The subjects of thermodynamics and statistical mechanicsStatistical mechanics are ordinarily considered separately from those of electricity and magnetismMagnetism. Nonetheless, in many chemical phenomena, it is the case that the thermal properties and the electromagnetic properties are intimately related. This connection occurs when the electromagnetic measurements involve macroscopic samples of matter held at constant temperatureTemperature and pressure. To account for the thermal properties of matter, this chapter includes discussions of the first and second laws of thermodynamicsThermodynamicssecond law and introduces the thermodynamic functions used to express these laws. The functions include the internal energy, the entropyEntropy, the enthalpyEnthalpy, the free energies, and the chemical potential. The laws of thermodynamics are applied to describe the properties of ideal gasesIdeal gas, which are then used as the basis for the description of the properties of ideal and real solutionsSolutionreal, including the concepts of activity and the activity coefficientActivityactivity coefficient. The consideration of solutions will lead us to the Gibbs–Duhem equation which relates the chemical potentials of the solventSolvent and soluteSolute. Upon completion of the survey of thermodynamics, statistical mechanicsStatistical mechanics will be introduced and shown to serve as the molecular basis for thermodynamics. Statistical mechanical topics that will be introduced include the canonical ensembleEnsemble, the grandEnsemblegrand canonical ensembleEnsemblecanonical, and the theory of fluctuationsFluctuation. With the connection between thermodynamics and statistical mechanics established, the chapter closes with a discussion of the osmotic compressibility, which is the thermodynamic source of the concentrationConcentration fluctuations that produce light scattering. The latter is observed in the case of solutions of macromolecules.