Heat and Temperature

Abstract

Whether a particular substance exists as a solid, a liquid or a gas at a given temperature and pressure depends upon the strength of the forces of attraction between its constituent atoms, ions or molecules. Thus, at atmospheric pressure and room temperature the van der Waals forces between the oxygen molecules in air are not strong enough to make them stick together, nor are the hydrogen bonds between water molecules strong enough for them to form ice. On the other hand, nearly all metallic and ionic/covalent materials are solids. Furthermore, simple solids tend to turn to liquids, and liquids to gases, if they are heated. These observations seem to point to the idea that the cohesion due to the forces of attraction between atoms, ions and molecules is opposed by the effect of heat.