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The state of matter intermediate between a solid and a gas, in which the molecules are relatively free to move with respect to each other but are restricted by cohesive forces to the extent that the liquid maintains a fixed volume. Liquids assume the shape of the vessel containing them, but are only slightly compressible.

Liquids have much higher densities than gases – comparing liquids with common gases under ordinary conditions the factor is of the order of 103. Their compressibility is low. They have no rigidity but their viscosity is of the order of 102 times greater than that of ordinary gases.

In liquids, the molecules are packed quite closely together so that each one is bonded to a number of neighbors. Given the position of one molecules it is possible to state how many more molecules should be found in contact with it. But still, as in the case of a gas, the pattern as a whole is a disordered one. The molecules are moving with just the same order of velocity as in a gas at the same temperature, though the motion is now partly in the form of rapid vibrations and partly translational. The configuration is therefore constantly changing.

This picture can be correlated with the macroscopic properties – the high density from the large number of molecules per unit volume, the lack of rigidity from the lack of order and the continual alteration of the arrangement. The comparatively close packing explains the low compressibility. The fairly high viscosity arises from the fact that the molecules have to wriggle past one another in this irregular but closely packed arrangement, rather like people moving past one another in a dense crowd, where slow relative movements are easy but rapid ones are difficult.

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Source: Properties of Matter, B. H. Flowers and E. Mendoza, Wiley & Sons, 1970.