Freezing Point Depression
Freezing point depression is one of four colligative properties (the others being boiling point elevation, vapor pressure lowering, and osmotic pressure) that causes a liquid to freeze at a lower temperature when a substance dissolves in it. It occurs because the dissolved particles interfere with the liquid’s ability to solidify. [1-4]
For example, salt is spread on roads in winter to prevent ice from forming, and antifreeze is used in car engines to prevent water from freezing. This simple idea shows how adding something to a liquid can change its freezing point, making it useful in many everyday situations.
Formula [1-5]
The freezing point depression (ΔTf) of a solution can be calculated using the following formula:
ΔTf = i x Kf x m
Where:
– m is the molality
– Kf is the freezing point depression constant
– i is the van’t Hoff factor
Let us look at the individual terms used in the equation and how they affect the freezing point.
Molality
Molality measures the concentration of the solution. It is defined as the number of moles of solute per kilogram of solvent and has the unit of mol/kg. Freezing point depression increases as the concentration of solute particles rises. It is because an increase in the quantity of solute particles increases the solvent’s ability to freeze. For example, a 1.0 mol/kg solution of salt will lower the freezing point more than a 0.5 mol/kg solution.
Freezing Point Depression Constant
The freezing point depression constant (Kf) determines how much the freezing point is lowered by adding a solute. Water has a Kf of 1.86 °C·kg/mol, and benzene has a Kf of 5.12 °C·kg/mol. It means that benzene will experience a greater freezing point depression than water at the same molal concentration of the solute. The extent of freezing point depression does not depend on whether the solvent is polar or nonpolar.
Van’t Hoff Factor
The van’t Hoff factor accounts for the number of particles the solute dissolves in the solution and, hence, determines the type of solute. For non-electrolytes like sugar, i is 1 because it does not break into smaller particles. For electrolytes like sodium chloride (NaCl), i is more than 1 because it dissociates into ions (Na+ and Cl−), so i = 2. The more particles a solute produces (higher i), the larger the freezing point depression.
Applications [1-5]
- Anti-freeze in Car Engines: Ethylene glycol lowers the freezing point of water in car radiators, preventing freezing in cold weather.
- Salting Icy Roads: Salt lowers water’s freezing point, helping to keep roads ice-free during winter.
- Making Ice Cream: Salt mixed with ice lowers the freezing point, allowing ice cream to freeze faster.
- Food Preservation: Salt and other solutes lower freezing points, helping preserve food at lower temperatures.
- Cryopreservation: Freezing point depression prevents ice crystal formation, protecting cells and tissues during freezing.