1. Introduction to Solutions

A solution is a homogeneous mixture of two or more substances. It consists of:

• Solute: The substance present in a lesser quantity.
• Solvent: The substance present in a larger quantity.

Types of Solutions

Concept Check

Q1: Define the term “solution” and differentiate between solute and solvent with examples.
Q2: Classify the following solutions as solid-liquid, liquid-liquid, or gas-liquid:
(a) Vinegar (b) Air dissolved in water (c) Brass

2. Concentration of Solutions

Concentration expresses the amount of solute in a given quantity of solvent or solution.

Methods of Expressing Concentration

1. Mass Percentage (% w/w) = (Mass of solute / Mass of solution) × 100
2. Volume Percentage (% v/v) = (Volume of solute / Volume of solution) × 100
3. Molarity (M) = Moles of solute / Volume of solution (L)
4. Molality (m) = Moles of solute / Mass of solvent (kg)
5. Mole Fraction (χ) = Moles of component / Total moles of all components

Concept Check

Q3: A solution is prepared by dissolving 5g of NaCl in 95g of water. Calculate the mass percentage of NaCl.
Q4: What is the molarity of a solution containing 0.5 moles of solute in 500 mL of solution?

3. Solubility and Factors Affecting It

• Solubility: The maximum amount of solute that dissolves in a given solvent at a particular temperature.
• Factors Affecting Solubility:
  • Nature of solute & solvent: “Like dissolves like” (Polar dissolves polar, non-polar dissolves non-polar).
  • Temperature:
    • Solubility of solids in liquids increases with temperature.
    • Solubility of gases in liquids decreases with temperature.
  • Pressure (Henry’s Law): Solubility of gases in liquids increases with pressure.

Concept Check

Q5: Explain why oxygen is more soluble in cold water than in hot water.
Q6: What happens to the solubility of a gas in a liquid when pressure is increased?

4. Henry’s Law (Solubility of Gases in Liquids)

Statement: The solubility of a gas in a liquid is directly proportional to the pressure of the gas above the solution.C=kPC = kPC=kP

Where:

• C = Solubility of gas
• k = Henry’s constant
• P = Pressure of the gas

Applications of Henry’s Law

• Used in carbonated drinks (CO₂ dissolved under high pressure).
• Important in scuba diving (avoiding decompression sickness).

Concept Check

Q7: A soft drink contains CO₂ at a pressure of 3 atm. If the pressure is reduced to 1 atm, what happens to the solubility of CO₂?
Q8: Why do deep-sea divers use a mixture of oxygen and helium instead of pure oxygen?

5. Raoult’s Law (Vapour Pressure of Solutions)

For an ideal solution, the vapour pressure of each component is proportional to its mole fraction.P1=X1P1°P_1 = X_1 P_1°P1​=X1​P1​°

For a two-component solution:Ptotal=P1+P2P_{\text{total}} = P_1 + P_2Ptotal​=P1​+P2​

• Positive Deviation: Higher vapour pressure than expected (e.g., ethanol + acetone).
• Negative Deviation: Lower vapour pressure than expected (e.g., water + nitric acid).

Concept Check

Q9: Define Raoult’s Law. How does it apply to ideal and non-ideal solutions?
Q10: Give an example of a solution showing negative deviation from Raoult’s Law.

6. Colligative Properties

Properties that depend only on the number of solute particles, not their identity.

Types of Colligative Properties

1. Relative Lowering of Vapour PressureP1°−P1P1°=X2\frac{P_1° – P_1}{P_1°} = X_2P1​°P1​°−P1​​=X2​Used to calculate molar mass of solute.
2. Elevation in Boiling Point (ΔTb)ΔTb=Kb×mΔT_b = K_b \times mΔTb​=Kb​×m(Boiling point increases due to solute).
3. Depression in Freezing Point (ΔTf)ΔTf=Kf×mΔT_f = K_f \times mΔTf​=Kf​×m(Freezing point decreases due to solute).
4. Osmotic Pressure (π)π=MRTπ = MRTπ=MRT(Used for determining molar mass of macromolecules).

Concept Check

Q11: Why does adding salt to water lower its freezing point?
Q12: Define osmotic pressure and its application in biological systems.

7. Van’t Hoff Factor (i) and Abnormal Molar Mass

• Some solutes undergo association or dissociation, leading to deviations in molar mass calculations.
• Van’t Hoff Factor (i) corrects these deviations:

i=Observed molar massCalculated molar massi = \frac{\text{Observed molar mass}}{\text{Calculated molar mass}}i=Calculated molar massObserved molar mass​

Examples:

• NaCl dissociates into Na⁺ and Cl⁻, so i = 2.
• Acetic acid dimerizes in benzene, so i < 1.

Concept Check

Q13: What is the Van’t Hoff factor for K₂SO₄ in water?
Q14: Why is the molar mass of acetic acid in benzene found to be twice its actual value?

8. Important Applications of Solutions

• Osmosis in Biological Systems: RBCs in saline solutions.
• Antifreeze in Car Radiators: Lowers the freezing point.
• Reverse Osmosis (RO): Water purification.

Concept Check

Q15: Why do doctors use saline solutions instead of pure water for IV fluids?
Q16: Explain the principle of Reverse Osmosis (RO) and its application in water purification.