Water of Crystallisation - Hydrated Salts, Hydration Enthalpy and Some Important Compounds of s-Block Elements, Practice Problems and FAQ

Have you ever stopped to look at the green bed of endless meadows? If you had, you would have noticed how beautifully the dew drops surrounded the long and sleek grass leaves. Dew drops or even raindrops on the grass would be significant as a single entity to a poet's eye. A philomath with a keen interest in chemistry would relate it to the phenomenon of crystalline possessing 'Water of crystallisation.' Hydrated salts exist in the same way that hydrated leaves do!

Take a look at a hydrated leaf and a hydrated salt. The droplets would most likely fall off if you shook the leaf. However, the water of crystallisation, which is present in some salt molecules, is embedded in them in such a way that no such light physical forces can force it out. Furthermore, bright sunlight would vaporise the water on the leaves, and for the hydrated salts– to remove the embedded water molecules present inside their crystalline structures (i.e., their water of crystallisation), a bit more effort (of 'Heating') is required than just natural sunlight.

Now let us try to decode more about the concept of ‘Water of Crystallisation’ and understand more about Hydrated Salts that possess water of crystallisation.

TABLE OF CONTENTS

What is ‘Water of Crystallisation’?
Hydrated Salts
Hydration of Alkali Metals
Hydration of Alkaline Earth Metals
Action of Heat on Hydrated Salts
Uses of Hydrated Salts
Practice Problems
Frequently Asked Questions - FAQ

What is ‘Water of Crystallisation’?

Water molecules found inside crystals are known as ‘water(s) of crystallisation’ or ‘water(s) of hydration’ in chemistry. The creation of crystals from aqueous solutions frequently includes the incorporation of water. The entire mass of water in a substance at a specific temperature is sometimes referred to as the water of crystallisation. It is usually present in a definite (stoichiometric) ratio.

Water in the crystalline framework of a metal complex or a salt that is not directly bound to the metal cation is known as ‘water of crystallisation’.
Hence, water that is chemically attached to a molecule crystal is known as ‘water of crystallisation’.
Water of crystallisation is also known as Hydration water. The salt molecules that contain water in their crystalline structures are also defined as hydrated salts.

Example: Copper sulphate pentahydrate (), sodium carbonate decahydrate ( ), Epsom salt ()

Quite a number of chemical compounds tend to incorporate water molecules into their crystalline frameworks when they crystallise from water or solvents containing water. Although a sample's water of crystallisation may usually be eliminated by heating it, the crystalline qualities are frequently lost.

Hydrated Salts

A hydrated salt is the one in which the salt molecule is linked to water molecules. A hydrated salt is a salt molecule that is linked to a particular amount of water molecules, according to another definition. Salt is created when an acid's anion and a base's cation combine to form an acid-base molecule.

The majority of salts are hydrated crystals, which means that a certain amount of water is connected to the crystal structure.
Hydrated salts are produced when ionic compounds face exposure to air and get bonded with molecules.
This is basically an ion-dipole interaction between the cation and the polar water () molecules.
A hydrated salt is the one in which the metal cations of the salt, in its crystalline structure, are coupled or surrounded by a fixed number of water molecules. This fixed number of water crystallisation molecules in a particular molecule of salt is called its ‘formula unit.’
This crystallisation water can be removed from the salt by heating it, resulting in an anhydrous structure.
An anhydrate is a salt molecule that has no water molecules attached to it, whereas a hydrated salt has water molecules bound to it.

Examples:

Sodium carbonate in a crystalline hydrated form is called washing soda which has the formula .
is available in natural mineral forms such as gypsum and is related to hydrates. Its two common hydrates are Plaster of Paris (.) and Gypsum (.).

Hydration of Alkali Metals

Hydration enthalpy is defined as the amount of the energy released when one mole of gaseous ions dissolves in a large amount of water i.e. infinitely diluted.

Hydration energy is released after attaining the stability gained because of the electrostatic attraction between water molecules and metal cations (ion-dipole interaction). The more the electrostatic attraction between the metal cation and the water molecules, the more the hydration energy.

The smaller the ion, the higher the charge density, the stronger the electrostatic attraction between water molecules and metal cations, and the higher the hydration enthalpy.
Hence, on moving down the group, the hydration enthalpy of alkali metal ions decreases with the increase in ionic size as the charge density decreases.

Thus, the correct order of hydration energy of alkali metals is

has the maximum degree of hydration or is the most soluble, and for this reason, lithium salts are mostly hydrated.

Example:

Smaller ions have a higher charge density and can be hydrated by more water molecules. This releases a higher enthalpy of hydration and makes the hydrated ions more stable.
Hydrated radius is defined as the radius of the ion and the closely bound water molecules (distance between the nucleus of a metal ion and the boundary of the water molecule). The ions with bigger size and low charge hold water molecules less tightly and thus have a smaller hydrated radius.
As the size of the cation decreases, the charge density of the cation increases and the degree of hydration increases.
As the degree of hydration increases, the hydrated ionic radius increases and the ionic mobility of the metal ions decreases.
The correct order of the ionic radius of alkali metals is

The correct order of the hydrated radii of alkali metal ions is

The correct order of the conductance for the alkali metals is

Hydration of Alkaline Earth Metals

Hydration enthalpies of Group II elements (alkaline earth metals) also decrease with increasing ionic radii down the group.

Order of hydration enthalpies of alkaline earth metal cations is

Hydration enthalpies of Group II metals (alkaline earth metals) are greater in magnitude with respect to Group I metals (alkali metals).
Hence, Group II metal ions have greater tendencies to form hydrated salts than Group I.

Example: Magnesium chloride and Calcium chloride exists as and .

Action of Heat On Hydrated Salts

Anhydrous salts i.e., salt crystals devoid of water of crystallisation is obtained when the hydrated salts are subjected to heat. When heated to high temperatures, hydrated salts lose their crystallisation water. When the water of crystallisation is removed, the hydrated salts lose their regular shape and colour, and they become colourless powdery particles. Because anhydrous salts lack the water of crystallisation, when water is supplied to them, they hydrate and return to their natural colour.

Example:

Plaster of Paris (.) is synthesised from gypsum (.). When gypsum is heated at , it loses its water molecules to become calcium sulphate hemihydrate (Plaster of Paris).

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

Again on adding water to anhydrous salts of copper sulphate, it turns blue. This principle is employed in detecting the presence of water or moisture in other liquids.

Uses of Hydrated Salts

Hydrated salts are used in a variety of industries. More than half of the products in the chemical sector have salt as a key ingredient. Hydrated salt finds utility in glass, paper, rubber, and textile manufacturing.

In both industrial and residential water softening systems, salt is used as a water softening salt. Furthermore, because of its ability to keep a consistent temperature for an extended length of time, hydrated salt is widely used in the alternative energy sector.

Gypsum (.) is used to make Portland cement.

Sodium carbonate decahydrate is used as washing soda.

One of the most well-known applications of hydrated salts in daily life is Epsom salts (). The human body requires many of the chemicals found in salts. However, some substances may be difficult to digest or only available through diet.

Traditional therapeutic baths have been taken in areas where hydrated salts naturally abound, with the belief that they offer medicinal properties. Epsom salt has commercial potential as a home cure, despite the fact that its therapeutic effects have not been proven.

Practice Problems

Q. 1. Among alkali metals, only lithium salts contain water of crystallisation, why?

Answer: Lithium is the smallest of the alkali metals. As a result, ions may polarise water molecules more easily than other alkali metals. As a result, water molecules form crystallisation water by bonding with lithium salts. As a result, hydrated lithium compounds such as trihydrated lithium chloride () have become very common. Ions' polarising power decreases as their size increases. Anhydrous salts of other alkali metal ions are prevalent as a result.

Q. 2. Why do the salts of elements of Group II (Alkaline earth metal) have more water of crystallisation than their Group II (alkali metal) counterpart elements?

Answer: Group II or alkaline earth metal cations have a smaller size as compared to Group I or alkali metal cations. The smaller the ionic radii of a cation, the higher will be its corresponding hydration enthalpy. Hence, the hydration energy of alkaline earth metal ions is comparatively greater than those of alkali metal ions. So, the salts of alkali metals have less water of crystallisation as compared to those of the alkaline earth metals.

Example: and exist in hydrated forms as and . and do not form such hydrated salts, i.e. they do not contain any water of crystallisation.

Q. 3. The formula for Epsom salt is:

Answer: Epsom salt is a hydrated magnesium salt, which is a Group II (alkaline earth metal). The formula is magnesium sulphate heptahydrate, that is .

So, option C) is the correct answer.

Q. 4. Among the alkali metals, which ion moves least rapidly in an aqueous solution?

Answer: Although has the smallest ionic radii of all, it moves least quickly in an aqueous solution. The reason for this observation is its highest extent of hydration owing to its smallest size, which makes it achieve maximum hydrated radii.

So, option C) is the correct answer.

Q. 5 Hydration enthalpy decreases down the group due to

a. Decrease in lattice energy
b. Increase in size
c. Decrease in electronegativity
d. None

Answer:

The smaller the ion, the higher the charge density, the stronger the electrostatic attraction between water molecules and metal cations, and the higher the hydration enthalpy.

Hence, on moving down the group, the hydration enthalpy decreases with the increase in ionic size as the charge density decreases.

So, option B) is the correct answer.

Frequently Asked Questions - FAQ

Why do the hydrated salts still appear as dry crystals and are not wetted?

Answer: Water of crystallisation present in hydrated salts is present inside their crystals in the forms of ion-dipole interactions. Hence, they are chemically bound water molecules and not free. So, they appear dry and not wet.

How does water crystallisation impact a salt?

Answer: Water of crystallisation affects the shape, structure and also certain chemical and physical properties like the colour of a particular crystalline salt. It also impacts how heating will affect it.

What happens when the water of crystallisation is lost?

Answer: When the water of crystallisation is lost, the hydrated salt becomes anhydrous in nature, and some of its physical properties are lost.

Example: Blue coloured copper sulphate pentahydrate on heating loses its water of crystallisation and turns colourless in its anhydrous form.

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