Separation of mixtures
2 marks · EasyExplain why crystallisation is a good technique for separating sodium chloride from its aqueous solution.
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
Crystallisation removes water by evaporating it, allowing sodium chloride to form solid crystals that are easy to separate from the solution.
Must show: evaporation of water, formation of sodium chloride crystals
Also accept: When water is partially evaporated from the solution, sodium chloride becomes less soluble and crystallises out.; As water is evaporated and the solution becomes saturated, sodium chloride leaves solution as crystals, allowing its separation.
Common mistakes: States only evaporation removes water (without mentioning crystals); Describes filtration instead of crystallisation
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Mentions evaporation/removal of water (or formation of a saturated solution).
A1 · 1 mark · States that sodium chloride forms crystals that can be separated.
Worked solution
Crystallisation works because as water is evaporated from sodium chloride solution, the solution becomes saturated, allowing sodium chloride to form crystals that can then be separated.