Difference Between Ksp and Qsp

Key Difference – Ksp vs Qsp
 

Ksp is the solubility product constant and Qsp is the solubility product quotient. The key difference between Ksp and Qsp is that  Ksp indicates the solubility of a substance whereas Qsp indicates the current state of a solution. The solubility product is the product of concentrations of ionic species present in a solution when a substance is dissolved in a solvent such as water.

The solubility product is determined when the solution is saturated with that substance. Solubility product quotient is the product of concentrations of ionic species in a solution at any time; before the saturation or after the solution is saturated. It is sometimes known as the ionic product.

CONTENTS

1. Overview and Key Difference
2. What is Ksp
3. What is Qsp
4. Relationship Between Ksp and Qsp
5. Side by Side Comparison – Ksp vs Qsp in Tabular Form
6. Summary

What is Ksp?

Ksp is the solubility product constant of a particular substance. It indicates the solubility of a substance (how much of solid is dissolved in a solution). The solubility product constant is given for a solution that is saturated with a substance. Higher the Ksp, higher the solubility of that substance. The solubility product is given as the product of concentrations of ionic species in a solution.

A saturated solution appears with cloudiness that indicates the beginning of the formation of a precipitate. It is the insoluble form of solute. The liquid portion of that system has the soluble solutes. The Ksp of that solution represents the equilibrium between these soluble and insoluble forms.

The factors affecting the value of solubility product constant are temperature, the presence of common ions, pH or acidity, etc. When the temperature is increased, the solubility of solid precipitate also increases. Then the product of concentrations of ionic species increases, resulting in a high value of solubility product constant. The presence of a common ion is described by the common ion effect. When a common ion is present, the Ksp is decreased. A common ion means one of the ionic species that are already present in that solution. For example, for an equilibrium system containing BaSO4 (barium sulfate) precipitate along with, Ba⁺² ions and SO₄^2- ions, the addition of either Ba⁺² or SO₄^2- ions can affect the equilibrium.

BaSO_4(s)↔         Ba⁺²_(aq)   +     SO₄^2-_(aq)

When one of these ions are added from an external source, then the above equilibrium shifts towards the left-hand side (a more insoluble form of the substance is formed decreasing the amount of ions present in the solution), decreasing the solubility of that substance.

What is Qsp?

Qsp is the solubility product quotient of a solution. It describes the current state of a solution. This means Qsp is given for an unsaturated (before saturation), saturated or a supersaturated solution. The Qsp is also called the ion product because it is the product of concentrations of ionic species at any moment (not at a specific moment such as saturation). Therefore, Ksp (the solubility product constant) is a special form of Qsp.

What is the Relationship Between Ksp and Qsp?

• If the Qsp value is less than the Ksp for a substance in a solution, more solids can be dissolved in that solution.
• When the Qsp and Ksp have equal values, then the solution has become saturated.
• If the Qsp is higher than the value of Ksp, a precipitate is formed.

What is the Difference Between Ksp and Qsp?

Summary – Ksp vs Qsp

Ksp and Qsp are very related terms in chemistry. Ksp is defined as a saturated solution which has equilibrium between ionic species and a solid precipitate (the moment at which the formation of a precipitate has begun). Qsp is given for any moment (not specified); before the saturation or after the saturation. The difference between Ksp and Qsp is that Ksp is the solubility product constant whereas Qsp is the solubility product quotient.