Solids and their Classification

Introduction

A solid is defined as that form of matter which possesses rigidity and hence possesses a definite shape and a definite volume. Unlike gases and liquids in which the molecules are free to move about and hence constitute fluid state, in a solids the constituent particles are not free to move but oscillate about their fixed positions.

Classification of Solids :

Solids are broadly classified into two types Crystalline Solids and Amorphous Solids.

A crystalline solid is a substance whose constituent particles possess regular orderly arrangement e.g. Sodium chloride, sucrose, diamond etc.

An amorphous solid is a substance whose constituent particles do not possess a regular orderly arrangement e.g. glass, plastics, rubber, starch, and proteins. Though amorphous solids do not possess long range regularity, in some cases they may possess small regions of orderly arrangement. These crystalline parts of an otherwise amorphous solid are known as crystallites.

An amorphous solid does not posses a sharp melting point. It undergoes liquefication over a broad range of temperature. The amorphous solid do not posses any characteristic heat of fusion. When an amorphous solid is cut with the help of sharp edged knife it results in an irregular cut.

Amorphous substances are also, sometimes, referred to as super cooled liquids because they posses disorderly arrangement like liquids. In fact many amorphous solids such as glass are capable flowing. Careful examination of the window panes of very old houses reveals that the panes are thicker at the bottom than at the top because the glass has flown under constant influence of gravity.

DISTINCTION BETWEEN CRYSTALLINE AND AMORPHOUS SOLIDS

CRYSTALLINE SOLIDS

1. The internal arrangement of particles is regular so they possess definite and regular geometry.

   
   




2. They have sharp melting points

   
   




3. There is regularity in the external form when crystals are formed

   
   




4. Crystalline solids give a regular cut when cut with a sharp – edged knife

   
   




5. They have characteristic heat of fusion.

   
   




6. Crystalline solids are rigid and their shape is not distorted by mild distorting forces

   
   




7. Crystalline solids are regarded as true solids

   
   




8. Crystalline solids are anisotropic. This implies that physical properties such as refractive index, conductivity, thermal expansion etc are different in different directions. This is due to orderly arrangement of particles

   
   

AMORPHOUS SOLIDS

1. The internal arrangement of particles is irregular. Thus they do not have any definite geometry.

   
   




2. They do not have sharp melting points

   
   




3. There is no regularity in the external form when amorphous solids are formed

   
   




4. Amorphous solids give irregular cut.

   
   




5. They do not have characteristic heat of fusion.

   
   




6. Amorphous solid are not very rigid. These can be distorted by bending or compressing forces.

   
   




7. Amorphous solids are regarded as super cooled liquids or pseudo solids

   
   




8. Amorphous solids are isotropic in nature. This implies that various physical properties are same in all the directions. This is because of random arrangement of particles.

   
   

USES OF AMORPHOUS SOLIDS

Amorphous solids such as glass and plastics are very important materials and are widely used in construction, house ware, laboratory ware etc. Amorphous silica is likely to be the best material for converting sunlight into electricity (photovoltaic). Another well known amorphous solid is rubber which is used in making tyres shoes soles etc.