In this chapter first we know about some terms as equilibrium state; equilibrium mixture; dynamic equilibrium; chemical equilibrium; ionic equilibrium. Then we discuss about equilibrium in physical processes(solid-liquidequilibrium, liquid-vapour equilibrium, solid-vapour equilibrium). Then we know about equilibrium involving dissolution of solid or gases in liquids, general characteristics of physical equilibrium. Then we study equilibrium in chemical processes – Dynamic equilibrium then law of chemical equilibrium and equilibrium constant then about homogeneous and heterogeneous equilibria. Then we learn the applications of equilibrium constants :- predicting the extent of a reaction, predicting the direction of reaction, calculating equilibrium concentration. Then we study about the relationship between equilibrium constant (K), reaction quotient (Q) and Gibbs energy (G). Then we know about factors affecting equilibria(Le Chatelier’s principle). Then we discuss ionic equilibrium in solution. Then we learn about acids, bases and salts : Arrhenius concept, Bronsted-Lowry concept, Lewis acid and bases. Then we study ionisation of acid and bases, buffer solution, solubility and solubility product constant and common ion effect on solubility of ionic salts.