Colligative Properties of Dilute Solutions

Introduction

When the amount of the solute is minimal compared to the amount of solvent, it is known as dilute solutions. They show more or less ideal behaviour as the volume and heat change, and also at the same time the mixing of solvent and solute, are inconsequential for all practical purposes. They consist of nonvolatile solute, which shows some types of special properties that are dependent upon the number of solute particles that exist in solution disregarding their nature. So, these types of properties are referred to as colligative properties of dilute solutions. For specifying an unknown compound molecular weight, colligative properties play an important role. 

Definition of the Colligative Properties:

“Colligative” has been derived from the Latin word “colligates” which mainly means “bound together”. 

These properties do not depend on the solution component’s chemical nature. Several qualities convey the concentration of a solution, like morality, polarity, and normality. Therefore these colligative properties can be linked. 

The various types of properties are:

• Lowering of the Vapour Pressure
• Elevation of the Boiling Point
• Depression of the Freezing Point
• Osmotic Pressure

Various Types of Colligative Properties

• Lowering of the Vapour Pressure

In a pure solvent, vapour pressure is reduced when a non-volatile solute is dissolved in it. The surface contains both solute molecules and as well as solvent molecules When a non-volatile solute is added to the solvent. Therefore the amount of surface that’s been covered by solvent molecules gets reduced eventually. 

So now, in case P is referred to as the solvent’s vapour pressure and Ps is referred to as the solution’s vapour pressure. Then, the difference between the ( P – Ps ) is known as the lowering of the vapour pressure and the ratio between P- Ps is known as the relative of the lowering of the vapour pressure. 

In 1886, François-Marie Raoult, a French chemist. Between vapour pressure and mole fraction, he established a relative lowering, and that relationship is referred to as Raoult’s law, which specifically says that the relative lowering in vapour pressure of a dilute solution is equal to the mole fraction of the solute that exists in the solution.

• Elevation of the Boiling Point

The vapour pressure rises when a liquid is heated, and the liquid boils when it equals the atmospheric pressure. 

Thus, for making the atmospheric pressure proportional to the vapour pressure we need to increase the solution’s temperature. The disparity between the boiling point of the pure solvent and the boiling point of the solution is understood as elevation in boiling point. 

So now, in case Tb is referred to as the boiling point of the solvent and T is referred to as the boiling point of the solution. Then the disparity in the boiling points (ΔT) is known as the elevation of boiling point.

• Depression of the Freezing Point

Temperature refers to the freezing point of a particular substance, in which the vapour of a corresponding solid is equal to the liquid of vapour pressure. 

Concerning Raoult’s law, when in a solvent, a non-volatile solid is added, its vapour pressure decreases. After that, it would come to be equal to that solid solvent at a lower temperature. 

Therefore, the disparity between the pure solvent’s freezing point and its solution is known as the depression in freezing point.

• Osmotic Pressure

When in between a solution and solvent, a semipermeable membrane is placed, it is observed that through the semipermeable membrane, the solvent molecules enter the solution, and the solution’s volume increases. Only the solvent molecules are allowed by the semi-permeable to pass through it, and it prevents the path of bigger molecules, for instance, solute. And this happening of the voluntary flow of solvent molecules through a semipermeable membrane, to get changed into a solution from a pure solvent or to get changed to a concentration solution from a dilute, is known as osmosis. 

When from the solution side, some extra pressure is applied to the semipermeable membrane, the flow of solvent molecules can be stopped. That extra pressure that is applied to stop the flow of solvent from the side of the solution is called the osmotic pressure of the solution.

Conclusion 

The significant factor of these colligative properties is that they are dependent only on the solution’s number of solute particles that are present. The meaning of the definition of each colligative property is that they are precisely associated with each other. Therefore, if only one property of these colligative is measured, the other can similarly be calculated. These colligative properties of dilute solutions are extremely significant as these properties provide useful methods for finding the weight of molecular weights and dissolved substances. In dilute solution, we mainly observe these colligative properties.