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
often.
Collide less often than in liquid.
