One of the great advantages of the chemical perspective on ocean processes is the ability to predict whether specific reactions between molecules and ions might occur in aqueous media. Given the extreme complexity of natural systems, this type of fundamental constraint is invaluable. Such predictions are based on concepts and energetic information that have been painstakingly generated over the past several centuries and assembled into the discipline known as thermodynamics. The purpose of this chapter is to present an introduction to the properties of water and ions, and the basic concepts of the thermodynamics of chemical reactions. Rather than cover the breadth of thermodynamics, we seek to demonstrate the applications of free energy to the prediction of equilibrium distributions of chemical species among gaseous, liquid, and solid phases. The goal is to establish the conceptual foundation and tools that can be applied to the following chapters on ocean processes.

The properties of water and ions

The structure of water

Water accounts for approximately 96.5 mass percent of seawater. The innate characteristics of water affect almost all the properties of the ocean (e.g. density, salinity, and gas solubility) and the processes (e.g. circulation, heat exchange, chemical reactions, and biochemical transformations) that occur within it. Water is so much a part of our world and daily lives that it is easy to overlook how unusual this substance is in its physical and chemical properties.