Classification of solids – Solids can be classified into two types on the basis of the arrangements of their constituent particles (atoms, molecules or ions). These are of two types: Crystalline Solid and Amorphous Solid.

Crystalline Solid

If a solid possesses long range, regularly repeating units, it is classified as a crystalline material. Crystalline solids are only produced when the atoms, ions, or molecules have an opportunity to organize themselves into regular arrangements, or lattices.

 

In other terms, Solids have a large number of crystals; each having a definite characteristic geometrical shape; are called crystalline solids.

The constituent particles of crystalline solid are arranged in regular pattern which is repeated periodically over the entire crystal. Crystalline solids are anisotropic in nature, i.e. many physical properties, such as electrical resistance, refractive index, etc. vary along different axes. Crystal of HCl, Quartz, Ice, Iron, common salt  etc. are some examples of crystalline solid.

Characteristics of crystalline solid –

1. Crystalline solids have definite characteristic geometrical shape.
2. Crystalline solids have sharp characteristic melting point.
3. Crystalline solids have definite and characteristic heat of fusion.
4. Crystalline solids produce pieces with plain and smooth surface when cut with a tool of sharp edge.
5. Crystalline solids are anisotropic in nature.
6. Crystalline solids are true solid.
7. Constituent particles of crystalline solids are arranged in long range order.

Amorphous Solid

Although the crystalline state is more thermodynamically-favorable than the disordered state, the formation of amorphous materials is favored in kinetically bound processes (e.g., chemical vapor deposition, sol-gel, solid precipitation, etc.).Thus, Solids having irregular shapes of particles are known as Amorphous Solids. The word ‘Amorphous’ came from Greek word ‘Amorphos’ which means no shape.

The arrangement of constituent particles is short in range, i.e. regular and periodical arrangement of particles is seen to a short distance only. The structures of amorphous solids are similar to that of liquids. Glass, rubber, plastics, etc. are some of the examples of amorphous solids. Amorphous solids are isotropic in nature, i.e. physical properties of amorphous solids are same in all directions.

In old buildings, it is often seen that glasses of windows get slightly thickened at bottom, this happens because glass which is an amorphous solid; flows down very slowly. Some very old glasses get milky appearance because of some crystallization. This happens because on heating, glasses get crystallized at some temperature. Due to the rigidity of the solid, but proclivity to remain in the amorphous state, these compounds have been incorrectly referred to as super cooled liquids.

Characteristic of amorphous solid –

1. Particles of amorphous solids have irregular shape.
2. Amorphous solids soften gradually over a range of temperature.
3. Amorphous solids produce pieces of irregular shapes when they are cut into two pieces.
4. Amorphous solids do not have definite heat of fusion.
5. Amorphous solids are isotropic in nature, i.e. they have same physical properties in all directions.
6. Amorphous solids are not true solids and hence these are also known as Pseudo Solid or Super Cooled Liquid.
7. The arrangement of constituent particles is in short range order.

Difference between Amorphous and Crystalline Solids:

Solid state
True solid or Crystalline solid	Pseudo solid or Amorphous solid or super cooled liquid
Are those solid in which particles or spheres are arranged in regular, periodic and predictable manner in 3-D space. Are anisotropic (the physical properties ex-refractive index density are different in different direction. Have sharp/fixed melting point Ex- Ice = 0°C = 273 K NaCl Have definite or fixed heat of fusion Can be cut into clean surfaces. Have long range order Are made up of unit cell. Ex- Ice, NaCl, KCl, ZnS, Diamond, Graphite, fullerene, Caf2, Li2O all metals	Are those solid in which particles are not arranged in regular, periodic & predictable manner in 3-D space. Isotropic – the physical properties are same in all directions Don’t have sharp melting point. They have melting range Glass – (110° - 170° C) Rubber – (90° - 135° C) Do not have fixed heat of fusion. Cannot be cut into clean surfaces. Have short range order. Are not made up of unit cell. Ex:- Glass, Rubber, Plastic, Carbon Allotropes (Charcoal, Coke).

Annealing: Amorphous Solids can be converted to its crystalline form by melting and then cooling the melt very slowly, this process is called annealing.