A small piece of potassium is added to a trough of water.
(a) Give two observations. [2]
(b) Write a balanced equation for the reaction. [1]
(c) Explain why potassium is more reactive than sodium. [3]
- (a) any two from: floats; moves around (on the surface) / darts about; fizzes / bubbles / effervescence (allow hissing); melts (into a ball); lilac flame / bursts into flame (allow catches fire); gets smaller / disappears (ignore sparks; reject 'gas produced' / 'hydrogen produced' / 'sinks')(1)
- (a) second observation from the list(1)
- (b) $2\text{K} + 2\text{H}_2\text{O} \rightarrow 2\text{KOH} + \text{H}_2$ (allow multiples; ignore state symbols)(1)
- (c) potassium atom is larger / has more (electron) shells / outer electron is further from the nucleus(1)
- (c) (so) weaker (electrostatic) attraction between the nucleus and the outer electron (accept more shielding)(1)
- (c) so the outer electron is lost more easily (reject 'gains electrons' / references to 'more electrons to lose')(1)
- Answer: (a) e.g. floats, lilac flame (b) 2K + 2H₂O → 2KOH + H₂ (c) outer electron further from nucleus, less attraction, lost more easily
- Examiner tip: 'Hydrogen is produced' is a conclusion, not an observation — write what you would SEE, e.g. fizzing.