Group 1 metals react with water. Describe what you would observe when a small piece of potassium is placed on water, and explain why potassium reacts more vigorously than lithium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Group 1 of the periodic table contains lithium, sodium and potassium. All of these metals react with cold water, producing a metal hydroxide and hydrogen gas.
Model answer (5 marks)
A small piece of potassium placed on water will:
1. Move or float on the surface of the water.
2. Catch fire, producing a bright lilac/violet flame.
3. Effervesce, giving off bubbles of hydrogen gas.
Potassium reacts more vigorously than lithium because it has more electron shells, so its outermost electron is further from the nucleus and is lost more easily.
1. Move or float on the surface of the water.
2. Catch fire, producing a bright lilac/violet flame.
3. Effervesce, giving off bubbles of hydrogen gas.
Potassium reacts more vigorously than lithium because it has more electron shells, so its outermost electron is further from the nucleus and is lost more easily.
Examiner tips
- Use the exact phrases: "moves/floats", "catches fire", "effervescence", "more electron shells", "outer electron is lost more easily".
- Show the sequence of observations before explaining the reason for the greater reactivity.
- Keep the answer concise – one sentence per point – to fit the 5‑mark allocation.
Common mistakes
- Writing the reaction as a single sentence without separating the observations and explanation.
- Using the wrong colour of flame (e.g. orange instead of lilac/violet).
- Forgetting to mention the effervescence or the electron‑shell explanation.
Mark scheme (5 marks)
- Potassium moves/floats on the surface of the water
- Potassium catches fire / burns with a lilac/violet flame
- Effervescence / bubbles of gas are produced (hydrogen gas given off)
- Potassium has more electron shells than lithium / potassium's outer electron is further from the nucleus
- The outer electron in potassium is lost more easily than in lithium, so potassium reacts more vigorously
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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