What is the main difference between evaporation and crystallisation?

Correct answer: Crystallisation focuses on obtaining pure crystals

The direct answer is option A: crystallisation focuses on obtaining pure crystals. Evaporation removes solvent, commonly water, by changing it into vapour. It may leave a solid residue, but rapid or complete evaporation can give impure or poorly formed solids. Crystallisation uses a suitable solution, often concentrates it and then cools it carefully, so a relatively pure solid forms as crystals while many impurities remain in the solution. Option A is correct because the main purpose of crystallisation is purification and recovery of a solid in crystal form. Option B is wrong: evaporation does not always use a magnet; magnets are unrelated to ordinary evaporation. Option C is wrong because crystallisation is commonly used for dissolved solids, not only gases. Option D is wrong because the methods have different purposes and are not identical. Memory cue: evaporation removes solvent; crystallisation aims at pure crystals.