text: Partial molar properties are useful because chemical mixtures are often maintained at constant temperature and pressure and under these conditions, the value of any extensive property can be obtained from its partial molar property. They are especially useful when considering specific properties of pure substances (that is, properties of one mole of pure substance) and properties of mixing (such as the heat of mixing or entropy of mixing). By definition, properties of mixing are related to those of the pure substances by:
  Here  denotes a pure substance,  the mixing property, and  corresponds to the specific property under consideration. From the definition of partial molar properties,
  substitution yields:
  So from knowledge of the partial molar properties, deviation of properties of mixing from single components can be calculated.