PHYSICAL CHEMISTRY - Structure of Lquids

When a gas is cooled, it changes into a liquid state and on further cooling the liquid freezes into solid state. Therefore liquid state is an intermediate state between the gaseous and solid states. The modern kinetic theory of matter, derived from the study of gases can be conveniently applied to study the behaviour of liquid and solids. It appears that for many substances the same sorts of molecules make up each state of matter becouse substances such as bromine and water can be changed back and forth from one state to anotherwith no pronounced change in the chemical nature of the substance. The great difference in proprties of gases and the other two states so called condensed phase can be accounted for by assuming that the molecules so widely separated in gaseous state are touching and so are affecting like chemical constituent of one another in the liquid or solid state.
In a perfect crystal a complete ordered arrangement of the atoms, ions or molecules constituting the crystal exists. The intermolecular forces are strong enough to hold them together. The constituent vibrate about there mean position but cannot execute translational motion. Hence both the short range as well as the long range orders exists in crystals. The molecule in a gas have complete random motion and the intermolecular forces between the molecules are small and are effective only at short distance. There is no possibility of any kind of ordered structured in gases. In liquid, on the other hand, the situation some what lies in between the two. The cohesive forces in liquids are stronger than those in gases. These forces, however, are not strong enought to disallow considerably the translational motion of the individual molecules. In terms of the arrangement of the constituents a liquid is having short range order but lacks long range order.