physical chemistry, branch of science that combines the principles and methods of physics and chemistry. It provides a fundamental theoretical and experimental basis for all of chemistry, including organic, inorganic, and analytical chemistry. In addition, it is the foundation of chemical engineering. Topics of interest are chemical equilibrium, reaction rates, solutions, molecular weights, molecular structure, and the properties of gases, liquids, crystals, and colloids. Among other factors, the influence of temperature, pressure, electricity, light, concentration, and turbulence are considered. There are three principal approaches in physical chemistry: thermodynamics, involving large numbers of molecules in equilibrium; kinetics, involving chemical changes with time; and molecular structure, involving the electronic and atomic arrangements that follow from the quantum theory. The latter approach is primarily theoretical and provides an understanding of the chemical bonds which are responsible for the structure of all materials.