Colligative Properties
Colligative properties are physical properties of solutions that depend only on the number of solute particles, not on their type. These properties arise when solute particles affect how solvent molecules behave. They are especially important in dilute solutions.
🔎 Key Principle: Colligative properties depend on the concentration of solute particles, not their chemical identity.
Four Main Colligative Properties
- Relative Lowering of Vapor Pressure: A non-volatile solute reduces the vapor pressure of the solvent.
- Elevation of Boiling Point: The solution boils at a higher temperature than pure solvent.
- Depression of Freezing Point: The solution freezes at a lower temperature than the pure solvent.
- Osmotic Pressure: This is the pressure needed to stop solvent flow through a semipermeable membrane into a solution.
Why Are Colligative Properties Important?
- They help us understand how solutions behave.
- We can determine the molar mass of unknown solutes.
- They are useful in industry, medicine, and environmental science.
- They reveal whether solutes associate or dissociate in solution (explained by the Van’t Hoff factor).
Key Formulas and Relationships
All colligative properties are directly related to the number of solute particles. In most cases, the concentration is measured in molality or molarity.
- ΔP ∝ mole fraction of solute
- ΔTb = Kb × m
- ΔTf = Kf × m
- π = C × R × T (for dilute solutions)
What is the Van’t Hoff Factor (𝑖)?
Sometimes, solutes break into more particles (like salts), or combine into fewer (like some organic acids). This changes the number of particles in solution. The Van’t Hoff factor (𝑖) helps us adjust our calculations for this effect.
𝑖 = Actual number of particles / Expected number of particles
It is important for calculating molar mass and explaining unusual results in experiments.
Real-Life Applications of Colligative Properties
- Antifreeze in cars lowers freezing point and prevents engine damage.
- Salt and sugar in food preservation kill bacteria through osmosis.
- Lab experiments use these properties to find molar masses.
- IV fluids are made isotonic to match blood osmotic pressure.
Conclusion: Colligative properties are a key concept in physical chemistry. They explain everyday phenomena and help us solve real-world problems—from preserving food to making safe medicines and clean water.
Colligative Properties – MCQ Quiz
Instructions: Choose the correct answer for each multiple-choice question below. Answers and explanations are given below each question.
1. Which of the following is not a colligative property?
- A. Boiling point elevation
- B. Freezing point depression
- C. Vapor pressure lowering
- D. Enthalpy of fusion
Answer: D. Enthalpy of fusion
Explanation: Enthalpy of fusion is a thermodynamic property, not a colligative one. Colligative properties depend only on the number of solute particles, not their nature.
2. Which unit is typically used for molality?
- A. mol/L
- B. g/L
- C. mol/kg
- D. mol/m³
Answer: C. mol/kg
Explanation: Molality is defined as moles of solute per kilogram of solvent. It is independent of temperature, making it ideal for colligative property calculations.
3. Which colligative property explains why salt is added to icy roads in winter?
- A. Boiling point elevation
- B. Osmotic pressure
- C. Freezing point depression
- D. Vapor pressure lowering
Answer: C. Freezing point depression
Explanation: Salt lowers the freezing point of water, preventing ice formation on roads even when temperatures drop below 0°C.
4. What is the effect on vapor pressure when a non-volatile solute is added to a solvent?
- A. Increases
- B. Remains unchanged
- C. Decreases
- D. Becomes zero
Answer: C. Decreases
Explanation: The addition of a non-volatile solute reduces the number of solvent molecules at the surface, thereby lowering the vapor pressure.
5. Which formula is used to calculate the elevation in boiling point?
- A. ΔTb = i × Kb × m
- B. ΔTf = i × Kf × m
- C. π = iMRT
- D. P₁ = X₁P₁⁰
Answer: A. ΔTb = i × Kb × m
Explanation: This is the correct equation for boiling point elevation. Here, i is the van’t Hoff factor, Kb is the ebullioscopic constant, and m is molality.
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