CRYSTALLIZATION-Definition, Principle, Demonstration, Application, Advantage of Crystallization, Practice Problems, and FAQs

Have you ever witnessed or seen in a cinema, a snowfall? If yes, then you must have felt or seen a snowflake.

Each snowflake acts as a tiny ice crystal, which tends to stick to each other and fall to the ground as a result of gravity. Low temperatures and the presence of moisture in the environment cause snowflakes to develop. The formation of tiny crystals of snow is an example of the crystallization process which is occurring in nature.

It is the process of obtaining a pure substance from a mixture in the form of crystals.

Over some time, a bottle of honey that is in liquid form and has a brownish colour starts to convert into a brownish white solid in winter. This is because the sugar molecules in honey start forming sugar crystals using the crystallization process.

Table of content:

Crystallization
Principle of crystallization
Choice of solvent
Demonstration
Application of crystallization
Advantages of crystallization
Practice problems
Frequently asked questions
Related Topics

Crystallization

Crystallization is the process through which a substance's atoms or molecules arrange themselves from a solution into a well-defined three-dimensional lattice, reducing the system's overall energy.

It is used for the purification of solid substances.

Examples: Fractional (multiple) crystallization can be used to purify sugar from its mixture with common salt using ethanol as a solvent. The salt is insoluble in ethanol and so precipitated out of the solvent.

The sugar can be crystallized from the ethanol solution.

Sometimes the crystals formed are coloured due to the presence of coloured impurities. In this situation, dissolve the crystals in the same solvent and add a small amount of charcoal to them. Then boil the mixture for 15 to 20 minutes and filter away the carbon. Charcoal absorbs all the coloured impurities and gives a colourless solution containing the pure substance.

Principle of crystallization

Crystallization is based on the principle of the difference in solubilities of compounds and impurities in a particular solvent.

Solutes usually tend to be more soluble in hot solvents than in cold solvents. If a saturated hot solution is allowed to cool without any disturbance, the decreased solubility at low temperatures forces the solute out of the solution slowly. The outgoing solute tends to grow into a pure crystal. The solid is filtered and dried. Slow cooling results in the formation of large crystals.

Choice of solvent

The solvent should be inert and not react chemically with an impure substance.
The solubility of the crystallizing substance should be relatively higher than other solutes.
The solubility should be high at higher temperatures.
The solvents which can be employed for crystallization are water, alcohol, benzene, acetone, ethyl acetate, petroleum ether, carbon tetrachloride, chloroform, etc.

Demonstration

Add impure benzoic acid to a beaker containing water.
Dissolve benzoic acid to make it a saturated solution.
Boil the saturated solution and add more benzoic acid to make it a supersaturated solution.
Allow the solution to cool without any disturbance for crystal formation.
The crystals so formed are then filtered with the help of filter paper.
Pure crystals of benzoic acid are obtained and collected.

Application of crystallization

Sea water purification
Alum purification
Chiral isomers are also separated using crystallization
Crystallization of salts is the most common use. It is also a low-budget procedure. Two utilizations of the procedure: purification of compounds and synthesis of crystals.

Advantages of crystallization

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 for this process is low.

Practice Problems

Q 1. Purification by crystallization is based on which principle:

a. The amount of solute that a solvent can dissolve increases as the temperature rises.
b. The amount of solute that can be dissolved by a solvent decreases with increases with temperature
c. Both A and B
d. None of the above

Answer: A)

One of the most important purification procedures is crystallization, which purifies compounds by removing undesirable by-products. This crystallization method is commonly used to purify crystalline substances. The amount of solute that can be dissolved by a solvent rises with temperature, which is the principle behind crystallization.

Q 2. Solid organic compounds are purified by which method?

a. Steam Distillation
b. Crystallization
c. Sublimation
d. Distillation under reduced pressure

Answer: B)

By using Crystallization, solid organic compounds are purified.

Q 3. What happens if the sample is exposed to too much solvent during crystallization?

a. The recovery of a pure sample increases
b. The recovery of pure samples happens more quickly
c. The recovery of the pure sample decreases
d. There was no change in the pure sample recovery

Answer: C)

At ambient temperature, impure chemicals are only slightly soluble in a solvent, but they become significantly soluble at higher temperatures. Heat is used to concentrate the solution, resulting in a supersaturated solution. When the pure compound cools, it crystallizes and is filtered away. The recovery of the material via crystallization will be reduced if too much solvent is utilized.

Q 4. Which of the following is a crystallization process example?

a. Alum purification
b. Seawater purification
c. Gas separation from air
d. None of the above

Answer: A)

The crystallization procedure purifies an impure sample of alum or copper sulfate. Distillation is used to purify seawater.

Q 5. What happens if the crystals given below are heated separately?

a. Blue vitriol
b. Benzoic acid

Answer:

1. Blue vitriol is copper sulfate decahydrate, on heating it loses its water of crystallization and changes to white powder
2. Benzoic acid undergoes sublimation when its crystals are heated.

Frequently asked questions

Q 1. What is the water of crystallization?

Answer: The number of water molecules present in the crystal and corresponding to each formula unit of the substance is referred to as the water of crystallization.

Q 2. What is the purpose of crystallization?

Answer: Crystallization allows a substance to be prepared in its purest form.

Q 3. What are the different methods of purification of a substance?

Answer: There are different techniques used for the purification of a substance. Some are distillation, sublimation, filtration, chromatography, crystallization, and differential extraction.

Q 4. Can we use the crystallization technique for the impurities having comparable solubilities?

Answer: Yes, by using repeated crystallization, a substance can be purified that contains impurities that have comparable solubilities.

Q 5. Why is heating done in the process of crystallization?

Answer: Heating is done to form a supersaturated solution, which on cooling back will throw out substances that will combine and grow into a big crystal.