In this lesson, we will learn:

- To recall the solubility product expression, K
_{sp}. - How to express solubility and saturation using equilibrium expressions.

__Notes:__- In Solubility and ion concentration, we updated our definitions of solubility and saturation. Their correct definitions are about an equilibrium between the dissolved and undissolved state. This means
__we can write solubility as an equilibrium constant expression__.- For example, a saturated solution of the salt MgCl
_{2}, can be described by the equilibrium equation:MgCl _{2 (s)}$\enspace \rightleftharpoons \enspace$ Mg^{2+}_{(aq)}+ 2 Cl^{-}_{(aq)} - The
*solubility product*expression for this equilibrium is:K _{sp}= [Mg^{2+}][Cl^{-}]^{2} - The
*solubility product constant*, K_{sp}, is used for any equilibrium constant between dissolved ions and the undissolved compound; it is the K_{eq}for saturated solutions.

- For example, a saturated solution of the salt MgCl
- Remember how K
_{a}is an acid strength rating and K_{eq}is basically a product rating in any equilibrium? K_{sp}is effectively a dissolved ions rating! A large K_{sp}value means the concentration of dissolved ions in the saturated solution is large.__Therefore the larger the K__._{sp}value, the more soluble a substance is - Its important to remember the concept of the solubility product constant when dealing with practical problems involving multiple saturated solutions:
- The reason it is called the solubility product CONSTANT is because
__when the solution is saturated, K__, regardless of how the solution was prepared or the relative concentrations. The K_{sp}is equal to the product of the concentration of dissolved ions_{sp}for any given compound at saturation does not change. Recall that:K _{sp}= [Mg^{2+}][Cl^{-}]^{2}

If MgCl_{2}was dissolved in solution until saturation occurred, we would have a stoichiometric ratio of Mg^{2+}and Cl^{-}ions, according to the equation:MgCl _{2 (s)}$\enspace \rightleftharpoons \enspace$ Mg^{2+}_{(aq)}+ 2Cl^{-}_{(aq)} - However, we could make the saturated solution of magnesium chloride another way. Separate compounds may be added in different quantities, such as Mg(NO
_{3})_{2}and NaCl which together contribute to the MgCl_{2}K_{sp}equilibrium.

The K_{sp}constant value could be reached if one ion concentration was larger while the other smaller.__Regardless of the proportion of ion concentrations, K__and it is the product of the separate dissolved ion concentrations multiplied together._{sp}is constant for a saturated solution

- The reason it is called the solubility product CONSTANT is because