Explaining the three states of matter

In a solid, the particles attract one another. There are attractive forces between the particles which hold them close together. The particles have little freedom of movement and can only vibrate about a fixed position. They are arranged in a regular manner, which explains why many solids form
crystals. 
It is possible to model such crystals by using spheres to represent the particles. If the spheres are built up in a regular way then the shape compares very closely with that of a part of a chrome alum crystal.

Studies using X-ray crystallography have confirmed how the particles are arranged in crystal structures. When crystals of a pure substance from under a given set of conditions, the particles present are always packed in the same way. However, the particles may be packed in different ways in crystals of different substances. For example, common salt (sodium chloride) has its particles arranged to give cubic crystals as shown in Figure.

In a liquid, the particles are still close together but they move around in a random way and often collide with one another. The forces of attraction between the particles in a liquid are weaker than those in a solid. Particles in the liquid form of a substance have more energy on average than the particles in the solid form of the same substance.

In a gas, the particles are relatively far apart. They are free to move anywhere within the container in which they are held. They move randomly at very high velocities, much more rapidly than those in a liquid. They collide with each other, but less often than in a liquid, and they also collide with the walls of the container. They exert virtually no forces of attraction on each other because they are relatively far apart. Such forces, however, are very significant. If they did not exist we could not have solids or liquids.

The arrangement of particles in solids, liquids, and gases is shown in Figure.

Solid
Particles only vibrate about fixed positions. Regular structure.








Liquid
Particles have some freedom and can move around each other. Collide often.









Gas
Particles move freely and at random in all the space available. Collide less often than in liquid.









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