Physical chemistry is the branch of chemistry that applies the laws of physics to elucidate the properties of chemical substances. It provides the theoretical foundation for understanding all chemical phenomena. Physical chemistry is concerned with the physical properties of substances, such as vapor pressure, surface tension, viscosity, refractive index, density, and atomic structure. It provides an understanding of thermal properties, equilibria, rates of reactions, and mechanisms of reactions. In its more theoretical aspects, physical chemistry attempts to explain spectral properties of substances in terms of fundamental quantum theory, the interaction of energy with matter, and the nature of chemical bonding.