Crystallization: Separation of Substances


What is Crystallization?

Crystallization is a technique used for the purification of substances. A separation technique to separate solids from a solution.

On adding a solid substance in a liquid and stirring it, the solid dissolves in the fluid. But when added more and more solid to the liquid, a point comes after which no more solid dissolves in the liquid. This point is called saturation point and the fluid is called saturation solution.

Crystallization process

  1. The solution is heated in an open container
  2. The solvent molecules start evaporating, leaving behind the solutes
  3. When solution cools, crystals of solute start accumulating on the surface of the solution
  4. Crystals are collected and dried as per the product requirement
  5. The undissolved solids in the liquid are separated by the process of filtration
  6. The size of crystals formed during this process depends on the cooling rate.
  7. Large number of tiny crystals are formed, if the solution is cooled at a fast rate
  8. Large crystals are formed at slow cooling rates


Separation technique of substance by Crystallization


Here is an experiment to understand crystallization clearly:

Step 1: Take 50 ml water in a beaker

Step 2: Add sugar in it and stir it

Step 3: Now heat the solution

Step 4: Repeat the process continuously

Step 5: After some time there will be a point at which no more sugar can be dissolved in water. This stage is the saturation point, and the solution is referred as a saturated solution

Step 6 : Now filter the sugar with the help of a filter paper

Step 7: Collect the filtrate in a glass bowl and cool it

Step 8: We will observe that some fine crystals are formed in the bowl

Step 9: The process of filtration can separate these crystals. The liquid left after the removal of crystals is known as mother liquor

Application of crystallization:

  • Purification of seawater

  • Separation of alum crystals from impure samples

  • In the pharmaceutical industry, crystallization is used as a separation and purification process for the synthesis and isolation of co-crystals, pure active pharmaceutical ingredients (API), controlled release pulmonary drug delivery, and separation of chiral isomers.

This was just a brief layout of the process of crystallization. To know about crystallization, other methods of purification of organic compounds and more, register with Byju’s and download our app.

Practise This Question

____________ is a process that separates a pure solid in the form of its crystals from a solution.