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The kinetic theory of matter

The ancients such as the philosophers Democritus and Lucretius held that matter was composed of minute particles. They also maintained that these particles were in a state of continuous random motion within solids, liquids and gases. The theory was therefore called the kinetic theory of matter, after the Greek work kinema - motion.

Strong evidence for the existence of molecules is provided by the following observations:
(a) the diffusion of gases and liquids - diffusion in solids has actually also been observed: a slab of lead was clamped to a slab of gold for some years, and diffusion of each metal a few millimetres into the other was demonstrated by chemical analysis.
(b) the mixing of two liquids to give a final volume which is less than the sum of their original volumes;
(c) dissolving a solid in a liquid.

Various phenomena dealt with elsewhere in detail may be explained in terms of the kinetic theory:
(a) evaporation - molecules in the surface of a liquid gain sufficient energy to escape from it
(b) saturated vapour pressure - there is a dynamic equilibrium between molecules entering and leaving a liquid surface in an enclosed space
(c) surface tension - the intermolecular forces at a liquid surface explain capillary rise, liquid drop shapes, the wetting of surfaces and so on
(d ) latent heat - energy is required to overcome the intermolecular attraction to change the state of a substance
(e) viscosity - the movement of molecules between adjacent layers of a moving fluid and their attraction gives the effect of viscous drag
(f) the behaviour of gases, both ideal and real, may be explained in terms of molecules.

© Keith Gibbs 2010