Introduction

Matter around us exists in three distinct physical forms -- solid, liquid and gas -- and every substance can, in principle, be pushed from one of these states into another simply by adding or removing heat (for example, ice to water to steam). The three states differ chiefly in how closely their particles are packed and how freely those particles can move. In a solid, atoms or molecules sit very close together (roughly $3$--$5$ Å apart), locked into a fixed, regular arrangement, so a solid keeps both its own shape and its own volume and is, for practical purposes, incompressible. In a liquid, particles are a little further apart (about $3$--$10$ Å) and are only loosely ordered; they can slide past one another within the body of the liquid, so a liquid takes the shape of whatever vessel holds it while still keeping a fixed volume, and it is only slightly compressible. In a gas, particles are separated by distances greater than $5$ Å -- in practice, far more, since gas molecules occupy only a tiny fraction of the space available to them -- and move about randomly and continuously in every direction; a gas therefore has neither a fixed shape nor a fixed volume, spreading out to fill and take the shape of its entire container, and it is highly compressible. Correspondingly, the strength of intermolecular forces of attraction runs from strongest in solids, to moderate in liquids, to weakest in gases, and this same trend explains why heating a solid enough eventually turns it liquid, and heating a liquid enough eventually turns it into vapour, while cooling reverses each step.