Physical chemistry, Branch of chemistry concerned with interactions and transformations of materials. Unlike other branches, it deals with the principles of physics underlying all chemical interactions (e.g., gas laws), seeking to measure, correlate, and explain the quantitative aspects of reactions. Quantum mechanics has clarified much for physical chemistry by modeling the smallest particles ordinarily dealt with in the field, atoms and molecules, enabling theoretical chemists to use computers and sophisticated mathematical techniques to understand the chemical nature of matter. Chemical thermodynamics deals with the relationship between heat and other forms of chemical energy.