Although surface site densities are very often considered physical constants associated with a particular surface structure, there are some cases when the surface site density may change with time. Ansys Chemkin allows the inclusion of reactions that do not conserve surface sites, if this is a desired feature of the problem. In such cases, the surface site density , for every phase whose total number of sites may not be conserved, becomes an additional solution unknown. The total number of surface phases on each material , for which this condition may hold, is defined as equal to the variable . For each material , there are additional equations to be solved, corresponding to additional unknowns. The equation for the time derivative of surface site densities is given by

(8–21)