Physical chemistry is the quantitative study of the physical properties of matter, including their transformations. The properties of macroscopic matter at equilibrium (such as the relationships among temperature, pressure, and volume in gases; heat capacity of solids; solubility of saturated solutions; melting and boiling temperatures; and behavior of electrolyte solutions, to name a few) are historically among the earlier interests of physical chemistry. All of the examples mentioned just above belong to the subject of thermodynamics, a remarkable science which unified a wide variety of empirical laws under the broader concepts of energy and entropy.