Chapter 1 Liquids and Solids

Gases can be treated as being ideal (i.e. the gas particles do not interact) in many situations given that the particles are far separated from each other under normal conditions. Particles that do not interact have an interaction energy of zero. If the particles are close enough together, an attractive interaction occurs and the interaction energy is negative. The plot below shows the interaction energy of some noble gas dimers as a function of particle separation.

Figure 1.1: Noble gases have attractive interactions.

As the noble gas atoms approach each other at short distances, an attractive interaction arises. The interaction is very weak but still present. It is because of this interaction that noble gases can be liquefied, albeit, at very low temperatures and high pressures due to the very weak nature of the interaction.

A liquid is a state of matter that consists of particles that do interact where the distance between the particles are small. Therefore, the treatment of particles in a liquid is far different from that of an ideal gas. The particles still retain the translational, vibrational, and rotational motions that we see in a gas and the positions/orientations of the particles are relatively more ordered than seen in the gas phase.