Introduction

We now know how to determine in which direction any chemical reaction will proceed at a given temperature and pressure, at least when all the products and reactants are pure phases. When even one of the products or reactants is a solute, that is, part of a solution, we would be stuck because although we have had a brief look at how calorimetry can be used with liquids and liquid solutions, we haven't yet seen how to use the data obtained. We will start considering this problem in the next chapter. Before going on, however, we should explore some relationships using the concepts we have defined so far, so as to make sure we fully understand them. Naturally, we will only be able to consider some simple properties of pure phases, and reactions between pure phases.

Some properties of water

Water is an extraordinarily important substance in many ways in Earth processes, both organic and inorganic. The fact that it is a liquid in the relatively narrow range of T and P found at the Earth's surface was of course essential to the evolution of life. The reason for this, that is, the polar structure of the H2O molecule and its many unusual properties such as hydrogen bonding, is a fascinating subject in physical chemistry. In geochemical thermodynamics, we are concerned only with its macroscopic properties and its role in mineral reactions, both at and near the surface and at depth in the crust, that is, the weathering, metamorphic, and igneous environments.