The vdW equation produces Z c = p c v c / ( R T c ) = 3 / 8 , while for most real fluids 0.23 < Z c < 0.31 . Thus most real fluids do not satisfy this condition, and consequently their behavior is only described qualitatively by the vdW equation. However, the vdW equation of state is a member of a family of state equations based on the Pitzer (acentric) factor, ω ω , and the liquid metals, Mercury and Cesium, are well approximated by it.