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Water of Crystallization

  • Last Updated : 11 Nov, 2021

Crystallization is a technique for separating solids from a solution or, to put it another way, a procedure for purifying things. This is the most frequent method for purifying seawater. Some salts have a few water molecules in their crystal structure as an essential component. The water of crystallisation refers to the water molecules that make up a crystal’s structure. Hydrated salts are salts that contain the water that causes crystallisation. Below is a detailed explanation of crystallisation water, hydrated and anhydrous salts and also the action of heat on the hydrated salts.

Water of Crystallization

The water of crystallization refers to the water molecules that make up the structure of a salt crystal. Water that has been chemically linked into a crystal structure is known as the water of crystallization. 

The production of crystals frequently requires the use of water. A fixed number of molecules in one formula of a unit of salt is referred to as water of crystallization. Hydrates are crystal salts that contain water during the crystallization process. The water of crystallisation is also known as the water of hydration or crystallisation water.

The formation of pure crystals from an aqueous solution causes the water to crystallize. Contaminants are not present in these crystals. Heat has a strong effect on these crystals. The water of crystallization, also known as hydration water, is made up of water molecules found inside crystals. In the production of crystals from aqueous solutions, water is frequently present. The water of crystallisation is the total quantity of water in a substance at a certain temperature and is usually present in a definite ratio in different situations. Water contained in the crystalline framework of a metal complex or a salt that is not directly bonded to the metal cation is known as “water of crystallization.” 

Many chemicals absorb water molecules into their crystalline frameworks after crystallization from water or water-containing solvents. Heating a sample may usually remove the water of crystallization, but the crystalline qualities are frequently lost. The dihydrate of sodium chloride, for example, is unstable at room temperature. Proteins crystallize with a lot of water in the crystal lattice, compared to inorganic salts. 

Proteins with more than 50% water of crystallization and copper sulphate pentahydrate with five molecules of water of crystallization are examples of water of crystallization.

What are Hydrated Salts?

A hydrated salt is a crystalline salt molecule that is loosely connected to a small number of water molecules. When the anion of an acid and the cation of a base are joined to form an acid-base molecule, salt is formed. An anhydrate is a salt molecule that is not bound to any water molecules, while a hydrated salt is one that is bound to water molecules. 

The water molecules in a hydrated salt are incorporated into the crystalline structure of the salt. A hydrated salt is one in which the ions in its crystalline structure are coupled with a number of water molecules. These water molecules are referred to as crystallisation fluids or hydration waters. Hydrated salts are salts that contain water during crystallisation. Every hydrated salt has a fixed number of crystallisation water molecules in its single ‘formula unit.’ 

The following are some examples of hydrated salts.

  • CuSO4.5H2O copper sulphate crystals, which include 5 molecules of crystallisation water in one formula unit.  Copper sulphate pentahydrate is the chemical name for it.
  • Calcium sulphate crystals, often known as gypsum crystals, are written as CaSO4.2H2O because they contain two molecules of water of crystallisation in one formula unit.  Calcium sulphate dihydrate is another name for it.
  • Sodium carbonate crystals, also known as washing soda crystals, are written as Na2CO3.10H2O because each formula unit contains 10 molecules of water of crystallisation. It’s known as sodium carbonate decahydrate

Copper sulphate, Calcium sulphate, and Sodium carbonate are some of the hydrated salts based on the above-mentioned examples. Hydrated salts can be found in a variety of conditions, including freshwater. Salt has a flexible crystalline structure that allows it to easily bond to water molecules and becomes hydrated. Sodium chloride or salt absorbs water vapour in the air or comes into touch with liquid water. Free-flowing chemicals, for example, generate salt molecules when the compounds in a particular area’s soil or rock dissolve and mix with the groundwater, eventually becoming hydrated with the water molecules.

Uses of Hydrated Salt

  • Epsom salts are the most well-known application of hydrated salts in daily life. Many of the compounds found in salts are required by the human body. Those compounds, however, may be difficult to absorb or receive only through diet. People have traditionally taken healing baths in regions where hydrated salts naturally occur, believing them to have curative effects. Epsom salts are an example of this. Even though its medicinal properties haven’t been proven, Epsom salt has commercial potential as a home remedy. Another important application of hydrated salt is in industry.
  • Salt, including hydrated salts, has a wide range of industrial applications. Many industries depend on hydrated salt. In the chemical industry, salt is the primary constituent in more than half of the products. Glass, paper, rubber, and textile industries also employ hydrated salt.
  • Salt is also employed as water softening salt in both industrial and home water softening systems. Furthermore, hydrated salt is employed extensively in the alternative energy sector because of its capacity to maintain a constant temperature for an extended period of time.

What are Anhydrous salts?

Anhydrous refers to a substance that is completely devoid of water. Anhydrous salts are salts that have lost their water of crystallisation. As a result, anhydrous salts lack crystallisation water. An anhydrous salt becomes hydrated when water is introduced to it. Calcium chloride, in its anhydrous state, has a variety of applications. It can also detect humidity in the air and vapour. Calcium chloride is used in several industrial safe-checks to measure road erosion or cracks.

In experiments, removing moisture is important because it typically controls side reactions or other unexpected consequences in the chemicals being studied. Organic compounds can be dried with drying agents like Na2SO4 and MgSO4. When these anhydrous materials come into touch with water, however, the water is absorbed rather than merely evaporated. As a result, such drying solutions are incomplete, traces of water may be left behind, and compounds may be regarded polluted as a result.

Action of heat on hydrated salts

When hydrated salts are heated to high temperatures, they lose their crystallisation water. The hydrated salts lose their regular shape and colour when the water of crystallisation is lost, and they become colourless powdery particles. Since there is no water of crystallisation in anhydrous salts, so when water is added to an anhydrous salt, it hydrates and returns to its original colour. The following example will help to clarify this.

The copper sulphate crystals are blue in colour. When copper sulphate crystals are heated to a high temperature, they lose all of their water and become anhydrous copper sulphate, which is white.

CuSO4.5H2O                   →                CuSO4                     +             5H2O
(Hydrated copper sulphate)               (Anhydrous copper sulphate)            (Water)

As a result of the loss of water during crystallisation, blue copper sulphate crystals turn white when heated vigorously. Copper sulphate crystal dehydration is a reversible process. As a result, adding water to anhydrous copper sulphate causes it to become hydrated and turn blue, resulting in the production of hydrated copper sulphate.

CuSO4                       +           5H2O                            CuSO4.5H2O
(Anhydrous copper sulphate)               (Water)                     (Hydrated copper sulphate)

When you add water to anhydrous copper sulphate, it turns blue. This property of anhydrous copper sulphate is utilised to detect the presence of moisture or water in a liquid. To white anhydrous copper sulphate powder, a few drops of the liquid to be tested are added. The presence of moisture or water in anhydrous copper sulphate is indicated by the appearance of blue colour.

Sample Questions

Question 1: Why do salts containing water of crystallization appear to be perfectly dry?


The water of crystallization is a part of the crystal structure of water. Since the water of crystallization is not free water, so it does not wet the salt. Thus, the salts containing water of crystallization appear to be perfectly dry.

Question 2: How is the water of crystallization useful for the crystals of salts?


The water of crystallization gives the crystals of salts their shape and in some cases imparts their colour. For example, the presence of the water of crystallization in iron sulphate crystals imparts them a green colour.

Question 3: Write the name and formula of a salt that has five molecules of crystallisation water in it.


Copper sulphate crystals include five molecules of water of crystallisation in one formula unit. Its formula is CuSO4.5H2O.

Question 4: What is the colour of FeSO4.7H2O crystals? What happens to the colour when it’s heated? 


FeSO4.7H2O  crystals are green in colour. It loses the water of crystallisation when heated, resulting in anhydrous ferrous sulphate. It has a reddish-brown colour to it.

Question 5: Is water present in anhydrous sodium chloride crystals? 


Since there are no water molecules in an anhydrous material, so there is no water present in anhydrous sodium chloride crystals. These anhydrous crystals were created by carefully eliminating the water of crystallisation from hydrated salts.

Question 6: Why do copper sulphate crystals change colour when heated?


The blue colour of the copper sulphate crystals turns white on heating because of the loss of water during crystallisation.

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