5 - States of matter: gases and liquids

Summary

Introduction

Matter exists in three states, gaseous, liquid and solid. It is convenient to discuss the properties of matter under these main headings, although certains types of substances, such as vitreous materials and liquid crystals, do not fall clearly into one or other of them.

The gaseous state is the most straightforward to study. The simplicity arises from the fact that the molecules of a gas are, normally, independent of one another, with negligible forces of interaction. Consequently, many of their properties are independent of their chemical nature and may be described by general gas laws; we have made use of this fact in the previous chapter and here we describe these laws in more detail.

Gases are characterized by large volume changes with change of temperature or pressure and by their ability to flow into and fill the space available to them; they are miscible with one another in all proportions. A liquid, like a gas, has no form, but it has a definite volume and takes up the shape of its containing vessel. It has a boundary surface that restricts its extent and which is responsible for many of its properties. The cohesive forces between molecules in a liquid are stronger than those between molecules in a gas, but weaker than those characteristic of solids. Under normal conditions, gases behave as an assemblage of freely moving molecules or atoms, whereas solids form rigid structures.