Depression of Freezing Point – Class 12 | Chapter – 2 | Chemistry Short Notes Series PDF for NEET & JEE
Depression of Freezing Point: Depression of freezing point is a colligative property that occurs when a non-volatile solute is added to a solvent. It is the phenomenon in which the freezing point of a solution is lower than the freezing point of the pure solvent. The depression of freezing point is directly proportional to the molality (number of moles of solute per kilogram of solvent) of the solution.
Depression of Freezing Point
The depression of freezing point can be explained by the same principle as boiling point elevation, which is Raoult’s law. When a non-volatile solute is added to a solvent, it decreases the mole fraction of the solvent in the solution, leading to a decrease in its freezing point. This decrease in freezing point is proportional to the molality of the solution and is given by the following equation:
ΔTf = Kfm
- ΔTf is the depression of freezing point
- Kf is the molal freezing point depression constant, which is a characteristic property of the solvent
- m is the molality of the solution.
The depression of freezing point has important applications in industry and everyday life. For example, it is used in the production of antifreeze, which is added to the cooling system of automobiles to prevent the engine coolant from freezing in cold temperatures. The antifreeze works by lowering the freezing point of the coolant, which allows it to remain a liquid at lower temperatures. Depression of freezing point is also used in the preservation of food, such as frozen vegetables, which are stored at temperatures below the freezing point of water to prevent bacterial growth and spoilage.
Like boiling point elevation, depression of freezing point can also be used to determine the molar mass of a solute in a solution. By measuring the depression of freezing point and knowing the identity of the solvent, it is possible to calculate the molar mass of the solute using the following equation:
M = (Kf x w2) / (ΔTf x w1)
- M is the molar mass of the solute
- w2 is the mass of the solute added
- w1 is the mass of the solvent
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