The dilute solutions of non-volatile solutes exhibit certain characteristic properties which do not depend upon the nature of the solute but depend only on the number of particles (molecules or ions) of the solute i.e., on the molar concentration of the solute. These are called colligative properties (colligative, from Latin: co means together, ligare means to bind). Thus, the properties of the solutions which depend only on the number of solute particles but not on the nature of the solute are called colligative properties.

The four important colligative properties are:

1. Relative lowering in vapour pressure
2. Elevation in boiling point
3. Depression in freezing point
4. Osmotic pressure

Which of the following is a colligative property?

Depression in freezing point

Depression in freezing point, also known as freezing point depression, is a colligative property that occurs when a solute is dissolved in a solvent. When a solute is added to a solvent, it disrupts the formation of the solid lattice structure that would normally occur during freezing.

The freezing process involves the conversion of a liquid into a solid, and it typically happens when the vapor pressure of the liquid is equal to the vapor pressure of the solid (in equilibrium). When a solute is present, it lowers the equilibrium solid-vapor pressure, requiring a lower temperature for the vapor pressure of the liquid to match the pressure outside. As a result, a solution with a solute has a lower freezing temperature compared to the pure solvent.

This phenomenon can be explained by the concept of entropy. When a solute is dissolved in a solvent, the randomness or entropy of the system increases. This increased entropy leads to a destabilization of the solid phase, requiring lower temperatures for the system to transition from the liquid to the solid state.

The depression in freezing point is directly proportional to the concentration of the solute particles in the solution. According to Raoult's law, the decrease in freezing point is proportional to the molal concentration of the solute. Mathematically, the freezing point depression (∆Tf) can be calculated using the equation:

\(\Delta T_f = K_f × m × i\)

where \(\Delta T_f\) is the change in freezing point, Kf is the cryoscopic constant (a property of the solvent), m is the molality of the solute (moles of solute per kilogram of solvent), and i is the van't Hoff factor, which represents the number of particles the solute dissociates into in the solution.

The depression in freezing point is a widely observed phenomenon and has practical applications. For example, it is utilized in antifreeze solutions, where the addition of a solute (such as ethylene glycol) to water lowers the freezing point of the mixture, preventing the liquid from freezing at low temperatures.

In summary, depression in freezing point is a colligative property that occurs when the presence of solute particles in a solvent lowers the freezing temperature of the solution compared to the pure solvent. It is a consequence of solute-solvent interactions and the resulting disruption of the solid lattice formation during freezing.