And the states of matter

Molecules are too small and are not seen under the most powerful microscope, but the movement of molecules can be made more real by a study of the so-called Brownian movement. Robert Brown, a botanist, observed in 1827 that small solid particles suspended in a liquid were in continual movement. In fact the chief characteristic of matter is its eternal restlessness, and the degree of restlessness is determined by the state of matter under observation; i.e. whether it is solid, liquid or gas. In gases molecules lead more or less independent existence and are moving with high speed in all directions; and the molecules must be continually colliding with each other and striking the walls of the vessel, and so exert pressure. A gas, therefore, has no definite shape or volume, and can be readily compressed. The molecules of a liquid can move sufficiently freely and allow the liquid to take up the shape of the vessel; but the attractive forces between the molecules, which are much closer together and moving more slowly than in a gas, do not permit the liquid to be compressed to any extent nor to vary its volume. A liquid, therefore, has a definite volume but not fixed shape, and is slightly compressible. In solids the freedom of the molecules must be very restricted and their motion is probably confined to small oscillations only, and must be greatly hampered by the nearness of neighbouring molecules. A solid, therefore, has both definite shape and volume, and is very difficult to compress. The rates of motion of molecules have an important influence on chemical changes. They occur most readily when molecules are moving freely, i.e. in gases; and chemical activity is least pronounced in solids, where the movement of molecules is slight.