Solid State

Solid state is a state of matter characterized by their definite volume and shape, rigidity and incompressibility.

Properties of solid state
  1. Solid has a fixed shape and volume.
  2. It has a strong intermolecular force of attraction.
  3. Particles of solid diffuse extremely slow with other solids as compared to liquid and gas.
  4. They are composed of atoms, ions or molecules.

Anisotropy: A solid whose physical properties are different in a different direction is called an anisotropic solid and the process is called anisotropy.

Isotropy: A solid whose physical properties are the same in all direction is called an isotropic solid and the process is called isotropy.

Classification of solid

On the basis of arrangement of repeating units such as atoms, ions or molecules, they are classified into two groups:

1. Crystalline solids: They are the solids that are composed of constituent particles (atoms, ions or molecules) arranged in a particular order in a three-dimensional region. eg. Blue vitriol, washing soda, sodium chloride crystals.

2. Amorphous solid: They are the solids which are composed of constituent particles (atoms, ions or molecules) not arranged in a particular order. eg. glass, plastic, rubber, etc.

Difference between crystalline and amorphous solid
Crystalline solidAmorphous solid
An ordered arrangement of structural units.Random arrangement of structural units.
They have a sharp melting point.They don’t have a sharp melting point.
They are anisotropic in nature.They are isotropic in nature.
They have fixed geometrical shape.They don’t have fixed geometrical shape.
They can be cleaved in a particular direction.They can not be cleaved in a particular direction.
They are true solid.They are supercooled liquid.

Water of crystallization

In some crystalline solid, a certain number of water molecules are associated as a part of a crystal. Such number of water molecules is called water of crystallization. The number of the water molecule which is associated with one molecule of hydrated crystalline solid is called water of crystallization. It is the amount of water required to maintain the crystalline properties of certain metallic solid. It is responsible for the geometrical shape and colour of crystal.

NameMol. formulaWater of crystallization
Blue vitriolCuSO4.5H2O5
Green vitriolFeSO4.7H2O7
White vitriolZnSO4.7H2O7
Epsum vitriolMgSO4.7H2O7
Gypsum vitriolCaSO4.2H2O2
Washing sodaNa2CO3.10H2O10
Plaster of parisCaSO4.1/2H2O1/2


The spontaneous process in which a hydrated crystalline solid loses one or more water of crystallization on exposure to air is called efflorescence.
Na2CO3.10H2O → Na2CO3.H2O + 9H2O


The spontaneous phenomenon in which a substance absorbs moisture and changes into solution when exposed to air is called deliquescence.
CaCl2 + H2O → CaCl2 solution


The spontaneous process in which a solid absorb a limited amount of moisture from the atmosphere and changes into a hydrated form is called hygroscopy.
CuSO4 + 5H2O → CuSO4.5H2O

Crystal lattice and unit cell

Crystals are homogeneous, usually solid, composed of the same repeating unit in a particular order forming at sharp edge and plane faces. They may be made of atoms, ions or molecules. The constituent particles are arranged in a regular repeating manner in three-dimensional space to form a crystal. The point occupied by constituent particles in three-dimensional space is called lattice point or lattice site. When such constituent particles occupy such lattice site and expand in three dimensions, it is called crystal lattice or space lattice.
The crystal lattice is an array of a structural unit of crystal in three-dimensional space.
The smallest repeating unit of crystal lattice having a definite geometrical shape is called a unit cell. It possesses all characters of the entire crystal. It is the fundamental unit of the crystal.

A unit cell is characterised by
i. Edge length (a, b and c)
ii. Interfacial angle (α, ß, and Γ).

interfacial angle of unit cell of solid state

Classification of unit cell

i. Simple unit cell: Lattice point occupy only corners of unit cell.

ii. Body centred unit cell: Lattice point occupies corners of the unit cell and centre of the body.

iii. Face centred unit cell: Lattice point occupy centre of each face and corners of unit cell.

Lattice point occupy center of each face and corners of unit cell.

Seven types of crystal system
NameEdgesInterfacial angle
Cubica = b= cα = ß = Γ = 90°
Tetrahedrala = b ≠ cα = ß = Γ = 90°
Orthorhombica ≠ b ≠ cα = ß = Γ = 90°
Rhombohedrala = b = cα = ß = Γ ≠ 90°
Hexagonala = b ≠ cα = ß = 90°, Γ = 120°
Monoclinica ≠ b ≠ cα = Γ = 90°, ß ≠ 90°
Triclinica ≠ b ≠ cα ≠ ß ≠ Γ ≠ 90°
unit cell of NaCl Solid state
Some Important Questions
  1. Differentiate between amorphous and crystalline solid.
  2. Define isotropy and anisotropy.
  3. What is the water of crystallization? Write any two examples of hydrated and anhydrous crystalline solid.
  4. Define efflorescence, deliquescence and hygroscopy.
  5. Define unit cell. Draw unit cell of NaCl.

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