This is the third edition of a physical chemistry textbook designed for a two-semester undergraduate physical chemistry course. The physical chemistry course is often the first opportunity that a student has to synthesize descriptive, theoretical, and mathe- matical knowledge about chemistry into a coherent whole. To facilitate this synthe- sis, the book is constructed about the idea of defining a system, studying the states in which it might be found, and analyzing the processes by which it can change its state