0.15 Phase transitions and phase equilibrium

Introduction

We have developed a very clear molecular picture of the gas phase via the Kinetic Molecular Theory. The gas particles (atoms or molecules) are very distant from one another, sufficiently so that there are no interactions between the particles. The path of each particle is independent of the paths of all other particles. We can determine many of the properties of the gas from this description; for example, the pressure can be determined by calculating the average force exerted by collisions of the gas particles with the walls of the container.

To discuss liquids and solids, though, we will be forced to abandon some of the most fundamental pieces of the Kinetic Molecular Theory of Gases. First, it is clear that the particles in the liquid or solid phases are very much closer together than they are in the gas phase, because the densities of these “condensed” phases are of the order of a thousand times greater than the typical density of a gas. In fact, we should expect that the particles in the liquid or solid phases are essentially in constant contact with each other. Second, since the particles in a liquid or solid are in close contact, it is not reasonable to imagine that the particles do no interact with one another. Our assumption that the gas particles do not interact is based, in part, on the concept that gas particles are too far apart to interact. Moreover, particles in a liquid or solid must interact, for without attractions between these particles, random motion would require that the solid or liquid dissipate or fall apart.

In this study, we will pursue a model to describe the differences between condensed phases and gases. To do so, we will begin by observing the transitions that occur between the liquid and gas phases. We will analyze the experimental conditions under which we expect to observe a substance in the liquid form or in the gas form, and we will also discover experimental conditions under which the two phases are present simultaneously. This will lead us to one of the ubiquitous concepts in Chemistry: equilibrium. By analyzing the conditions of phase equilibrium, we will develop a kinetic molecular view of liquids and of the equilibrium between a liquid and gas.

Foundation

The "phase" of a substance is the particular physical state it is in. The most common phases are solid, liquid, and gas, each easily distinguishable by their significantly different physical properties. A given substance can exist in different phases under different conditions: water can exist as solid ice, liquid, or steam, but water molecules are H ₂ O regardless of the phase. Furthermore, a substance changes phase without undergoing any chemical transformation: the evaporation of water or the melting of ice occur without decomposition or modification of the water molecules. In describing the phase changes of a substance, we will also assume an understanding of the principles of the Atomic Molecular Theory and the Kinetic Molecular Theory.