Decoupling chemical and thermal freeze outs in hydrodynamics

From an analysis of various types of data obtained in relativistic nuclear collisions. the following Picture has emerged in thermal and hydrodynamical descriptions: as the fluid expands and cools, particles first undergo a chemical freeze out at T-ch.f. similar to 160-200 MeV, then a thermal freeze out at T-th.f. similar to 100-140 MeV, In this paper we show how to incorporate these separate freeze outs consistently in a hydrodynamical code via a modified equation of state (general case) or via a modified Cooper-Frye formula (particular case of T-ch.f. close to T-th.f. or few particle species undergoing early chemical freeze out). The modified equation of state causes faster cooling and may have sizable impact on the predicted values of observables.