At an azeotrope \(x_i = y_i\) for each component \(i\)

where \(x_i\) = the mole fraction of component \(i\) in the liquid phase

\(y_i\) = the mole fraction of component \(i\) in the vapor phase

For a binary mixture, at an azeotrope, either \(P > P_1^{sat} \,\,and \,\,P_2^{sat}\) or \(P < P_1^{sat} \,\,and \,\,P_2^{sat}\)

For a binary mixture, at an azeotrope, either \(T < T_1^{sat}\,\, and\,\, T_2^{sat}\) or \(T > T_1^{sat}\,\, and\,\, T_2^{sat}\)

where \(P_i^{sat}\) = saturation pressure of component \(i\)

\(T_i^{sat}\) = saturation temperature of component \(i\)

At an azeotrope \(\frac{dP}{dx_i} = 0\) and \(\frac{dT}{dx_i} = 0\)

Modified Raoult’s law \(x_i \gamma_i P_i^{sat} = y_iP\)

where \(\gamma_i\) = activity coefficient of component \(i\)