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The word colligative comes from the Latin word ‘colligatus,’ which means ‘bound together.’ In common parlance, it refers to the ratio of how the properties of a solvent are bound to the concentration of solute in the solvent.
A colligative property is defined as the property of the solution determined by the ratio of the number of solute and solvent molecules in a solution. Colligative properties are completely independent of the nature of the chemical species of the solution’s components. The presumption that solution properties are not dependent on the nature of solute particles is exact only for ideal solutions (that exhibit thermodynamic properties compared to those of an ideal gas); it is approximate for dilute real solutions.
The colligative properties can be linked with quantities that express the concentration of a solution, such as molarity, normality, and molality. All the colligative properties are inversely proportional to the molar mass of the solute for all mass ratios of solute and solvent.
Wilhelm Ostwald classified solute properties in three categories:
- Colligative properties – They depend only on solute concentration and temperature and are independent of the nature of the solute particles.
- Additive properties – Mass is the perfect example of additive properties. Additive properties are the sums of properties of the constituent particles of a solution, and they depend on the composition (molecular formula) of the solute particles.
- Constitutional properties – Constitutional properties are the properties that depend on the molecular structure of the solute in the solvent.
Example of colligative properties
Adding alcohol to water lowers the solution’s freezing point below the normal temperature.
Similarly, if we add a pinch of salt to a glass of water, the freezing temperature is significantly lower than the normal temperature. Moreover, the boiling point of the solution increases, and the solution will have a lower vapour pressure and a change in osmotic pressure.
Various types of colligative properties of a solution
The different types of colligative properties of a solution are-
- Relative lowering of vapour pressure ( Raoult’s law)
- Elevation of boiling point
- Depression of freezing point
- Osmotic pressure
Relative lowering of vapour pressure
The entire surface of a pure solvent is covered with the molecules of the solvent. On adding a non-volatile solute to the solvent, the vapour pressure of the solvent gradually gets lowered. This occurs because after adding the non-volatile solute, the surface is occupied by both solute and solvent molecules at a given temperature.
Relative lowering of vapour pressure = Vapour pressure lowering/vapour pressure of the pure solvent.
Elevation of boiling point
When we add a non-volatile liquid (solute) to a pure solvent, the vapour pressure of the solution decreases. So, to make the vapour pressure equal to the atmospheric pressure, the temperature of the solution needs to be increased. This variation in the boiling point of the pure solvent and the solution is called the elevation in boiling point.
Remember, the boiling point of a liquid is the temperature at which the vapour pressure is equal to the atmospheric pressure.
Both the boiling point elevation and the freezing point depression are proportional to the lowering of vapour pressure in a solution.
If T0b is solvent’s pure boiling point and Tb is the solution’s boiling point, then the increase in boiling point is given as
Depression in freezing point
At the freezing point, the vapour pressure of solid and liquid are at equilibrium. So when a non-volatile solid (solute) is added to a pure solvent, it reduces the vapour pressure of the solvent. To achieve equilibrium, the temperature of the solvent needs to be lowered.
Measurement of this difference is called cryoscopy.
If T0f is the pure solvent’s boiling point and Tf is the solution’s boiling point, the depression in freezing point is given as
Osmotic Pressure
Osmosis is a process where liquids pass through a semipermeable membrane. A semipermeable membrane placed between a solution and a solvent allows the solvent molecules to pass through it while preventing the bigger molecules like solute particles. This spontaneous flow of solvent molecules through the semi-porous membrane is called osmosis.
To stop the flow of solvent molecules through the semi-porous membrane, we need to apply some extra pressure from the solution side. This pressure applied to stop the flow of solvent molecules is termed as the osmotic pressure of the solution.
Osmotic pressure is a colligative property because it is determined by the amount of solutes existing instead of the type of solute. It has been experimentally demonstrated that osmotic pressure (π) is directly proportional to molarity (C) and temperature (T).
π = (n2/V) RT, where
π= CRT
where R is the gas constant.
Conclusion
A colligative property is the property of a solution that depends only on the number of solute particles and not on their nature. Colligative properties of a solution include relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure. They are studied mostly for dilute solutions, whose behaviour may be approximated as that of an ideal solution.
The properties mentioned above are colligative only within the dilution limit. At higher concentrations, the freezing point depression, boiling point elevation, vapour pressure elevation or depression, and osmotic pressure depend on the chemical nature of the solvent and the solute.
