The major differences between the gas species and the liquid species in Ansys Chemkin are the format of the thermodynamic properties and the density model. The gas species thermodynamic properties, that is, the heat capacity at constant pressure C_p , are given in either the NASA 7-parameter polynomial format or in the NASA 9-parameter format. The liquid heat capacity will follow the formats of well-established databases such as the Design Institute for Physical Properties (DIPPR) database [9] and the NIST Chemistry Webbook [10]. In addition to the heat capacity, the liquid species require other thermodynamic properties such as vapor pressure, heat of vaporization, and the critical properties. The fitting equations need the critical properties for evaluating the liquid properties.

The gas density is determined either by the ideal gas law or by the cubic equation of state (Real Gas Model). However, Chemkin calculates the density of a liquid species from the equation fitted to the measured data, therefore the equation of state is not used to calculate the density of the liquid species or mixture.