Introduction

This chapter reviews the basic differences between gases, liquids, and solids, the transitions between them and the importance of symmetry. Some attention is given to the earliest equation of state put forward for non-ideal gases that is based on a molecular view of matter: the van der Waals equation of state. This equation of state predicts the condensation of a gas into a liquid and the existence of a critical point. The concept of universality emerges naturally from it