- One correct observation: the potassium moves around / fizzes / melts into a ball on the surface of the water
- Second correct observation: potassium burns with a lilac / purple flame OR the potassium ignites
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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