Chemistry: Solid State: Nature and Classification of Solid State (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • The matter exists in three different states i.e.. solid, liquid and gas.
  • In all 3 states the constituent particles such as atoms, molecules or ions are held together by different forces of attraction between them, but, the nature and magnitude of the forces varies.
  • The solids are distinguished from a liquid or gas by their rigidity which makes them occupy definite volume and have a well-defined shape.
  • In solid state, the constituent particles are in close contact and have strong forces of attraction between them.

Nature of Solid State

  • According to Kinetic Molecular Theory, the gases consist of number of molecules, which are in constant random motion in all directions in the available space.
  • These molecules have very weak forces of attraction between them.
  • A sample of gas can be compressed, as there is a lot of free space between the molecules as shown in figure below:
Nature of Solid State

In liquids (as shown in figure below) the molecules are also in constant motion, but this motion is relatively restricted.

Solid State
  • In Liquid, there is very little free space available between the molecules, so they are relatively incompressible.
  • In solid state the constituent particles are arranged in a closely packed ordered arrangement (as shown in figure below) with almost no free space.
Molecules of a Gas
  • They can just vibrate about their fixed positions. These are in close contact and cannot move around like the molecules of a gas or a liquid. So, the solids are incompressible, rigid and have a definite shape.
  • Like liquids, the volume of a solid is independent of the size or the shape of the container in which it is kept.

Classification of Solids

  • On the basis of nature of arrangements of the constituent particles the solids are classified into amorphous and crystalline solids.
  • In crystalline solids, the constituent particles are arranged in a regular and periodic pattern and give a well-defined shape to it.
  • The term ‘crystal’ comes from the Greek word, crystals meaning ice.
  • The regular pattern extends throughout the solid and such solids are said to have long range order.
  • Sodium chloride and sucrose are common examples of crystalline solids.
  • Some solids have only a short range of order that the particles are arranged regularly in only some regions of the solid and are relatively disordered in other regions. They are called as amorphous solids.
  • In Greek, a means without and morph means form, so, amorphous means without form.
  • Glass, fused silica, rubber and high molecular mass polymers are some examples of amorphous solids.
  • An important difference between the amorphous and crystalline solids is that amorphous solids are isotropic in nature (i.e.. , these exhibit same value of some physical properties in all directions) .
  • The crystalline solids are anisotropic (i.e.. , the values of some physical properties are different in different directions) .
  • The refractive index and coefficient of thermal expansion are typical physical properties, which have different values when measured along different directions of a given crystal.
  • The crystalline solids have a sharp or definite melting point, whereas the amorphous solids do not have definite melting point, these melt over a range of temperature.
  • The crystalline solids can be further classified on the basis of nature of interaction between the constituent particles.

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