Text. The kinetic theory and the three states of matter

Many scientific facts can be explained by assuming two things:

1) Molecules always possess some motion. This is the basis of the kinetic theory, since "kinetic" means "in motion".

2) Molecules exert forces of attraction on each other they are close enough together^1. A force of attraction is something which urges the molecules together and tends to prevent their escape from one another. It is believed that the forces are only really large if the molecules are extremely close to each other. The attraction weakens very rapidly with distance. These two assumptions enable the three staates of matter to be explained.

The gaseous state. A gas is a substance which: a) has no set shape of its own, b) fills any vessel into which it is put c) exerts a pressure on anything with which it comes into contact.

Now suppose that in a certain container there is a col lection of molecules which are moving about with very high velocities in all directions. Then they will continually со lide with each other, knocking each other violently aside They will therefore keep each other at a distance and the average distance apart will be sufficiently large for the force of attraction between them not to matter.* The mole cules will make full use⁴ of all the space available in the containing vessel. In their constant collisions with the walls of the vessel or anything placed in their path, they will exert a pressure there. They rebound from each other immediately on collision, they can exert no lasting forces of attraction and so will not take up any fixed shape of their own.

The liquid state. A liquid is a substance which: a) pos-sesses a definite volume, yet b) has no shape of its own, but takes up the shape of that part of the containing vessel wit which it comes into contact. c) Also, if heated, it will evap-orate and eventually boil; while if it is cooled, it will event ually freeze and become solid.

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Suppose the velocities of the molecules referred to above are by some means progressively reduced. Then the violence of their mutual collisions will steadily weaken. Soon the atractive forces will cause them to collapse together and their average distance apart to become very small. Each mole cule will have little chance of escaping from the forces of attraction of its neighbouring molecules, though it can wander about amongst them. As a result, all the molecules will be weakly held together and will thereby occupy a much smaller but definite space. These are just the fundamental properties of the liquid state.

The solid state. A solid is a substance which: a) possesses a definite volume, b) has a fixed shape, c) shows a measure of mechanical strength which resists any effort made⁷ to change its shape.

If the motions of molecules are reduced enough, individual molecules may not be able to escape from the attractive forces of their immediate neighbours. On the average, the molecules will become closer together than they were in the liquid state — almost touching, and the forces between them will be correspondingly greater. The motion of each molecule will have been reduced to a mere vibration to and fro⁸ about some fixed position in the solid structure. Molecules can no longer wander amongst one another as they could in the liquid state. Each forces its neighbours to stay in their places, and thus the collection as a whole retains its own shape. This state of affairs represents the solid state.

Notes

¹ are close enough together — располагаются очень тесно

друг к другу

² knocking each other violently aside стремительно от­талкиваются друг от друга в стороны