An introduction to the gas phase An Introduction to the Gas Phase is adapted from a set of lecture notes for a core first year lecture course in physical chemistry taught at the University of Oxford. The book is intended to give a relatively concise introduction to the gas phase at a level suitable for any undergraduate scientist. After defining the gas phase properties of gases such as temperature pressure and volume are discussed. The relationships between these properties are explained at a molecular level and simple models are introduced that allow the various gas laws to be derived from first principles. Finally the collisional behaviour of gases is used to explain a number of gas phase phenomena such as effusion diffusion and thermal conductivity.