This lecture covers the concept of variance in physico-chemical systems, introduced by Josiah Willard Gibbs. It explains the factors of equilibrium, the calculation of variance, and its importance. Applications of the Gibbs rule in homogeneous and heterogeneous chemical equilibria are discussed, along with the qualitative aspects of equilibrium shifts following Le Châtelier's principle. The lecture concludes by emphasizing the significance of variance in determining the equilibrium state of a system.