For critical radius (fig:5.2) we can set the

or, ; for critical condition .

; This is known as Kelvin's equation.

is the critical energy barrier that is required to form the nucleus. Higher the degree of super saturation(x) lesser will be the value . This value of corresponds to an unstable equilibrium because when the radius of the nucleus is less than the liquid will evaporate back to form vapors, whereas when the radius is larger than the radius continues to increase further.