In general, liquids can be obtained from gases by cooling the latter below their respective critical temperatures, followed by the treatments of high pressure. The effect of cooling is to decrease the thermal energies of molecules and the effect of high pressure is to decrease the volume of the system so as to allow the molecules to come closer, there by increasing the binding energies amongst them. Alternatively, liquids can be obtained by heating solids up to or beyond their melting points. In solids, molecules do not possess any transnational energy but possess only vibrational energy. The binding forces amongst them are very strong. The effect of heating solids is to impart sufficient energy to molecules so that they can overcome these strong binding forces. Thus, we see that the properties of liquids lie in between those of solids and gases. For example, liquids are less compressible than gasses but a little more compressible than solids. They are less dense than solids but more dense than gases.
The two important properties of liquids, namely, fixed volume but no fixed shaped, arise mainly because of the following two facts:
- The energies binding the molecules are large than their average thermal energy.
- These binding energies are not strong to stop the motion of the molecules altogether, as is the case in solids, with the result that molecules can move from one place to another but cannot escape from the liquid unless they are present at the surface.
Properties of Liquids
In this section, we will discuss only three properties of liquids, namely,
- Vapour pressure,
- Viscosity, and
- Surface tension.