The equilibrium constant expression is the ratio of the concentrations of a reaction at equilibrium. Each equilibrium constant expression has a constant value known as K, the equilibrium constant. When dealing with partial pressures, \(K_p\) is used, whereas when dealing with concentrations (molarity), \(K_c\) is employed as the equilibrium constant. Reactions containing pure solids and liquids results in heterogeneous reactions in which the concentrations of the solids and liquids are not considered when writing out the equilibrium constant expressions.