Physical chemistry is a branch of chemistry that focuses on understanding the physical properties of molecules, the forces that act upon them, and the energy changes that occur during chemical reactions. It combines principles of physics and chemistry to study how matter behaves on a molecular and atomic level. Key areas within physical chemistry include thermodynamics, which examines the heat and energy transformations in chemical processes; kinetics, which studies the rates of chemical reactions; quantum chemistry, which uses quantum mechanics to explain the behavior of electrons in atoms and molecules; and spectroscopy, which analyzes the interaction of light with matter to determine molecular structures and dynamics.