AAMC FL Practice Exam

What causes a product to precipitate in a saturation reaction?

1. When Ksp equals the product concentration

2. When the product concentration is less than Ksp

3. When the product concentration exceeds Ksp

4. When temperature increases

The correct answer is: When the product concentration exceeds Ksp

In a saturation reaction, a product precipitates when its concentration exceeds the solubility product constant, known as Ksp. The Ksp is a constant that reflects the maximum concentration of ions that can exist in a solution at equilibrium. When the concentration of the product (or the ions forming the product) surpasses this Ksp value, the solution becomes supersaturated, leading to the formation of solid precipitate as the excess ions are unable to remain dissolved. Therefore, the condition of exceeding the Ksp indicates that the solution cannot maintain all the dissolved ions in liquid form, prompting them to come together to form a solid, or precipitate. This concept is crucial in understanding the dynamics of solubility equilibria and precipitation reactions in chemistry. In contrast, if the product concentration is equal to or less than Ksp, the system remains in equilibrium or is undersaturated, which does not favor the formation of precipitate. Changes in temperature can affect the Ksp itself, but they don't directly define when a precipitate will occur in the context of a given concentration related to a fixed Ksp.