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Applications of Colligatives Properties: Introduction, Applications of Colligative Properties, Practice Problems & Frequently Asked Questions

Applications of Colligatives Properties: Introduction, Applications of Colligative Properties, Practice Problems & Frequently Asked Questions

Due to the drop in temperature and snow development, frozen roadways are a common winter sight in Kashmir and other cold regions. Without clearing the snow off the roads and railroad tracks, logistics comes to a screeching halt. Though machines can be employed to remove the snow, de-icing the roads covered is being done by applying salts.

Do you know what these salts are and how they are used to de-ice the roads?


By lowering the freezing point of water below zero degrees, the salts de-ice the roadways by causing the water to thaw and flow off the surfaces. You may have noticed that adding salt to ice causes it to melt; this is a typical technique for de-icing snow-covered roads. The salts that are used to de-ice roadways the most frequently are sodium chloride and calcium chloride. The main justification for sprinkling salt on an ice road is that a solution of water containing dissolved salts has a lower freezing point than pure water. Normal water freezes at 0°C, but adding salt lowers the freezing point. The total freezing point is decreased at higher salt concentrations.

This concept page details the explanation of the depression in freezing point and other applications of the principle called colligative properties.

Table of Contents

  • Colligative Properties - Introduction
  • Applications of Colligative Properties
  • Practice Problems
  • Frequently Asked Questions(FAQs)

Colligative Properties - Introduction

The addition of a non-volatile solute to a volatile solvent reduces the vapour pressure of the solution. A relation of the lowered vapour pressure to the concentration of the solute and solvent in the solution was given by Raoult’s law. The decrease in vapour pressure proportionally affects other properties of the pure solvent like freezing point, boiling point, and osmotic pressure.

1. Relative lowering of the vapour pressure of the solvent
2. Depression in the freezing point of the solvent
3. Elevation in the boiling point of the solvent
4. The osmotic pressure of the solution

The above-mentioned characteristics of solutions are those that are totally determined by the proportion of solute particles to solvent molecules in a specific solution and are totally unrelated to the type of chemical substances present. These characteristics are known as the "colligative properties," which are derived from the Latin words "co" meaning "together" and "ligare," which means "to bind."

Relative lowering of vapour pressure:

The pressure that the vapours apply to the liquid in equilibrium at a specific temperature is known as vapour pressure.

A non-volatile solute dissolved in a volatile solvent lowers its vapour pressure. When p indicates the solution's vapour pressure and PAo represents the vapour pressure of a pure solvent, we get

The lowering in vapour pressure of the solvent (△PA) is expressed as,

PA=P0A-PA …………………. (1)

The relative lowering in vapour pressure in a solution containing various nonvolatile solutes is proportional to the total of the mole fractions of the individual solutes.

χB=PAP0A=P0A-PAP0A …………….. (2)

Mole fraction of solute (𝜒B) = Vapour pressure of pure solvent - Vapour pressure of solutionVapour pressure of pure solvent

The relative lowering of vapour pressure denoted by equation (2) is equal to the mole fraction of the solute.

Depression in Freezing point:

The temperature at which both a solid and a liquid substance have equal vapour pressures is known as the freezing point.

In general, the freezing point depression is directly proportional to the molality of the added solutes. The depression in the solution's freezing point can be represented as

ΔTf  m

ΔTf= Freezing point of the pure solvent - Freezing point of the solution

ΔTf=Kf m

Where Kf is the freezing point depression constant or cryoscopic constant. Kf unit is K kg mol-1.

Elevation in boiling point:

The temperature at which a liquid's vapour pressure equals its atmospheric pressure is known as the boiling point of the liquid.

In general, the boiling point elevation is directly proportional to the molality of the added solutes. The elevation in the solution's boiling point can be represented as,

ΔTb  m

ΔTb= Boiling point of the solution - Boiling point of the pure solvent

ΔTb=Kb m

Where Kb is the molal elevation constant of ebullioscopic constant. Kb unit is K kg mol-1.

Osmotic Pressure:

Osmosis is the movement of solvent molecules through a semipermeable membrane from an area of higher solvent concentration to an area of a lower solvent concentration.

The passage of solvent molecules across the semipermeable membrane from the solvent end to the solution end can be prevented by adding more pressure to the solution side. The osmotic pressure of the solution is that additional pressure that only prevents the solvent from flowing. A solution's osmotic pressure is directly proportional to its concentration.

Π= C R T

Where = Osmotic pressure

R = Universal gas constant


Π= (nBV)R T

Where V is the volume of the solution in L containing nB mol of solute.

Applications of Colligative Properties:

1. LPG cylinders (Relative lowering of vapour pressure):

Liquid petroleum gas is referred to as LPG. If there is not enough vapour pressure, an LPG cylinder cannot supply fuel to the burner. By turning the burner knob, the valve opens and the vapour pressure rises. The fuel vapours are then capable of moving in the burner's path. When the burner is turned to the opposite position, the valve also closes and lowers the vapour pressure. This causes a disruption in the fuel vapour movement.

2. Sugar solution (Relative lowering of vapour pressure):

In a cup of pure water, every molecule on the exposed surface is constituted of water. When water and a solute, such as sugar, are combined, some of the surface-bound sugar particles will be sugar. As a result, the water's exposed surface area is effectively decreased, rendering it a little less evaporation and lowering the vapour pressure.

3. Frozen ocean (Depression in freezing point):

Seawater's freezing point is lowered by the significant amount of dissolved salts present. The freezing point depression is the name of this phenomenon. Because of the existence of these dissolved salts and contaminants, seawater does not completely freeze.

4. Antifreeze in automobiles (Depression in freezing point):

An everyday characteristic is a depression in the freezing point. Most antifreeze used in the automotive sector has a lower freezing point than is typical, enabling car engines to run in below-freezing weather.

5. Cooking by using salt (Elevation in boiling point):

In our cooking, salt is the main component. In addition to giving our food a taste, it also enhances the boiling point of water. This is an illustration of an elevation in boiling point. The chemistry responsible for this is that salt, or sodium chloride, separates into sodium and chloride ions when it is added to water. The intermolecular interactions between water molecules are altered by these ions. Additionally, introducing any solute to water boosts its temperature due to the elevated boiling point even in the absence of a charged solute. The boiling point is a colligative property that depends on the quantity of particles created in the solution, therefore the more salt you add, the higher the boiling point will be.

6. Milk boiling (Elevation in boiling point):

Milk contains fat in the form of an emulsion, protein in a colloidal phase, and lactose as a real solution, water is a pure liquid that does not include any solids. When milk is boiled, proteins in the form of creams and fats, which are lighter than water, float to the top of the liquid. If milk is heated too much, however, water vapour expands, creating pressure that pulls the creamy layer upward and finally causes milk to flow out. Although milk and water have boiling points that are reasonably near to one another, milk has a somewhat higher boiling point due to the increase in the boiling point.

7. Killing snails and slugs (Osmosis):

You've actually heard that spraying salt on slugs and snails will kill them. They are simply killed by the osmosis process. They lose water because the liquid within them leaks out in an attempt to retain the mucus layer and reduce the salt concentration. If slugs and snails are subjected to too much salt, they will perish.

8. Wrinkled fingers (Osmosis and Osmotic pressure):

When we sat in the tub or submerged our fingers in water for an extended period of time, our fingertips wrinkle because of osmosis. Our fingers become pruned or wrinkled as a result of the skin on our fingertips absorbing water, expanding, and bloating.

Practice Problems:

Q1. Those solutions with the same osmotic pressures are referred to as

(A) Isotonic
(B) Hypertonic
(C) Hypotonic
(D) None of these

Answer: (A)

Solution: Isotonic solutions are those that have the same osmotic pressure. If a solution has a higher osmotic pressure than another solution, it is said to be hypertonic. If the osmotic pressure of one solution is lower than the osmotic pressure of another solution, the solution is said to be hypotonic.

Q2. Colligative properties completely depend on

(A) Pressure
(B) Volume
(C) Number of solute properties
(D) Nature of solute particles

Answer: (C)

Solution: The properties of a diluted solution that depend only on the quantity of solute particles present, not the nature of solute particles, are known as colligative properties.

Q3. If 40 g of solvent are used to dissolve 2 g of solute (whose molar mass is 40 g mol-1), and the elevation in boiling point is 2 K. The solvent's molal elevation constant is

Solution: ΔTb=  Kb m




Q4. Which of the following factors is observed when a non-volatile solute is added to a solvent?

(A) Rise in vapour pressure
(B) Decrease in osmotic pressure
(C) Rise in freezing point
(D) Lowering vapour pressure

Answer: (D)

Solution: The vapour pressure of a solvent decreases when a non-volatile solute is dissolved in it.The solvent's freezing point is lowered when a non-volatile solute is dissolved in it. The boiling point of the liquid rises when a non-volatile solute is dissolved in it. The osmotic pressure of the solvent rises when a non-volatile solute dissolves in it.

Frequently Asked Questions(FAQs):

Q1. Why is freezing exothermic?
Fusion, vapourization, and sublimation are examples of endothermic reactions, whereas freezing, condensation, and deposition are examples of exothermic processes. Water progressively loses heat to the surroundings when it is frozen. The water molecules start to move slowly as they start to lose energy and eventually gather closer together to produce ice. As a result of this procedure, heat is released into the environment by the water. Freezing is therefore an exothermic process.

Q2. Why can the water in large bodies of water not quickly evaporate?
Large bodies of water have low vapour pressure, which raises the surface tension of the water and prevents water molecules from binding and evaporating quickly, which is why the water in these bodies of water does not evaporate readily. In addition, water molecules found in large bodies of water like lakes, ponds, and seas have a tendency to evaporate and condense simultaneously and can contain a specific amount of water.

Q3. Why is vapour pressure lowering not a Colligative property?
Colligative properties of a solution are those that depend on the quantity of solute in a solution rather than the nature of the solute. Because the lowering of vapour pressure is determined by the nature of the solute rather than its quantity, it is not a colligative property. However, the relative lowering in vapour pressure is a colligative property because it is determined by the quantity of solute rather than its nature.

Q4. What occurs when a cell is placed into a hypertonic solution?
Water travels out of the cell when it is placed in a hypertonic solution (a solution that contains more solutes than the cell), causing the cell to shrink initially from the cytoplasm and subsequently from the vacuole. This is known as exosmosis, and it results in plasmolysis.

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