Chemistry Reference
In-Depth Information
●
Solids, liquids and gases
A
solid
, at a given temperature, has a definite volume
and shape which may be affected by changes in
temperature. Solids usually increase slightly in size
when heated (
expansion
) (Figure 1.2) and usually
decrease in size if cooled (
contraction
).
A
liquid
, at a given temperature, has a
fixed volume and will take up the shape of any
container into which it is poured. Like a solid, a
liquid's volume is slightly affected by changes in
temperature.
A
gas
, at a given temperature, has neither a definite
shape nor a definite volume. It will take up the shape
of any container into which it is placed and will
spread out evenly within it. Unlike those of solids
and liquids, the volumes of gases are affected quite
markedly by changes in temperature.
Liquids and gases, unlike solids, are relatively
compressible
. This means that their volume can be
reduced by the application of pressure. Gases are
much more compressible than liquids.
The main points of the theory are:
●
All matter is made up of tiny, moving particles,
invisible to the naked eye. Different substances
have different types of particles (atoms, molecules
or ions) which have different sizes.
●
The particles move all the time. The higher the
temperature, the faster they move on average.
●
Heavier particles move more slowly than lighter
ones at a given temperature.
The kinetic theory can be used as a scientific model
to explain how the arrangement of particles relates to
the properties of the three states of matter.
Explaining the 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 (Figure 1.3a). 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 (Figure 1.3b).
Figure 1.2
Without expansion gaps between the rails, the track would
buckle in hot weather.
a
A model of a chrome alum crystal.
b
An actual chrome alum crystal.
Figure 1.3
Studies using X-ray crystallography (Figure 1.4) have
confirmed how the particles are arranged in crystal
structures. When crystals of a pure substance form
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 1.5.
●
The kinetic theory
of matter
The
kinetic theory
helps to explain the way in which
matter behaves. The evidence is consistent with the
idea that all matter is made up of tiny
particles
. This
theory explains the physical properties of matter in
terms of the movement of its constituent particles.
