Three states or phases of matter
Matter commonly exists in either the solid, liquid or gaseous state or phase. In a solid substance the molecules vibrate about their zero resultant force tions, alternately attracting and repelling one another. All true solids have a line structure in which the atoms are arranged in a regular pattern called the (Fig. 13.2 to 13.4).There is, however, a borderline class of materials which ap be solids but actually are very viscous liquids. Pitch is a good example. When
with a hammer, it readily splinters, but if placed in a funnel and left for several years it slowly flows out. In a liquid, the molecules are also vibrating to and from alternatively attracting and repelling one another with forces which can be just as strong as those in a solid (see action of a siphon, page 125). At the same time, however, the liquid molecules can move freely among one another, exchanging partners as they go. It is this freedom of movement which enables a liquid to take up the shape of any vessel in which it is placed. It is worth mentioning that experimental evidence indicates that small groups of liquid molecules can arrange themselves for very short periods of time into the same kind of regular pattern found in solids.
In a gas the molecules are much further apart than those in solids and liquids. They move at high velocities colliding with one another and with the walls of their containing vessel. Except at the moment of collision, the short-range intermolecular forces we have been describing do not come into action. Unless the gas is high! compressed, the molecules are, for the greater part of the time, so far apart that the attractive force is effectively negligible. Consequently, a gas is perfectly free to expand and completely fill the vessel containing it. The average distance moved by a molecule between collisions is called its mean free path. Rudolf Clausius applied the laws of mechanics to these collisions of the molecules with the walls of the containing vessel and showed how they explained the relation between the pressure and volume of a gas.