  The SU(2) and SU(3) chiral phase transitions in a hot gas made of pions,
kaons and etas are studied within the framework of Chiral Perturbation Theory.
By using the meson meson scattering phase shifts in a second order virial
expansion, we are able to describe the temperature dependence of the quark
condensates. We have estimated the critical temperatures where the different
condensates melt. In particular, the SU(3) formalism yields a lower critical
temperature for the non-strange condensates than within SU(2), and also
suggests that the strange condensate may melt at a somewhat higher temperature,
due to the different strange and non-strange quark masses.
