This lecture delves into the chemical equilibria involved in the dissolution of mountains, focusing on the processes of physical and chemical weathering. It explains the dissolution of minerals like Feldspar in granite, detailing the reactants and products involved. The lecture also covers how to write a balanced chemical reaction and calculate the equilibrium constant. Various types of chemical equilibria, including solubility reactions, acid dissociation constants, and complexation reactions, are discussed. The importance of equilibrium constants in regulating the concentration of aqueous and gaseous species in the environment is emphasized.