Concept: Intermolecular forces are forces of attraction and repulsion between interacting molecules. Vander Waal forces are very short-lived forces weak forces and exist between all molecules when they are quite close to each other. Explanation: Vander Waal forces are of three types namely London forces, dipole-dipole forces, dipole induced dipole force. A) London forces: Observed in non-polar molecules like H2, F2, Cl2, etc. Weakest of all intermolecular forces. At a particular point of time, the distribution of electrons gets shifted towards one side causing temporary separation of positive and negative charge. B) Dipole-dipole forces: Molecules have permanent dipole moment. In polar molecule centre of positive and negative charge do not coincide and the ends possess partial charges. When such molecules come close to each other they orient themself in a manner that results in an attraction between these dipoles. C) Dipole induced dipole forces: Observed in mixture containing Polar and non-polar molecules. When non-polar molecules come close to polar molecules the positive ends of polar molecules attract electrons of non-polar molecules thus leading to a separation of charge in nonpolar molecules which is referred to as induced dipole.