COLLIGATIVE PROPERTIES
INTRODUCTION
“Have you ever wondered why adding salt to water raises its boiling point, or why antifreeze prevents car engines from freezing in winter?”These everyday phenomena are explained by an interesting concept in chemistry called colligative properties.
Colligative properties are unique because they depend not on what substance is added to a solution, but on how many particles are present. Whether it’s sugar, salt, or any other solute, the effect on the solution is determined purely by the number of dissolved particles.
In this article, we’ll explore what colligative properties are, their types, and how they play an important role in both chemistry and real-life applications.
What are Colligative properties?
Colligative properties are a set of properties of solutions that depend only on the number of solute particles present, not on the nature (identity) of the solute.
Definition
Colligative properties are those properties of a solution that depend on the ratio of the number of solute particles to solvent particles, irrespective of what the solute actually is.
Key ideas
If we dissolve different substances (like sugar or salt) in the same amount of solvent, as long as the number of particles is the same, the colligative effect will be the same.
Conceptual Question
Why do colligative properties depend only on number of particles and not nature?
Answer
Because these properties arise from dilution of solvent molecules, not chemical identity. They depend only on particle concentration, not type of solute.
List of Colligative properties
There are four main colligative properties:
1. Relative Lowering of Vapour Pressure
When a non-volatile solute is added to a solvent, the vapour pressure of the solvent decreases. This happens because solute particles occupy space at the surface, reducing solvent molecules escaping into vapor.
Explanation:
Fewer solvent molecules are available to evaporate → vapour pressure drops.
Conceptual numerical
A solution is prepared by dissolving 1 mole of a non-volatile solute in 4 moles of solvent. Calculate the relative lowering of vapour pressure.
Solution
mole fraction of solute,
xₛₒₗᵤₜₑ= 1/(1+4)= 1/5 = 0.2
Relative lowering = 0.2 (20%)
2. Elevation of Boiling Point
The boiling point of a solution is higher than that of the pure solvent. Since vapour pressure is lowered, more heat is needed to reach boiling.
Explanation:
Lower vapour pressure → higher temperature needed to boil → boiling point rises.
Conceptual Question
Calculate the boiling point of a solution containing 0.5 mol solute in 1 kg water. Given:Kb= 0.52 Kkg/mol
Solution
∆T_b = K_b × m
= 0.52 × 0.5
=0.260
Normal boiling point of water = 100°C
New boiling point = 100 + 0.26 = 100.26°C
3. Depression of Freezing Point
The freezing point of a solution is lower than that of the pure solvent. Solute particles interfere with the formation of the solid structure.
Explanation:
Solute prevents orderly crystal formation → freezing occurs at lower temperature.
Conceptual Question
What will be the freezing point of a solution containing 1 mol glucose in 1 kg water? Given: Kf = 1.86 K kg/mol.
Solution
∆Tf = Kf × m
= 1.86 × 1
= 1.86
Normal freezing point of water = 0°C
New freezing point = 0°C - 1.86= -1.86°C
4. Osmotic Pressure
Osmotic pressure is the pressure required to stop the flow of solvent through a semipermeable membrane into the solution.
Explanation:
Solvent naturally moves from dilute to concentrated solution → pressure is needed to stop this flow.
OSMOSIS
REVERSE OSMOSIS
Conceptual Question
Calculate osmotic pressure at 300 K for a solution of concentration 0.2 mol/L. (R = 0.0821 L atm/mol K)
Solution
Summary Table
| s.no. | Properties | Effect |
|---|---|---|
| i. | Vapour Pressure | Decreases |
| ii. | Boiling Point | Increases |
| iii. | Freezing Point | Decreases |
| iv. | Osmotic Pressure | Increases |
Important Notes
These properties depend on
- number of particles, so electrolytes (like NaCl) show greater effects because they dissociate into ions.
- Governed by laws like Raoult’s Law and concepts from Physical Chemistry.
50 MCQs
Concept-Based MCQs (1–15)
- Colligative properties depend on:
- Which is NOT a colligative property?
- Vapour pressure of a solution is:
- Which law explains vapour pressure lowering?
- Boiling point elevation occurs because:
- Freezing point depression occurs due to:
- Osmotic pressure depends on:
- Unit of osmotic pressure is:
- Which solute gives maximum colligative effect?
- van’t Hoff factor (i) for glucose is:
- van’t Hoff factor for NaCl (ideal):
- Colligative properties are important to determine:
- Molality depends on:
- Which property is used in reverse osmosis?
- Which is most affected by electrolytes?
- Mole fraction of solute when 1 mol solute + 9 mol solvent = ?
- If molality doubles, ΔTb becomes:
- ΔTf is proportional to:
- If Kb = 0.5 and m = 2, ΔTb = ?
- If i = 2, Kf = 1, m = 1 → ΔTf = ?
- Osmotic pressure increases with:
- If concentration doubles, osmotic pressure:
- ΔTb for electrolyte is:
- Relative lowering of vapour pressure =
- Freezing point of solution is:
- If solute is volatile, vapour pressure:
- Osmotic pressure formula:
- Unit of Kb:
- If m = 0, ΔTf =
- If i increases, ΔTb:
- If solvent amount increases, molality:
- If solute particles double, effect:
- ΔTf depends on:
- Osmotic pressure at 0 concentration:
- If T increases, π:
- A: Vapour pressure decreases in solution
R: Solute blocks solvent escape - A: Boiling point increases
R: Vapour pressure decreases - A: Freezing point decreases
R: Solute aids crystal formation - A: Osmotic pressure depends on concentration
R: More particles increase pressure - A: Electrolytes show greater effect
R: They dissociate - A: Glucose shows i=2
R: It dissociates - A: Molality independent of temperature R: Based on mass
- A: Vapour pressure increases
R: Solute is non-volatile - A: π = CRT
R: Derived from gas law - A: Colligative properties depend on nature
R: They depend on number - Which has highest ΔTf?
- Which shows least vapour pressure?
- Reverse osmosis uses:
- Which property helps find molar mass?
- Ideal solution obeys:
A) Nature of solute
B) Number of solute particles
C) Type of solvent
D) Temperature only
A) Osmotic pressure
B) Boiling point
C) Vapour pressure lowering
D) Surface tension
A) Higher than solvent
B) Equal to solvent
C) Lower than solvent
D) Unpredictable
A) Boyle’s law
B) Raoult’s law
C) Charles law
D) Henry’s law
A) Vapour pressure increases
B) Vapour pressure decreases
C) Density increases
D) Volume increases
A) Increased intermolecular forces
B) Disruption of crystal formation
C) Increase in pressure
D) Increase in solubility
A) Temperature only
B) Volume only
C) Concentration
D) Density
A) Kelvin
B) atm
C) mol
D) kg
A) Glucose
B) Urea
C) NaCl
D) Ethanol
A) 0
B) 1
C) 2
D) 3
A) 1
B) 2
C) 3
D) 0
A) Colour
B) Molecular mass
C) Density
D) Shape
A) Volume
B) Temperature
C) Mass
D) Pressure
A) Vapour pressure
B) Osmotic pressure
C) Freezing point
D) Boiling point
A) Colour
B) Density
C) Colligative properties
D) Viscosity
Numerical-Based MCQs (16–35)
A) 0.1
B) 0.9
C) 1
D) 0.5
A) Half
B) Double
C) Same
D) Zero
A) Molarity
B) Molality
C) Density
D) Pressure
A) 1
B) 2
C) 0.5
D) 4
A) 1
B) 2
C) 3
D) 4
A) Decrease in T
B) Increase in T
C) Decrease in concentration
D) Increase in volume
A) Halves
B) Doubles
C) Same
D) Zero
A) Kb m
B) iKb m
C) Km
D) i/m
A) Mole fraction of solvent
B) Mole fraction of solute
C) Mass fraction
D) Volume fraction
A) Higher
B) Lower
C) Same
D) Zero
A) Always decreases
B) Always increases
C) May increase or decrease
D) Zero
A) PV=nRT
B) π=CRT
C) V=nRT
D) P=RT
A) K kg mol⁻¹
B) mol kg⁻¹
C) atm
D) K
A) 1
B) 0
C) Infinite
D) Negative
A) Decreases
B) Increases
C) Same
D) Zero
A) Increases
B) Decreases
C) Same
D) Zero
A) Half
B) Double
C) Same
D) Zero
A) Nature of solute
B) Number of particles
C) Colour
D) Shape
A) Zero
B) Infinite
C) High
D) Negative
A) Decreases
B) Increases
C) Same
D) Zero
Assertion–Reason (36–45)
A) Both true, R correct
B) Both true, R wrong
C) A true, R false
D) A false
Mixed & Tricky (46–50)
A) 1M glucose
B) 1M NaCl
C) 1M urea
D) 1M sucrose
A) Pure solvent
B) Solution
C) Dilute solution
D) Concentrated solution
A) High pressure
B) Low pressure
C) Heat
D) Vacuum
A) Density
B) Colligative property
C) Colour
D) Boiling point only
A) Dalton law
B) Raoult law
C) Boyle law
D) Charles law