Depression of Freezing Point

Freezing Point

The freezing point is the particular temperature when a liquid turns into a solid substance. Freezing point depression means lowering the freezing point of solvents in a solution by adding solutes to it. 

It is one of the colligative properties of a solution. It is proportional to the molality of the dissolved solute of the solution. 

The reason for the depression of the freezing point is that according to Raoult’s Law, the vapour pressure of a pure solvent reduces by dissolving a solute into the solution. 

We know that the vapour pressure of a non-volatile solvent is zero. Therefore, the total vapour pressure of the solution is lower than the vapour pressure of the pure solvent.

For instance, the freezing point of water is 0°C.

The Formula of Depression of Freezing Point

Freezing point Depression is explained by the following mathematical formula –

∆Tf = Kf • m • i

Where,

∆Tf – freezing point depression

Kf – the cryoscopic constant

m – molality

i – van’t Hoff factor

Explanation:

• Here, we can define ∆Tf  as Tf  (Pure Solvent ) – Tf  (Solution)
• The cryoscopic constant depends on the characteristics of the solvent
• We will need a higher Kf value to observe bigger drops in the freezing point in an experiment
• The molality refers to the amount of mole solute per kilogram of solvent
• The Van’t Hoff factor implies the number of ions present per formula solute unit

How does the Freezing Point Depression Occur?

By adding solute to the solution, the depression of the Freezing Point of a solvent occurs. We can illustrate this aspect below:

• Suppose the solvent in the solution is at its freezing point
• In that case, an equilibrium state occurs between the liquid and solid states of the solvent
• Consequently, the vapour pressure of the solid and the liquid states are equal
• Now a non-volatile solute is added to the solution
• Then the vapour pressure of the solution is reduced to that of the pure solvent
• Hence, the solid and the solution reach equilibrium at a lower temperature
• Furthermore, the increase in the molality of the solute leads to freezing point depression forward of the solvent

Examples of Freezing Point Depression in Day to Day Life

• When we put salt on an ice-covered road, the salt mixes with minimal water to prevent the ice from refreezing
• The property is applicable for making ice cream
• In the dairy industry, we apply the measurement of freezing point depression (FPD) to provide a perfect amount of water to be added to the milk; for instance, we consider milk with over 0.509°C EPD as pure milk
• By using this property, alcoholic beverages like vodka do not freeze in the freezer
• The property is useful in Radiator fluids used in automobiles to prevent freezing up of water in cold climates
• The approach utilises ethylene glycol and water
• Animals living in icy climates have some natural antifreeze substances within themselves like glycerol, sorbitol
• This aspect helps lower the freezing point of the water in their bodies to survive
• The freezing point depression causes the ocean water to remain unfrozen even when the temperature is below 0°C

List of Some useful Solvents and their Corresponding Freezing Point

Following are some useful solvents with their normal freezing point as well as their freezing point depression

Importance of Freezing Point Depression in Chemistry

• The freezing point depression formula determines the molar mass of a particular solute used in the solution
• Freezing point depression is helpful to estimate the standard to which a particular solute can dissociate in a solvent
• A measurement called ‘Cryoscopy’ is often useful in freezing point depression
• Here ‘cryo’ means cold, and ‘scopy’ means to observe the cold. It depends on the error-free calculation of the freezing point
• It is also helpful as a purity analysis tool in thermoanalytical techniques like Differential Scanning Calorimetry (DSC)
• Most importantly, freezing point depression illustrates why adding a solute to a solvent reduces the freezing point of the solvent
• The depression of the freezing point occurs whether the solute and the solvent are in a liquid, solid, or gaseous state
•  It can also happen in solid-solid mixtures
• The formula used in this equation is known as Blagden’s Law by combining Raoult’s Law and the Clausius Clapeyron Equation

Limitations of Freezing Point Depression

Here are certain limitations when applying freezing point depression in some instances.

• The solute must be in a lower quantity than the solvent
• The formula and its implementation of depression of freezing point are applicable to dilute solutions only
• The solute used here must be non-volatile
• It only depends on the solute concentration in an ideal solution

Conclusion

In freezing point depression, a substance’s liquid and solid states have the same vapour pressure. The solution’s freezing point is always less than that of the pure solvent. It implies that a solution must be cooled down to a lower temperature than the pure solvent for freezing. The depression of the freezing point is a colligative property of a solution. It depends on the number of solute particles present in a solution. Hence, the more ions in a solute, the greater the dimensions of freezing point depression.