Kinetic Theory of Matter

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This theory explains the physical properties of matter in terms of motion of its molecules. According to this theory, every substance (solid, liquid or gas) consists of a large number of minute particles called molecules. A molecule may be defined as the smallest particle of a substance that can exist in free state and has all the characteristics of the present substance.

The molecules are in continuous random motion. They possess all possible velocities in all possible directions. When a body is cooled, there is a decrease in the molecular motion. When a body is heated, there is an increase in molecular motion.

The energy possessed by molecules is of two forms - kinetic energy and potential energy. The kinetic energy manifests itself in molecular motion. The potential energy manifests either in the expansion of a substance or in change of its state. The heat supplied to a body partly increases the kinetic energy and partly increases the potential energy of the molecules.

To sum it up, the kinetic theory of matter is based upon the following points:

• Matter consists of molecules. These are the smallest particles, which are capable of free existence and retain all the chemical properties of the parent substance.

• The molecules are always in a state of random continuous motion.

• The molecules exert forces on one another. These forces depend upon intermolecular distance.

The intermolecular distances are greater in gases than in solids or liquids. As a result, the intermolecular forces of attraction are very weak. So, the molecules of a gas are free to move about in the entire space available to them. That is why a gas neither has a fixed volume nor a fixed shape.

When a gas is heated, the random motion of the molecules increases. This increases the rate of collision and as a consequence, increases the pressure exerted by the gas on the walls of the container.

The kinetic theory of gases attempts to develop a model of the molecular behavior, which should result in the observed behavior of an ideal gas.