Solid State

The three states of matter : solid, liquid and gas are the result of competitive molecular interaction and thermal energy.

For any substance to be in a particular state of matter is a matter of strength of intermolecular forces which when strongest bring the molecules closest (as much as possible) and substance adopts definite volume and shape at room temperature.

The same substance at three different temperatures:

(i) Temperature below its melting point (m.p.).


(ii) Temperature above its m.p. but below b.p.


(iii) Above its b.p.

In solid state, the atoms are arranged in an orderly manner and are at fixed positions. There is only one form of molecular motion in solids, namely, vibration motion, by virtue of which the particles move about their fixed positions and cannot easily leave the solid surface.

Solids can be classified in two major categories on the basis of pattern of arrangement of constituent particles.

(a) Crystalline Solid; (b) Amorphous Solid

(a) Crystalline Solid: Solids in which constituent particles are arranged in a well and long range ordered pattern. e.g. Salt, Diamond, Iodine, etc.

(b) Amorphous Solid: Solids in which pattern is short range ordered or disordered. e.g. Plastic, Glass, Rubber etc. Amorphous solid are also known as super cooled liquid.

Difference between Crystalline and Amorphous Solid

Property	Crystalline Solid	Amorphous Solid
1. Crystal Geometry	These have definite crystal shapes because of ordered arrangement.	Do not have definite geometrical shape because of irregular arrangement
2. Symmetry	These can have (a) plane of symmetry (b) center of symmetry (c) axis of symmetry.	Do not have any symmetry.
3. Anisotropic or isotropic nature	In these, some properties are direction dependent e.g. refractive index i.e., these are anisotropic.	Properties are not direction dependent i.e.these are , Isotropic
4. Melting Point	Sharp M.P.	These have range of melting temperature.
5. Physical State	Hard and rigid	Soft
6. Crystal System	These have seven types of definite crystal systems.	These do not have regular crystal systems.

Crystalline solids are classified on the basis of interparticle forces.

Types of Solid	Constituent particle	Nature of Forces	Examples
1. Ionic	Positive and Negative ions	Electrostatic forces	Ionic compounds
2. Covalent	Covalently bonded atoms	Covalent forces in 3D cross network	Diamond-graphite carborundum, silicon
3. Metallic	Metal ions (kernels) surrounded by mobile electrons.	Metallic bonds	Metals and some alloys
4. Molecular	Molecules	van der Waal forces	12, solid C02, Ice, Sulphur

Note: In this chapter, when we will be talking about solids, it must be understand to be crystalline only.