If crystal growth does not start on its own after the solution in the flask returns to room temperature, identify the best ways to promote this process. Select one or more: Add a bit of solid as a seed crystal. Scratch the bottom of the flask gently with a stirring rod. Place the flask on a hot plate to evaporate some solvent. Add ice cubes to the solution.

The best way to promote the process are:

--> Add a bit of solid as a seed crystal.

--> Scratch the bottom of the flask gently with a stirring rod.

Explanation:

A crystal growth is seen is SUPERSATURATED solutions which contains more solute than it can normally dissolve at that given temperature. It is usually very UNSTABLE and capable of releasing the excess solute if disturbed, either by shaking or seeding with a tiny crystals.

Crystallization can be used for the separation of two salts with different solubilities as well as for purification of a soluble salt that contains insoluble solid impurities. Recrystallization improves the validity of the process. Crystallization can be initiated by:

--> Scratching the bottom of the flask gently with a stirring rod: scratching initiates crystallization by providing energy from the high-frequency vibrations.

--> Adding a bit of solid as a seed crystal: Seed crystals create a nucleation site where crystals can begin growth.