Rubidium and caesium
4 marks · NormalWrite a balanced chemical equation for the reaction of potassium with water.
Describe what you would observe when a small piece of sodium is placed in water.
Suggest one safety precaution that should be taken when carrying out this reaction.
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(a) 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
(b) Sodium melts into a ball, moves quickly across the surface, fizzes/bubbles, and eventually disappears.
(c) Wear eye protection/safety goggles (or: use a safety screen, use small pieces only, keep away from open flames).
Common mistakes: Unbalanced equation; Potassium observations instead of sodium; Omitting eye protection as a safety measure
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correct balanced equation for potassium and water
B1 (b) · 1 mark · Sodium melts/moves on water surface OR fizzing/bubbling
B2 (b) · 1 mark · Second correct observation for sodium and water
B3 (c) · 1 mark · States any proper safety precaution
Worked solution
When sodium is placed in water, it melts to form a ball, moves rapidly across the surface, fizzes, and eventually disappears.
Safety precaution: Wear eye protection. Alternatives: use small pieces, keep away from flames, use safety screens.