Question

why crystallisation technique is better than simple evaporation?​

Answer 1

Not all compounds can be separated by evaporation due to their similar boiling point some solid decomposes

Answer 2

Crystallization is a better technique than evaporation to purify a solid due to the following reasons.
1.During evaporation, the solution is heated to dryness. some solids may discompose or some substance like sugar may get charred during direct heating.
2. Some impurities may still be present in the solution after filtration and they make the solid impure.
3.Direct heating or evaporation does not give crystals. Only a solid residue is left in the dish.