A solution’s freezing point is less than the pure solvent’s point of freezing. This indicates that for a solution to freeze, it must be chilled to a lower temperature than the pure solvent. The freezing point of a solvent in a solution varies when the concentration of a solute in the solution changes (although it is unaffected by the identity of the solvent or the solute(s) particles in the solution (kind, size, or charge). So, let us learn about the depression of freezing point and see some solved examples using the freezing point depression formula.

Depression of Freezing Point: Detailed explanation

The Depression of Freezing Point is a decrease in the temperature at which a material freezes, which occurs when a lesser amount of another non-volatile chemical is added to the mixture. Examples include the addition of salt to water (which is used in ice cream makers and for de-icing roads), the addition of alcohol to water, the addition of ethylene or propylene glycol to water (which is used in antifreeze in automobiles), the addition of copper to molten silver (which is used to form solder that flows at a lower temperature than the silver pieces being connected), and the blending of two substances, such as contaminants into a finely powdered medicine.

In all circumstances, the solute is the material added/present in lesser proportions, whereas the solvent is the original component present in higher quantities. As a result, a mixed solution or solid-solid combination is at a lower freezing point temperature than a pure solvent or solid. Also, the chemical potential of the mixture’s solvent is less than that of its pure solvent, which is proportionate to the mole fraction. 

A comparable phenomenon occurs when the chemical potential of vapour over a solution is lesser than that of vapour above a pure solvent, increasing to the boiling point. At temperatures below 0 °C (32 °F), the freezing point of pure water, freezing-point depression causes seawater (a combination of salt and other chemicals in water) to remain liquid.

Non-Volatile Solutes

Whether the solution comprises a non-volatile or volatile solute, the solvent’s freezing point in a solution will have a lower freezing point than the pure solvent. However, we’ll study only non-volatile solutes in this section for better understanding.

We know from experiments that the difference in the freezing point between a solution and a pure solvent is exactly proportionate to the molal concentration of the solute:

Kf m = T

where T is the change in the solvent’s freezing point.

The molal depression of the freezing point constant is represented by Kb.

The molal concentration of the solute in the solution is denoted by the letter m.

It’s worth noting that the molal depression of the freezing point constant, Kb, has a particular value that depends on the solvent’s identity.

The typical freezing point of water (solvent) is molality in numerous sucrose-containing solutions. It’s worth noting that when the concentration of sucrose rises, so does the freezing point of water.

The calculation for dilute solutions

When the solution is viewed as an ideal solution, the amount of the freezing-point depression is only determined by the solute concentration, which can be evaluated using a simple linear connection with the cryoscopic constant (“Blagden’s Law”):

ΔTf = Kf · b · i,

where:

• The freezing-point depression is defined as Tf (pure solvent).
• The cryoscopic constant, Kf, is determined by the solvent’s characteristics rather than the solute’s. (Note: A higher KF number makes it easier to see bigger reductions in the freezing point while doing tests.) Kf = 1.853 Kg/mol for water.] ).
• The molality is denoted by the letter b. (moles solute per kilogramme of solvent).
• The Can’t Hoff factor (number of ion particles per formula unit of solute, e.g. I = 2 for NaCl, 3 for BaCl2) is the number of ion particles per formula unit of solute.).

Calculation of the Molal Depression Constant

Problem:

In 20.0 g of benzene, 1.60 g of naphthalene (C10H8) is dissolved. Pure benzene has a 5.5°C freezing point, whereas the combination has a 2.8°C freezing point. What is the Kf of benzene’s molal depression constant freezing point?

Strategy:

1. Step 1: Determine benzene’s depression of freezing point.

Tf = (freezing point of pure solvent) – (freezing point of pure solvent) (Freezing point of solution)

2.7 oC = (5.5 oC) – (2.8 oC)

1. Step 2: Determine the solution’s molal concentration.

Molality is defined as the number of moles of solute per kilogram of solvent. Naphthalene moles = (1.60 g) (1 mol/128 g) = 0.0125 mol naphthalene

(0.0125 mol) / (0.0200 kg) = 0.625 m molality of a solution

1. Step 3: Calculate Kf of the solution.

   Tf = (Kf) (m)

   (2.7 oC) = (Kf) (0.625 m)

   Kf = 4.3 oC/m

Uses of Depression in Freezing Point:

1. In cold areas where the temperature drops below 0°C, sodium chloride (NaCl) is spread over the roads to prevent ice formation. NaCl lowers the freezing point of water. Hence, ice doesn’t accumulate over the road.
2. In areas where the atmospheric temperature drops to 18° C, Calcium Chloride (CaCl2) is used instead of sodium chloride. Calcium chloride Associates into three ions, which causes more depression in the freezing point of water and helps to melt ice on roads. 
3. During cold seasons, there is a chance of the radiator getting frozen. We use radiator fluids in automobiles. These fluids are generally made of ethylene glycol and water and help prevent the freezing of radiators.
4. It is used as a purity analysis device. It is analysed by differential scanning calorimetry. This method is very effective and gives pure results.
5. It is used in the dairy industry. This property makes sure that extra water is not there in the milk. Milk with a freezing point depression of 0.509° is considered pure.
6. This property is also used in making ice cream. It helps to make a freezing mixture by adding NaCl or another salt to lower its melting point.

Conclusion

Here, we learned all about the concept of depression of freezing point in an elaborate manner. With this article, we also explored the definition of the non-volatile solutes and molal depression constant. Further, we also discussed the various uses of depression of freezing point.