Three States of MatterThe various kinds of substances that make up matter can be broadly divided into three categories, namely, gases, liquids and solids. These are known as the three states of matter. These states can be considered to arise as a result of competition between two opposing molecular forces, namely, the forces of attraction between the molecules and the thermal energy of molecules.
If the thermal energy is much larger than the forces of attraction, we have matter in the gaseous state. Molecules in this state move with very large speed and the forces of attraction amongst them are not sufficient to bind the molecules at one place, with the result that the molecules move practically independent of one another. Because of this feature, the volume of gas is very sensitive to the changes in temperature and pressure. There exists no boundary surface and, therefore, gases tend to fill completely any available space, i.e. they do not possess a fixed volume.
If the forces of attraction are greater than the thermal energy, we have matter in the liquid state. Molecules in the liquid state too have kinetic energy but they cannot go very far away because of the larger forces of attraction amongst them. Due to this feature, liquids have a definite volume, but they do not have a definite shape. They take the shape of the vessel in which they are placed. In general, liquids are denser and less compressible than gases.
If the forces of attraction between molecules are much greater than the thermal energy, the molecules are held at fixed positions and we have matter in the solid state. Molecules in the solid state, therefore, do not possess any translational energy, but have only vibrational energy as they vibrate about their mean positions. Because of large forces of attraction amongst molecules, the solids differ very much from liquids and gases with respect to size, shape and volume. Solids, in general, have a definite size, shape and volume.