Heat and Thermodynamics

6.5 The Steam Line, The Hoar Frost Line and The Ice Line

Any possibility of a spontaneous change in an isothermal isobaric process is conditioned by the corresponding negative change in the thermodynamic potential. This means that for such transformation

 

dG < 0

Or, by Eq. 6.21     dm (G₁ – G_v) < 0

 

If G₁ > G_v, then dm must be negative, that is, a quantity of liquid must evaporate. Conversely if G₁ < G_v, dm must be positive, that is, a quantity of vapour must condense to the liquid state. There will evidently be no evaporation or condensation if G₁ = G_V This is, therefore, the condition of equilibrium between the liquid and the vapour phases. The line I in the (p − T) space over which the functional relation G₁ = G_V holds is called the steam ...