What is Freezing Point Depression?
“Freezing point depression refers to the lowering of the freezing point of solvents upon the addition of solutes”. It is a colligative property of solutions that is generally proportional to the molality of the added solute. The depression in the freezing point of a solution can be described by the following formula.
ΔTf = i*Kf*m
- ΔTf is the freezing point depression,
- i is the Van’t Hoff factor,
- Kf is the cryoscopic constant, and
- m is the molality.
As per Raoult’s law, “the vapor pressure of a pure solvent decreases with the addition of a solute”. Since the vapor pressure of a non-volatile solvent is zero, the overall vapor pressure of the solution is lesser than that of the pure solvent.
Why does the Freezing Point Depression Occur?
The reason for the depression of the freezing point of a solvent upon the addition of a solute is explained below.
- At the freezing point of a solvent, there exists an equilibrium between the liquid state and the solid state of the solvent.
- This implies that the vapor pressures of both the liquid and the solid phase are equal.
- Upon the addition of a solute which is non-volatile, the vapor pressure of the solution is found to be lower than the vapor pressure of the pure solvent.
- This causes the solid and the solution to reach equilibrium at lower temperatures.
A graph detailing the freezing point depression of water upon the addition of sucrose to it is provided below.
From the graph, it can be observed that the increase in the molality of sucrose causes further depression in the freezing point of the solvent (water).
Freezing Point Examples
- The freezing point of seawater is below 0oC; it remains liquid at temperatures below the freezing point of pure water. This is caused by the salts that are dissolved in it.
- Another example of freezing point depression of a solvent can be observed in vodka. It can be considered to be a solution of ethanol in water, and its freezing point is lower than water but higher than pure ethanol.
- Many organisms can survive in extremely cold climates because their bodies can produce compounds such as glycerol and sorbitol, which helps in decreasing the freezing point of the water in their bodies.
The normal freezing point and the corresponding freezing point depression is tabulated below.
|Solvent||Normal Freezing Point, oC||Freezing Point Depression, Kb, oC m-1|
Uses of Freezing Point Depression
Some important uses of freezing point depression are listed below.
- In cold areas where the temperatures range from 0oC to -15oC, sodium chloride is spread over the roads in order to lower the freezing point of water and prevent the buildup of ice.
- If the temperatures are below 18oC, calcium chloride is used instead of NaCl to melt the ice on the roads. This is because CaCl2 dissociates into 3 ions, causing a greater depression in the freezing point of water.
- Radiator fluids used in many automobiles are generally made up of ethylene glycol and water. This prevents the freezing of the radiator during cold seasons.
- The molar mass of a given solute can be determined from the freezing point depression formula.
- The degree to which a solute dissociates in a solvent can also be measured with the help of this formula.
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