Colligative properties of solutions

Colligative properties of a solution refer to the properties that solely depends on the number of particles in the solution. The number of particles differs between different solutions and these differences are due to the mere presence of solute particles in the solution, rather than to the specific identity of the solute. 4 important colligative properties of solutions are vapor pressure lowering, boiling-point elevation, freezing-point depression and the osmotic pressure.

Vapor pressure lowering

Vapor pressure lowering is the pressure exerted by vapor that is in dynamic equilibrium with its liquid in a closed system. A solution that is nonvolatile, not easily vaporized, always has a lower vapor pressure than a pure solvent. For instance, glucose is a molecular substance and NaCl is an ionic substance. Here, both glucose and sodium chloride would lower the vapor pressure of a pure solvent. Clearly, when a non-volatile solute is added to a solvent, the vapour pressure reduces. From Raoult’s law: p1=x1 p10. The reduction in the vapor pressure of solvent is given as:



Elevation of Boiling point

Boiling point of a substance is the temperature at which the vapor pressure of the liquid phase is equivalent to the atmospheric pressure. Addition of nonvolatile solute to a liquid solvent reduces the vapor pressure of a given solvent. Due to the decrease in vapor pressure, additional kinetic energy must be added to raise the vapor pressure of  the liquid phase of the solution to atmospheric pressure.  Thus, the boiling point of the solution is higher than the boiling point of pure solvent. Boiling point elevation therefore is the difference in temperature between the boiling point of a solution and that of the pure solvent. The increase in boiling point Tb=Tb- Tb0 where Tb0 represents the boiling point of pure solvent and Tb represents the boiling point of solution commonly referred to as elevation of boiling point.

 Tb= Kbm

Where Kb is the boiling point elevation constant/molar elevation constant.

Freezing point elevation

Generally, when a substance freezes, the particles of the solid take on an orderly pattern. The presence of a solute in water disrupts the formation of this pattern due to the shells of water solvation. As a result, more kinetic energy must be withdrawn from a solution than from pure solvent. Freezing point depression is the difference in temperature between the freezing –point of a solution and that of the pure solvent. The amount of the freezing point depression is found to be proportional to the number of solute particles dissolved in the solvent and is independent of its identity. When nonvolatile solid is added to the solvent, its vapor pressure decreases to become equal to that of solid solvent at lower temperature. Hence, the freezing point of the solvent reduces.

Tf = Tf0 – Tf where Tf0 represents the freezing point of pure solvent while Tf represents the freezing point of a nonvolatile solute after it has been dissolved in the solvent.

Tf= Kf m

Where Kf is the freezing point depression constant/Molar depression constant.

Osmotic pressure

Osmosis is the process through which solvent molecules moves through a semi-permeable membrane from pure solvent to solution. Osmotic pressure is referred as a colligative property as it depends on the number of solute particles and is independent of their identity. In science, a semi-permeable membrane is a special type of membrane that allows only small particles/molecules to penetrate through but  blocks the larger particles from passing through it. In case we use a dilute solution, the osmotic pressure will be directly proportional to the molarity, C of the solution at a given temperature. Mathematically represented as:

π = CRT

In this case, π represents the osmotic pressure of the simulation and R is the gas constant. Hence, the molar mass can be calculated by using the following relationship:

π V= w2RT/M2

Isotonic solutions

Isotonic solutions refer to two solutions containing same osmotic pressure at a given temperature. A solution with lower concentration or simply lower osmotic pressure is referred to as hypotonic w.r.t to concentrated solution while the solution with higher concentration or higher osmotic pressure is called a hypertonic solution w.r.t the dilute solution.

Reverse osmosis process

Reverse osmosis is the process by which molecules of the solvent pass through a semi-permeable membrane from the solution to the pure solvent by applying excess pressure on the solution side.

Abnormal molar masses

Molar mass of a solution is said to be abnormal if and only if its value is either lower or higher than the expected value. When the molar mass of a given substance deviates from the theoretical value(s), the substance is said to have an abnormal molar mass. The molar mass of a given substance can be abnormal under the following conditions:

  1. If the solute undergoes association in the solution.

  2. If the solute undergoes dissociation in the solution.

Read more

  1. Ideal and non-ideal solutions

  2. Solubility laws and vapor pressure

  3. introduction to solutions

  4. Bond breaking and inductive effect in inorganic compound

Check your understanding

  1. What do you mean by colligative property of a solution?

  2. Mention the 4 colligative properties of solutions.

  3. What is osmotic pressure of a solution?

  4. Distinguish between hypertonic solution and hypotonic solution.

  5. What is boiling point of a substance ?






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