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It is the technique used in the purification of organic compounds. It is based on the difference in solubilities of the compound and the solvent impurities. The impure compound is dissolved in a solvent and the solution is concentrated to form a saturated solution. While cooling the solution, the pure compound crystallizes out and is removed by the process of filtration. This filtrate contains many impurities and a small quantity of the compound. The crystallization is carried out in a mixture of this solvent when the compound is highly soluble in one solvent.

Repeated crystallization becomes important for the purification of compounds that contain impurities of solubilities. In this process, the impure compound is converted into crystals. The conceptual steps of Crystallization are -

1. Choice of Solvent

  • The solvent cannot react chemically with an impure substance.
  • The solvent dissolves more substance.
  • The solvents which can be employed for crystallization are water, alcohol, benzene, acetone, ethyl acetate, petroleum ether, carbon tetrachloride, chloroform, etc.

2. Preparation of Solution

The impure material is powdered and then heated into a clinical flask with solvent.

3. Filtration of Solution

The hot solution is formed after preparing the solution. Then it is filtered through filter paper or a hot water funnel.

4. Crystallization and Separation of Crystals

After completing the above process, the solution is allowed to cool for some time till the crystal begins to separate.

5. Drying the Crystals

The formation of crystals occurs by drying them in air or by placing them in a steam oven.

6. Removal of Color

Sometimes the crystals formed are colored due to the presence of colored impurities. In this situation, dissolve the crystals in the same solvent and add a small amount of charcoal and in it. Then boil the mixture for 15 to 20 minutes. Charcoal absorbs all the colored impurities and forms a pure substance.

Process of Crystallization

We have summarized the process of crystallization as follows -

  • The desired solution is transferred in a container and is heated. The container must be open.
  • Once the heating begins, the solvent part of the solution begins to evaporate. And all the solute particles are left behind.
  • Once the solution cools down, all the solute crystals formed start to accumulate on the solution's surface.
  • Then all the crystals that formed are collected. Every crystal is dried upon its requirement of the product formed.
  • Using the filtration procedure, all the solid particles that do not dissolve in the liquid are distinguished.
  • The crystal size that formed depends upon the rate of cooling.
  • The number of tiny-sized crystals is formed when the cooling of a solution is performed at a rate that is fast.
  • Whereas, when the rate of cooling is slow, it results in the formation of large crystals.

Fractional Crystallization


It is a method used to separate or purify two or more compounds having different solubilities in one solvent. The process of separating different components of a mixture by repetition of crystallization is known as fractional crystallization.

Precautions To Be Taken During the Experiment

  • Ensure that crystals are washed properly.
  • Avoid overheating the solution.
  • The filtrate should evaporate slowly.
  • Don't use a fast cooling procedure.

Advantages of Crystallization

Some major advantages of Crystallization are -

  • High purity products are obtained from minimal steps by the process of Crystallization.
  • The dry products which are obtained from this process can be directly packed or stored.
  • The energy requirement is low for this process.
  • It helps in purification of seawater.
  • It is used to separate alum crystals from impure material.


  • Crystallization of salts is the most common use. It is also a low-budget procedure. Two utilizations of the procedure: Purification of compounds & Synthesis of crystals
  • Seawater purification
  • Alum crystals are separated from the sample that is impure.

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