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# Crystallization

Crystallization is a technique for purification of the substances. It is a technique that separates a solid from its solution. The process in which the solid dissolve in the liquid arranges itself in well-defined 3-D structures called the crystal is called crystallization. When any substance undergoes crystallization its molecule arranges itself in a fixed structure at an angle to form a 3-D structure called the crystal and then the crystal is removed from the solution. The crystal is a pure substance and thus the process of crystalization is widely used in the purification of material from its impure solution.

## What is Crystallization?

Crystallization is a natural process that happens when the materials solidify from a liquid, it can also occur when a solid precipitates from a liquid or gas. The crystallization process occurs resulting in a change in the physical property of the liquid, such as the change in temperature, change in its acidity.

The crystallization process is carried out on the basis of the size and shapes of the molecules involved, and their chemical properties. Crystals can be made out of 1 species of atom, different species of ions, or even huge molecules like proteins. The unit cell is the smallest unit of the crystal. It is the base form of atoms or molecules upon which more units can be attached.

## Examples of Crystallization

Some common examples of where the crystallization process occurs are listed below:

• The crystallization of water forms ice cubes and snow.
• The crystallization of honey when it is placed in a jar and exposed to suitable conditions.
• The formation of stalagmites and stalactites (especially in caves).
• The deposition of gemstones is an example of crystal formation.

## Crystallization Process

The crystallization process consists of two major events. One is nucleation and the other is crystal growth which both are driven by thermodynamic properties also as chemical properties. Nucleation is the primary step where the solute molecules or atoms dispersed within the solvent start to collect into clusters, that become stable under the present operating conditions. These stable clusters constitute the nuclei. Therefore, the clusters got to reach a critical size so as to become stable nuclei. It’s at the stage of nucleation that the atoms or molecules are arranged in a defined and periodic manner that defines the crystal structure.

Crystal growth refers to the increase in the size of the nuclei which is able to achieve the critical cluster size. Many compounds have the power to crystallize with some having different crystal structures, a phenomenon called polymorphism. Certain polymorphs can be metastable, meaning that although it’s not in thermodynamic equilibrium, they are kinetically stable and requires some input of energy to initiate a change to the equilibrium phase.

### Theory of Crystallization

The three major stages in the process of Crystallization are:

• Supersaturation of Solution: It can be done in three ways: Heating the solution, Cooling the solution, and Salting it out.
• Nucleation: This takes place in several steps. During their random motion, the atoms/ molecules/ ions will come closer to one another and form aggregates called Clusters. These clusters will combine to form an EMBRYO. In this stage, only the lattice formation begins. These embryos combine to form NUCLEI. From nuclei, crystals are formed.
• Crystal Growth: Once the crystals are formed, nuclei formation stops and crystal growth begins.

### Water of Crystallization

The number of water molecules that combine with other molecules in definite proportions to form a crystal structure is called the water of crystallization. This water of crystalization is responsible for the shape and color of the crystal. For Example in Washing Soda, Na2CO3.10H2O, the water of crystallization is 10.

## Crystallization of Salt

The crystallization of salt occurs when the liquid in the solution tends to evaporate and the temperature of the solution decreases making the nucleus of the crystal of the salt grow with the passage of time if the crystal is big enough it can be easily removed by the filtration process.

For example, in a brine solution when the water evaporates, the temperature of the solution decreases and the nucleus of the NaCl crystal started to form marking the start of the crystallization process which further increases with the passage of time till all the NaCl in the solution is crystallized. However, it has some extra compounds such as sodium sulfate, calcium chloride, and potassium chloride which are later purified by industrial processes.

## Types of Crystallization

The process of Crystallization can occur in three different ways and they are,

• Cooling Crystallization
• Evaporative Crystallization
• Reactive Crystallization

Now let’s about each of them in detail.

### Cooling Crystallization

When the product’s solubility increases significantly with rising temperature, cooling crystallization is attractive. In similar circumstances, cooling crystallization usually uses less energy than evaporative crystallization. Saturated solution at 30°C by cooling it to 0°C the precipitation occurs when a large crystal mass is precipitated after the hydration of water.

### Evaporative Crystallization

In evaporative crystallization, the solvent’s evaporation causes the crystallization to occur. At constant temperature the precipitation of crystals by increasing the solute concentration above the solubility threshold. The solute/solvent mass ratio is increased using the technique of evaporation.

### Reactive Crystallization

In reactive crystallization, the crystals are formed when the reactant reacts to form the solute. The crystallization process is faster in reactive crystallization than in any other type of crystallization.

## Factors Affecting Crystallization

Various factors affecting the crystallization process are,

• Presence of Another Substance
• Solvent Used
• Nucleation
• Crystal Growth
• Rate of Cooling

Now let’s learn about each in detail, and how it affects the crystallization process.

### Presence of Another Substance

Sodium chloride crystallized from aqueous solutions produces cubic crystals. If sodium chloride is crystallized from a solution containing a small amount of urea, the crystals obtained will have octahedral faces.

### Solvent Used

The solvent with moderate solubility is preferred for crystallization. The presence of benzene can help crystal growth. Avoid highly volatile solvents.

### Nucleation

Nucleation refers to the formation of the very first atom of a crystal. The first atom that grows is called Nucleus. It acts like a seed that allows the growth of more atoms around it. Crystals initially form via “nucleating events”. After a crystallite has nucleated it must grow. Nucleation sites are necessary for the formation of crystals. Excess nucleation sites cause smaller average crystal sizes.

### Crystal Growth

Crystals grow by the ordered deposition of the solute molecules onto the surface of a pre-existing crystal. Crystal growth is facilitated by the environment changing slowly over time. Keep crystal growth vessels away from sources of mechanical agitation (e.g. vibrations). Set up away from vacuum pumps, hoods, doors, drawers, and so on.

### Rate of Cooling

Quality crystals grow best over time in near-equilibrium conditions. The longer the time, the better the crystals. Faster crystallization is not as good as slow crystallization. Faster crystallization higher chance of lower quality Crystals.

## Differentiate between Crystallization and Evaporation

Crystallization is a process in which a solid pure particle comes together in the form of crystals and in case of evaporation the liquid of the solution is removed and we are only left with the salt dissolved in it. Boh crystallization and evaporation are processes used for the purification of the substance and the differences between them are discussed in the table below.

## Applications of Crystallization

Crystallization is a key component of almost all processes in the manufacturing of small-molecule pharmaceuticals. It is essential in both processing and pharmaceutical development. Since the properties of a solid material (polymorphism) can dramatically affect the process or the product’s compliance and effect (dissolution rate for example), monitoring and controlling the isolation of solids for the various applications through crystallization is of paramount interest.

So crystallization is used in:

• Purification of Seawater.
• Purification of Drugs.
• In the pharmaceutical industry in the preparation of organic and inorganic API 8- separation of API from galenical extracts.
• In the separation of Alum crystals from impure samples.

## FAQs on Crystallization

### Q1: What is the Definition of Crystallization?

Crystallization is a process that happens when the materials solidify from a liquid, in this process, the molecules of the solute dissolved in the liquid combined together to form a crystal.

### Q2: What is the Water of Crystallization?

The fixed amount of water associated with hydrated crystals that is an integral part of the crystal is known as the water of crystallisation.

Example: In Blue Vitriol CuSO4.5H2O, the water of crystallization is 5 and in Glauber Salt Na2SO4.10H2O the water of crystallisation is 10.

### Q3: What is a Decahydrate Crystalline Salt?

Washing Soda is a decahydrate crystalline salt. Its molecular structure is (Na2CO3.10H2O) as we see it has 10 water molecules and hence its name is decahydrate.

### Q4: What are the Advantages of Crystallization?

The key advantages of crystallization are listed below.

• A product of high purity can be obtained from one single step via the process of crystallization.
• The dry products formed from crystallization can be directly packaged and stored.
• The energy requirements and the operating temperatures of this process are relatively low.

### Q5: What is the Crystallization Point?

The temperature at which the crystallization of the solid starts, i.e. its nucleus begins to form is called the crystallization point.

### Q6: What are some Examples of Crystallization?

• Crystallization of water to form ice cubes
• Crystallization of honey placed in a jar
• Formation of stalagmites and stalactites