Reactions of metals with water
4 marks · NormalPotassium, sodium, lithium, and calcium all react with cold water, but in different ways.
Describe two observations you would make when potassium is added to cold water.
Explain what happens to calcium granules when they are added to cold water.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Potassium moves rapidly on the surface and burns with a lilac flame; there is fizzing (hydrogen gas produced).
(b) Calcium sinks, then rises as hydrogen bubbles form; a white solid (calcium hydroxide) is produced and the water may turn cloudy.
Common mistakes: Not mentioning hydrogen bubbles; Confusing colourless solution with white solid; Describing flame for sodium or lithium instead of potassium
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Potassium burns with a lilac/purple flame.
B2 (a) · 1 mark · Fizzing/effervescence/movement/rapid motion on water; hydrogen gas produced.
B3 (b) · 1 mark · Calcium granules sink, but rise due to bubbles of gas (hydrogen) forming.
B4 (b) · 1 mark · A white solid (calcium hydroxide) is formed; water may become cloudy.
Worked solution
a) When potassium is added to cold water:
- It burns with a lilac flame (1).
- Potassium moves rapidly on the surface, fizzing as hydrogen gas forms (1).
b) When calcium granules are added to cold water:
- The granules initially sink, then rise as hydrogen bubbles form on their surface (1).
- White calcium hydroxide solid is produced (may turn water cloudy) (1).