AQA GCSE · Question 02.2 · Atomic structure and the periodic table
Group 1 elements react with water.
Give two observations made when potassium reacts with water.
How to approach this question
Recall the reactions of Group 1 metals with water. Potassium is very reactive. Think about what you would see. What gas is produced? What happens to the metal itself due to the heat of the reaction? What colour flame is associated with potassium ions?
Full Answer
1. Fizzing / effervescence / bubbles of gas produced.
2. Potassium melts into a sphere / ball.
3. Moves around on the surface of the water.
4. A lilac/purple flame is produced.
When potassium reacts with water, it is a very vigorous reaction. The observations are:
1. **Fizzing:** Hydrogen gas is produced rapidly, causing fizzing or effervescence. (2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g))
2. **Melting:** The reaction is highly exothermic (releases a lot of heat), which is enough to melt the potassium (melting point 63°C) into a silvery ball.
3. **Movement:** The hydrogen gas produced propels the molten ball of potassium across the surface of the water.
4. **Lilac Flame:** The heat from the reaction is sufficient to ignite the hydrogen gas, which burns with a characteristic lilac (pale purple) flame due to the presence of potassium ions.
Common mistakes
✗ Saying "it dissolves" - it reacts, it doesn't dissolve.
✗ Saying "disappears" - this is not specific enough.
✗ Confusing the flame colour with sodium's (yellow/orange).
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