Crystallisation: The procedure involved in the production of crystals is referred to as crystallisation. When you hear the word crystal, what comes to mind is a solid, shining substance that could be rare or valuable. The impure substance is dissolved in a suitable solvent until it reaches a nearly saturated solution at a temperature higher than room temperature, then allowed to cool during the crystallisation process. To learn more about crystallization, read the below article.

Define Crystallisation

The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallisation. Complete evaporation yields only the residue, which may contain impurities. The process of crystallisation, on the other hand, can produce large crystals of pure substances from their solutions. As a result, the process of crystallisation is commonly used to purify solid compounds.

Crystallisation is a technique for obtaining large crystals of a pure solid substance from an impure solid substance. There are two types of impurities in an impure solid substance: insoluble impurities and soluble impurities. Filtering the solution removes the insoluble impurities, while crystallisation removes the soluble impurities.

For example, common salt obtained from seawater is an impure substance containing numerous impurities. As a result, common salt is purified through the crystallisation process. Crystallisation is also used to purify an impure alum sample.

Crystallisation Process

The crystallisation process is used to obtain a pure solid substance from an impure sample. This is accomplished as follows:

1. A solution is formed by dissolving the impure solid substance in the minimum amount of water to form a solution. 
2. To remove insoluble contaminants, the solution is filtered.
3. The clear solution is gently heated on a water bath until a concentrated solution or saturated solution is obtained (this can be tested by periodically dipping a glass rod in hot solution. Small crystals form on the glass rod, indicating that the solution is saturated). Then turn off the heat.
4. Allow the hot, saturated solution to gradually cool.
5. Pure solid crystals are formed. Impurities remains dissolved in solution.
6. Using filtration method, separate the pure solid crystals and dry them.

Separating Substances Via Crystalisation

The crystallisation method can be used to purify impure copper sulphate to obtain pure copper sulphate. This is done as follows:

1. To make copper sulphate solution, take about \(10\,{\rm{g}}\) of impure copper sulphate and dissolve it in a small amount of water in a china dish.
2. To remove insoluble impurities, filter the copper sulphate solution.
3. Gently heat the copper sulphate solution on a water bath to evaporate water and obtain a saturated solution (this can be tested by periodically dipping a glass rod in hot solution). The solution is saturated when small crystals form on the glass rod). Then turn off the heat.
4. Allow the hot, saturated copper sulphate solution to cool slowly.
5. Crystals of pure copper sulphate are formed. Impurities remain behind in the solution.
6. Filter and dry the copper sulphate crystals from the solution.

Why is Crystallization better than Evaporation?

Copper sulphate can also be recovered from the solution obtained after filtration by direct heating. As a result, the water evaporates, leaving behind the solid copper sulphate. However, the crystallisation technique is always superior to evaporation for the following reasons.

1. During direct heating, some solids may decompose and some substances, such as sugar, may become charred.
2. Some minute particle impurities may still be present in the solution obtained after filtration. They will be found in the solid residue that remains after evaporation. The crystals formed during crystalllation are always pure.
3. Direct heating or evaporation does not produce crystals. Only a solid residue remains in the dish.

Applications of Crystallisation

1. It is used for separating common salt from sea water.
2. It is used to separate alum crystals (phitkari) from crude samples.
3. It is used for separating two salts that are soluble in water from a mixture of two.

Water of Crystallisation

Some salts have a few water molecules as an essential component of their crystal structure. The water molecules which form part of the structure of a Crystal are called water of crystallisation. The salts which contain water of crystallisation are called hydrated salts. Every hydrated salt has a ‘fixed number’ Water of crystallisation molecules in its single ‘formula unit’.

It should be noted that water of crystallisation is a component of a ‘crystal structure of a salt’. Because crystallisation water is not water, it does not wet the salt. As a result, the salts containing water of crystallisation appear to be completely dry. The water of crystallisation gives the salt crystals their ‘shape’ and, in some cases, their ‘colour.’

For example,

1. Copper sulphate crystals contain \(5\) molecules of water of crystallisation in a single formula unit, and are thus denoted as \({\rm{CuS}}{{\rm{O}}_4}.\,5{{\rm{H}}_{\rm{2}}}{\rm{O}}.\) It is known as copper sulphate pentahydrate.
2. Sodium carbonate crystals (washing soda crystals) contain \(10\) molecules of water of crystallization per formula unit and are thus written as \({\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3}.10{{\rm{H}}_2}{\rm{O}}.\) This is called sodium carbonate decahydrate (‘decahydrate’ means ‘ten molecules of water’).
3. Calcium sulphate crystals (gypsum crystals) contain two molecules of water of crystallisation in a single formula unit and are thus denoted as \({\rm{CaS}}{{\rm{O}}_4}.2{{\rm{H}}_2}{\rm{O}}\) is called calcium sulphate dihydrate (‘Dihydrate’ means ‘two molecules of water’).
4. Iron sulphate crystals contain \(7\) molecules of water of crystallisation per formula unit and are thus written as \({\rm{FeS}}{{\rm{O}}_4}.7{{\rm{H}}_2}{\rm{O}}.\) It is called iron sulphate heptahydrate (‘Heptahydrate’ means ‘seven molecules of water’).

Experiment to show the Action of Heat on Copper Sulphate Crystals

1. Place some crystals of copper sulphate in a dry boiling tube (These are blue in colour).
2. Strongly heat the crystals by holding the boiling tube over the flame of a burner for some time.
3. When heated, the blue copper sulphate crystals turn white and form a powdery substance. In the boiling tube, we can also see tiny droplets of water (These droplets have formed from the water of crystallisation which was removed from copper sulphate crystals during heating).
4. After cooling the boiling tube, add \(2\) or \(3\) drops of water to the white copper sulphate powder that has formed above.
5. The crystals of copper sulphate are restored their original blue colour. They become blue again. 

The colour changes that occur when blue coloured copper sulphate crystals are heated to form ‘white’ anhydrous copper sulphate and then changed back to blue when water is added.

Summary

The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallisation. Crystallisation method is better than evaporation. Crystallisation is a technique for obtaining large crystals of a pure solid substance from an impure solid substance. The crystallisation method is used to separate common salt from seawater, alum crystals from crude samples and for separating two salts that are soluble in water from a mixture of two.

Frequently Asked Questions on Crystallisation

Frequently asked questions related to crystallisation is listed as follows:

Q.1. What is crystallisation? What is it used for?
Ans: The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallization.
Uses of crystallization are as follows:
a. It is used for separating common salt from sea water.
b. It is used to separate alum crystals (phitkari) from crude samples.
c. It is used for separating two salts that are soluble in water from a mixture of two.

Q.2. What is procedure of crystallisation?
Ans: The procedure of crystallisation is as follows:
a. A solution is formed by dissolving the impure solid substance in the minimum amount of water to form a solution. 
b. To remove insoluble contaminants, the solution is filtered.
c. The clear solution is gently heated on a water bath until a concentrated solution or saturated solution is obtained (this can be tested by periodically dipping a glass rod in hot solution. Small crystals form on the glass rod, indicating that the solution is saturated). Then turn off the heat.
d. Allow the hot, saturated solution to gradually cool.
e. Pure solid crystals are formed. Impurities remains dissolved in solution.
f. Using filtration method, separate the pure solid crystals and dry them.

Q.3. What happens during crystallisation?
Ans: To produce a pure solid, the solution is heated in a water bath until saturated. The solution is saturated when particles of a regular and definite shape, known as crystals, begin to appear. After that, the solution is removed from the flame and allowed to cool. Crystals with distinct geometrical shapes begin to form as the liquid cools.

Q.4. What are the examples of crystallization?
Ans: The examples of crystallization: The formation of ice cubes and snow from the crystallisation of water. When honey is placed in a jar and exposed to suitable conditions, it crystallises.

Q.5. What is the purpose of crystallisation?
Ans: Crystallization is a versatile and powerful technique for separating components from a liquid mixture.