A solid is a state of matter characterized by its high density, low compressibility, and definite structure compared with the liquid, gaseous states. It indicates that the molecules or ions in solids are relatively close together. Solid can vary easily from the liquid state by their definite structure, high rigidity, and considerable mechanical strength.
Solids are classified into two broad categories such as crystalline and amorphous solids. The term solid is generally employed for substances that are crystalline in nature.
Crystalline solids: They are materials whose constituents such as atoms, ions, or molecules are arranged in a definite order. The definite and ordered arrangement of constituents extends over a large distance in the crystal lattice. Crystalline solids such as NaCl, ice, or sugar possess a sharp melting point.
Amorphous solids: Amorphous solids such as glass, pitch, rubber, or plastics have many characteristics of crystal such as definite shape, rigidity, and hardness but they do not contain regular structure and melt gradually over a range of temperatures.
