Colligative properties of solutions

Colligative properties are the properties that originate in solutions upon addition of solutes. These are the physical changes dependent on the quantity of both solute and solvent constituting the understudy solution.

Colligative properties are dependent on the number of solute molecules, although they are independent of their quality and identification. This assumption is an ideal one, as no system can be entirely independent of the chemical nature of its species. In other words, colligative properties are the set of properties perfectly expressed in dilute ideal solutions.

The word colligative comes from Latin ‘colligatus’ meaning bound together. It means that all colligative properties are actually bound together or they are the different ways of expressing the same change i.e. change in properties of solvent due to solute molecules.

Prerequisites
Ideal and real solutions

There are four properties known as colligative properties.

  1. Relative lowering of vapor pressure
  2. Elevation in boiling point
  3. Depression in freezing point
  4. Osmotic pressure

1. The Relative lowering of vapor pressure

Liquids can be changed into the gaseous phase when their vapor pressure equals the exerted (atmospheric) pressure. Vapor pressure depends upon factors like temperature, pressure, intermolecular forces, etc. The temperature being in direct relation with vapor pressure increases with an increase in temperature. Pressure and intermolecular forces on the other hand are inversely proportional to vapor pressure.

In a closed system, solute molecules cover the intermolecular spaces between solvent molecules, increasing the network of intermolecular bonding. This leaves solvent molecules with no option other than staying in. In this way, the solute added to the solvent lowers its vapor pressure.

Colligative properties of solution: Relative lowering of vapor pressure

Raoult’s law

To calculate, how much the solute affects the vapor pressure of the solution (solvent),  we imply Raoult’s law.

Psolution = X solvent . Posolvent

where,

  • Psolution = Vapor pressure of the solution
  • X solvent = Mole fraction of solvent
  • Posolvent = Vapor pressure of the pure solvent

Limitations of Raoult’s law

Raoult’s law is only applicable on

  • Dilute or low concentration solutions
  • Solutions of non-volatile solutes

2. Elevation in boiling point

The boiling point is the temperature at which the vapor pressure of the solution becomes equal to the atmospheric (external) pressure. A non-volatile solute when added to a pure solvent, decrease the evaporation rate of the solution leading to low overall vapor pressure. In this state, the boiling point shifts to a higher range, and an increase in temperature is required which could result in increased vapor pressure to reach boiling point.

In this way, a phenomenon of elevation in boiling point takes place.

Colligative properties of solution: Elevation of boiling point

Tob is the boiling point of pure solvent while Tb is the boiling point of (non-volatile solute + solvent) solution. The difference between these temperatures can be calculated as:

ΔTb = Tb – Tob

3. Depression in freezing point

Freezing point is defined as the temperature at which the vapor pressure of the liquid state solvent becomes equal to the vapor pressure of the solid state solvent. By the addition of a non-volatile solute, depression is observed in the vapor pressure of the solvent. It is because its intermolecular forces become stronger.

An orderly arrangement of molecules is required for a substance to exist in solid state. When a nonvolatile solute is added to a solvent, its network of intermolecular forces increases, and as a result, the solid state becomes more favorable at a lower temperature than the original freezing points.

Colligative properties of solution: Depression in freezing point

Tf is the freezing point of the solution and Tof is the freezing point of the pure solvent. Delta Tf can be found out by:

ΔTf = Tof – Tf

4. Osmotic pressure

Osmosis is the net movement of solvent particles from a lower region of solution (higher region of solvent) to a higher region of solution (lower region of solvent) through a semi-permeable membrane. Solvent molecules pass through the semi-permeable membrane due to their smaller size as compared to solute molecules.

Colligative properties of solution: Osmosis

Osmotic pressure is the extra pressure that is applied to the solution side which exactly stops the movement of solvent molecules coming from the higher region of solvent to the lower region of solvent. Molecules stop their movement when they are in equilibrium.

Colligative properties of solution: Osmotic pressure

Osmotic pressure is represented by the symbol pi (π). Its formula is represented as:

π = i.c.R.T

where,

π = osmotic pressure

i = Dimensionless Vont Hoff’s index

c = molar concentration of solute

R = Ideal gas constant

T = Absolute temperature (usually in Kelvins)

Applications of Colligative properties

  1. Colligative properties can be used to determine the molecular weight of the compounds. The dissolved solute can be evaluated by any of the colligative properties and the change is used to calculate the molecular weight of the solute.
  2. Osmotic pressure helps to retain plant posture. When water is absorbed by the plants due to the concentration gradient across the cell membrane, the cells expand. Due to this expansion, there is a pressure exerted on the cell wall of the plant which helps to stand in an upright position.
  3. Osmotic pressure is used for the desalination and purification of seawater.
  4. Dehydration of meat cells via osmotic pressure. This is done to extend meat life even without a refrigerator.
  5. Salt is put on streets to melt ice during winters. This increases the osmotic pressure resulting in the melting of ice.
  6. Making ice cream by achieving lower temperature than the freezing point of water by dissolving salt.
  7. Cooking time is significantly decreased by the use of salt since it increases the osmotic pressure.

Concepts Berg

Why is molality used in colligative properties?

Molality is always used as a concentration unit in colligative properties because the mass of the solvent does not change while adding solute to the solution. Molarity cannot be used due to a change in the volume of the solution when more solute is added.

What are the other colligative properties of the solution?

There are four types of colligative properties of the solutions.

  1. Relative lowering of vapor pressure
  2. Boiling point elevation
  3. Freezing point depression
  4. Osmotic pressure

Are colligative properties physical or chemical?

Colligative properties depend on the number of solutes in a solution. They do not depend on the nature of solute meaning they are physical properties.

What are colligative properties?

Colligative properties are the properties that depend on the concentration of solute or the number of particles of solute in a solution. The identity of solute does not affect the colligative properties. Examples are osmotic pressure, freezing point depression, boiling point elevation, and lowering of vapor pressure, etc.

How do colligative properties work?

When particles of solute are added to the solution, the solute molecules affect create colligative properties by making strong intermolecular interactions with solvent molecules. As a result, a decrease in vapor pressure, depression in freezing point, elevation in boiling point, and osmotic pressure, etc occur.

Is osmotic pressure a colligative property?

Osmotic pressure is a colligative property because it depends on the number of solute molecules and not on their nature. It is just like freezing point depression, boiling point elevation, etc.

Is lowering of vapor pressure a colligative property?

Yes, lowering of vapor pressure is a colligative property because when solute molecules make strong interactions with solvent molecules, the solvent molecules feel resistance in escaping out from the surface of the solution, That’s why the vapor pressure gets lowered.

Can colligative properties be considered extensive properties?

Extensive properties depend on the amount of matter (mass and volume) while intensive properties depend on the nature of matter. Colligative properties also depend on the numbers of solutes in the solution so they are considered extensive properties.

What are some applications of elevation in boiling point?

The property of elevation in boiling points of solvents is used in several applications like determination of the molecular weight of solute, refining of sugar, synthesis of antifreeze chemicals,  and cooking, etc.

Why osmotic pressure is the best colligative property?

As compared to other colligative properties, osmotic pressure is the most useful one because it has the advantage of dealing with temperature. It is studied at room temperature while all others are either elevated or reduced temperatures. Moreover, it has a large magnitude even for dilute solutions so it is more observable. Osmotic pressure also uses molarity as a unit of concentration instead of morality so it is far more useful than other colligative properties.

What actually makes colligative properties independent of the type of solute?

The fact that solute molecules are somewhat similar to solvent molecules and it makes them able to fit inside hollow intermolecular spaces of solvent molecules. The solute molecules stay put due to the intermolecular forces which have nothing to do with the nature or type of the solute as almost all of them can have these forces.

Reference links