What thermodynamics tells us about the QCD plasma

As long as a rapid change of the entropy density s(T) takes place across the critical temperature T-c of the QCD phase transition, the pressure P(T), and the energy density e(T) normalized by their Stefan-Boltzmann values generally deviate from unity for T much greater than T-c, even if there are no interactions among quarks and gluons at T>T-c. We shall demonstrate this both analytically and numerically for a general class of s(T) consistent with thermodynamica constraints and make a qualitative comparison of the result with the lattice QCD data. Quantities related to ds(T)/dT such as the specific heat and sound velocity are also discussed.