Different Types of Colligative Properties of Solution

A colligative property is a quality of a substance determined by the number of particles (molecules or atoms) present but not by the nature of the particles. Two examples of colligative properties are the pressure of an ideal gas and the lowering of a solvent’s freezing point due to dissolved particles.

Colligative properties are useful when defining a solution since they show how the properties of the solution are related to the solute concentration in the solution.

Colligative Properties Examples

• Elevation in boiling point
• Depression in freezing point.
• Relative lowering of vapour pressure
• Osmotic pressure

Elevation in boiling point

It is one of the colligative properties. Others are freezing point, osmotic pressure, vapour pressure. The solute-solvent ratio, and not the solute’s nature, determines boiling point elevation, which is a colligative feature of matter. This amount of solute added to a solution influences the change in the rise of the boiling point of the solution, that is, the higher the solute concentration (quantity of the molecules or atoms) in a solution, the higher the boiling point elevation. Consider an example of the increased boiling point of water when 100 gm of salt (NaCl) was added to 1 litre of water, resulting in an increase of 4-5 degrees in the boiling point of the water.

Boiling Point Elevation Formula

The formula can be expressed by taking the example of a solution dissolved with non-volatile solutes.

• The boiling point of a solution is equal to the boiling point of the pure solvent added with the elevation of the boiling point.
• The elevation in boiling point (ΔTb) is proportional to the number of solute particles (molecules or atoms) in the solution. The measurement of the same is possible with the given equation:

ΔTb = (i).(Kb). (m)

Where,

ΔTb: Elevation in boiling point

i: The Van’t Hoff factor

Kb: The ebullioscopic constant

m: The molality of the solute. 

Depression in Freezing Point

Before discussing depression, let us know what this term means. So, the depression of the freezing point is a colligative property due to the addition of solute molecules to any solvent. So, to be very precise, depression of freezing point is a term that refers to the lowering of the freezing point of solvents due to the addition of solute molecules to it. Due to a decrease in temperature, a substance starts freezing, and its intermolecular forces take over, arranging themselves in a pattern and eventually turning into a solid. 

Depression in freezing point formula

Let us look at an example to have a better understanding. When we keep water cool in a temperature which is below the freezing point of water, the hydrogen bonding begins to stick more and thus results in the formula for depression of freezing point:

△Tf  = i  x Kf x m

Here, 

△Tf stands for depression of freezing point

 i  stands for Van’t Hoff Factor

 Kf stands for cryoscopic constant and

 m represents molality

Relative lowering of vapour pressure

In a pure solvent, vapour pressure is reduced when a non-volatile solute is dissolved. The surface contains both solute molecules and solvent molecules when a non-volatile solute is added to the solvent. Therefore, the amount of surface 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, the difference between them (P-Ps) is known as the lowering of the vapour pressure, and the ratio between P and Ps is known as the relative of the lowering of the vapour pressure. 

In the year 1886, Raoult came up with a relationship between the relative lowering of vapour pressure and mole fraction and termed it Raoult’s law. This law states that the relative lowering of vapour pressure is equal to the mole fraction of the solute in the solution by the relation:

Po- Ps / Po = n / n+N

Osmotic pressure

Osmotic pressure is the pressure necessary to prevent water from diffusing across a barrier created via osmosis. To put it another way, it relates to how strongly the water would have to “push” through the barrier to disperse to the other side. The diffusion of water across a semipermeable membrane is known as osmosis. As a result, in osmosis, the solutes cannot move because they are unable to flow through the membrane.

Osmotic pressure is calculated using the following formula:

π = iCRT

Where,

π = osmotic pressure

i = van’t Hoff index

C = molar concentration of the solute 

R = ideal gas constant

T = temperature in Kelvin

Conclusion

The significant factor of these colligative properties is that they are dependent only on the solution’s number of solute particles 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 they provide useful methods for finding the weight of molecular weights and dissolved substances. In dilute solutions, we mainly observe these colligative properties.