Why Solids Aren't in Ksp: Simple Chemistry Explained

When a solid ionic compound dissolves in water, it breaks apart into its constituent ions.This process creates a dynamic equilibrium between the solid and its dissolved ions.As the solid dissolves, positive and negative ions enter the solution.The solubility product constant, Ksp, represents this equilibrium mathematically.Notice that only the concentrations of dissolved ions appear in the Ksp expression.This equilibrium is dynamic, meaning ions constantly move between the solid and solution phases.The amount of solid present doesn't affect the equilibrium position, as long as some solid remains.This equilibrium continues until the product of ion concentrations equals the Ksp value.Let's examine the practical application of Ksp using silver chloride as our example.When silver chloride dissolves, it forms silver and chloride ions in solution.The Ksp expression only includes the aqueous ion concentrations, with a value of one point eight times ten to the negative ten.We omit the solid silver chloride from our Ksp expression because its concentration remains constant as long as any solid is present.To calculate solubility, we let s represent the concentration of each ion in solution.Since silver and chloride ions are produced in equal amounts, their concentrations are both equal to s.Substituting into our Ksp expression, we get s squared equals Ksp.Taking the square root of both sides gives us our solubility.We can also predict whether precipitation will occur by comparing the ion product Q to Ksp.For example, if silver ion concentration is two point zero times ten to the negative five molar,and chloride ion concentration is one point zero times ten to the negative five molar,we calculate Q by multiplying these concentrations.Since Q is greater than Ksp, precipitation will occur.