Written as a mathematical equation in terms of the work done in each process this is ∫ ∫ v g v f p d v + ∫ ∫ v f v g p s d v = − − ∫ ∫ v f v g p d v + p s ( v g − − v f ) = 0 for T = constant This equation together with the equation of state written for each of the states f and g p s = p ( v f , T ) p s = p ( v g , T ) are three equations for the four variables, p s , T , v f , v g , so given any one of them, say T , the other three are determined. In other words, there is a unique value of p s ( T ) , as well as v f ( T ) and v g ( T ) , at which the phase transition can occur.