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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.

Matter can occupy three different states — gas, liquid and solid. Changes of state depend on the motion of sub-microscopic particles. The motion of these particles depends on energy. Cooling particles takes away energy and slows them down. Heating particles adds energy and speeds them up.

In fact the chief characteristic of matter isits eternal restlessness, and the degree of restlessness is determined by the stale of matter under observation; i.e. whether it be 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.

We use gases, such as a natural gas, in many important ways. Cooling a gas into a liquid decreases its volume dramatically. This makes it possible to store and transport it more effectively.

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.

Crystals are highly ordered form of solid matter. They were one of the first clues to the arrangement of particles in the solid state.

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.

The states of matter are few. But the ways in which they are realized, the number of different substances around us, are many. Let me give you an example. On breathing oxygen — the life-giver — is obviously a gas. But here is another element, sulphur that chemically very closely related to oxygen. And yet, it is obviously different: it is a solid at room temperature. Now, there are obviously different forces that work between the atoms of molecules of sulphur and oxygen within these two substances. We want to know why this is so. We have to probe deeper, we have to, then, ask: What is the nature of the atom? What is it that makes oxygen and sulphur similar or different?

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