Depression in Freezing Point - Definition, Formula, Graphical Representation, Practice Problems and FAQ

Let's pretend we're going on an arctic expedition. We can see sheets of ice and glaciers as we sail over the ocean.

"Is the entire ocean frozen?" is a common query.

The response is a resounding "no."

The large concentration of dissolved salts in seawater lowers its freezing point. This phenomenon is called the depression in freezing point. Seawater does not totally freeze due to the presence of these dissolved salts/impurities.

TABLE OF CONTENTS

Definition of Freezing Point
Definition of Depression in Freezing Point
Formula for Depression in Freezing Point
Graphical Representation of Depression in Freezing point
Determination of Molal Depression Constant
Example of Depression in Freezing Point
Practice Problems
Frequently Asked Questions - FAQ

Definition of Freezing point

The temperature of a liquid at which it changes its state from liquid to solid at atmospheric pressure is called the freezing point.

Factors affecting the freezing point are:

If the intermolecular forces between the liquid molecules are strong, then its freezing point is high.
If the intermolecular forces between the liquid molecules are weak, then its freezing point is relatively low.

Definition of Depression in Freezing Point

Depression in freezing point refers to the lowering of the freezing point of the solvents upon the addition of solutes. It is a colligative property.

Formula for Depression in Freezing Point

Generally, depression in freezing point is directly proportional to the molality of the added solutes. The depression in the freezing point of the solution can be described by the following formula;

Where Depression in freezing point

Molal depression constant or cryoscopic constant or freezing point depression constant

Molality of the solution.

Mass of the solute

Molar mass of the solute

Mass of the solvent in

Graphical Representation of Depression in Freezing Point

The liquid phase is in dynamic equilibrium with the solid phase at the freezing point of the substance. Freezing of the solution takes place when its vapour pressure equals the vapour pressure of the pure solid solvent as shown in the figure.

According to Raoult’s law, at low temperature, when a non-volatile solute is added to the solvent, its vapour pressure decreases and becomes equal to the vapour pressure of the solid solvent. Thus, the freezing point of the solution decreases.

Let be the freezing point of the pure solvent and be its freezing point when a non-volatile solute is dissolved in it. The decrease in freezing point is known as depression in freezing point.

Determination of Molal Depression Constant or Cryoscopic Constant or Freezing Point Depression Constant

is the characteristic of the solvent and can be calculated from the thermodynamic relationship,

Where, latent heat of fusion in of solvent.

freezing point of the solvent

universal gas constant

Example of Depression in Freezing Point

Salting of icy roads

You may have noticed that applying salt to ice causes it to melt, and this is a popular process for de-icing snow-covered roads. The most popular salts used to de-ice roadways are sodium chloride () and calcium chloride (). The fundamental rationale for sprinkling salt on an ice road is that a solution of water containing dissolved salts has a lower freezing point than pure water due to the solution's lower freezing point. Water generally freezes at (), but when salt is added, the temperature reduces. The overall freezing point is lower when the salt concentration is higher.

Practice Problems

Q1. Find the freezing point of the solution obtained by mixing of glucose in of water given that

Answer.

Q2. Find the depression in freezing point of sucrose that must be added to of sucrose in of water given that

Answer.

Q3. If the latent heat of fusion of ice is , calculate the molal depression constant for water.

Answer.

Q4. An aqueous solution of a non-electrolyte boils at . The freezing point of the solution will be ,

Answer.

0.52

Frequently Asked Questions - FAQ

Question 1. Is freezing exothermic or endothermic?

Answer. Endothermic processes include fusion, vapourisation, and sublimation, whereas exothermic processes include freezing, condensation, and deposition. When water is placed in the freezer, it gradually loses heat to the environment. As the water molecules lose energy, they begin to move slowly, getting closer and closer until they form ice. The water releases heat into the environment as a result of this process. So, freezing is an exothermic reaction.

Question 2. What are the six types of phase changes?

Answer. The six-phase changes are sublimation, deposition, condensation, evaporation, freezing and melting.

Question 3. What are supercooled liquids?

Answer. For the majority of substances melting point and freezing point have to be the same but a few substances have a slight difference between their melting points and freezing points. These substances can be cooled beyond their freezing point while retaining their liquid form. Hence, super-cooling is a state where liquids do not solidify even below their normal freezing points.

Question 4. How does snowfall occur?

Answer. Everyone is enthralled by the sight of snowfall. Have you ever been curious about the process of snow formation? Do you understand the science going on here?

Snowfall is based on the concept of freezing. The creation of snow occurs when the atmospheric temperature is at or below the freezing point of water, which is , and there is a minimum amount of moisture present in the air. Because warm air has the capacity to contain more water vapours, the heaviest snowfall occurs when there is relatively warm air near the ground. Snowfall is rare in dry locations since it requires moisture and freezing temperatures to produce. The dry valleys of Antarctica, for example, have low humidity and strong, cold winds that remove moisture from the air. As a result, a small amount of snow falls in this severely cold region.

Related Topics

Distillation Curves	Elevation in Boiling point
Steam distillation	Osmosis and Osmotic pressure
Van’t Hoff Equation	Raoult's Law - Applications
Azeotropic mixtures	Henry's Law - Applications and Limitations