This book is for one-semester or two-semester courses in the application of chemical methods to problems in the earth sciences. The first five chapters treat fundamental concepts of physical chemistry such as volume, energy, entropy, equilibrium, and rates. The above is an introduction to thermodynamic logic from a macroscopic and an atomic point of view. The remaining material deals with the most common types of chemical reactions encountered in the earth sciences. In these chapters, theories are first presented and then applied to particular geochemical problems. Special features include integration of theory and applications in the belief that understanding why a formula works is necessary to proper application of the formula, treatment of isotopic reactions and isotopic fractionations from a theoretical and practical point of view, and comparison of the behavior of aqueous solutions at elevated temperatures and pressures with the behavior at one atmosphere.