Solutions Class 12 Notes (2026-27) — CBSE
Class 12 Chemistry Chapter 1 notes: concentration terms, Henry's law, Raoult's law, ideal and non-ideal solutions, colligative properties and van't Hoff factor.
Solutions — Class 12 Chemistry Notes
Chapter Snapshot
A solution is a homogeneous mixture of two or more substances. This chapter covers how to express concentration, the solubility of gases (Henry's law), the vapour pressure of liquid mixtures (Raoult's law) with ideal/non-ideal behaviour and azeotropes, and the four colligative properties used to find molar masses — corrected by the van't Hoff factor.
Board relevance: ~7 marks. Expect a colligative-property numerical (usually ΔTf or π) and a Raoult's/Henry's law question. Molality — not molarity — appears in the colligative formulas.
Key Concepts & Definitions
Concentration terms:
Term Formula Note
Molarity (M) moles of solute / litre of solution Depends on temperature
Molality (m) moles of solute / kg of solvent Independent of temperature
Mole fraction (x) moles of component / total moles Sum of all x = 1
Mass percentage (mass of solute/mass of solution) × 100 —
ppm (mass of solute/mass of solution) × 10⁶ For very dilute solutions
Molality is preferred in colligative work precisely because it does not change with temperature.
Henry's law — the solubility of a gas in a liquid at a given temperature is proportional to the pressure:
p = KH · x
- A higher KH means lower solubility.
- KH increases with temperature, so gases become less soluble on heating (why warm soda goes flat and why aquatic life suffers in warm water).
- Applications: soda bottles are sealed under high pressure; deep-sea divers get bends from dissolved N₂; climbers suffer anoxia at altitude.
Raoult's Law and Types of Solutions
Raoult's law (two volatile liquids): each component's partial vapour pressure is proportional to its mole fraction:
pA = pA° xA, pB = pB° xB, ptotal = pA° xA + pB° xB
For a non-volatile solute in a solvent, only the solvent contributes:
p = pA° xA , and the relative lowering of vapour pressure = (pA° − p)/pA° = xsolute
Ideal vs non-ideal solutions:
Type Raoult's law ΔHmix, ΔVmix Example
Ideal Obeyed at all concentrations Both zero Benzene + toluene; n-hexane + n-heptane
Positive deviation p higher than predicted ΔH 0, ΔV 0 (weaker A–B forces) Ethanol + water; ethanol + acetone
Negative deviation p lower than predicted ΔH < 0, ΔV < 0 (stronger A–B forces) Chloroform + acetone; HNO₃ + water
Azeotropes — constant-boiling mixtures that distil unchanged:
- Minimum-boiling azeotrope ← positive deviation (e.g. ethanol–water, 95%).
- Maximum-boiling azeotrope ← negative deviation (e.g. nitric acid–water).
Formulas — Colligative Properties
Colligative properties depend only on the number of solute particles, not their identity.
Property Formula Constant
Relative lowering of vapour pressure (p° − p)/p° = x₂ —
Elevation of boiling point ΔTb = Kb · m Kb = molal elevation constant (ebullioscopic)
Depression of freezing point ΔTf = Kf · m Kf = molal depression constant (cryoscopic)
Osmotic pressure π = CRT = (n/V)RT R = 0.0821 L·atm/mol·K
Molar mass determination: M₂ = (Kb × w₂ × 1000)/(ΔTb × w₁), and similarly with Kf. Osmotic pressure is the most accurate method (measurable at room temperature and gives large, easily measured values, useful for polymers and proteins).
Osmosis terms: osmosis is solvent flow through a semipermeable membrane; reverse osmosis (applying pressure π) is used to desalinate sea water. Solutions with equal π are isotonic; higher is hypertonic, lower hypotonic.
van't Hoff factor (i):
i = observed colligative property / calculated value = normal molar mass / observed molar mass
- i 1 → dissociation (NaCl → i ≈ 2, K₂SO₄ → i ≈ 3).
- i < 1 → association (acetic acid dimerises in benzene, i ≈ 0.5).
- Corrected formulas: ΔTb = i·Kb·m, ΔTf = i·Kf·m, π = i·CRT.
Worked Examples
Example 1 — Molality: Find the molality of a solution with 18 g glucose (M = 180) in 500 g water.
moles = 18/180 = 0.1; molality = 0.1/0.5 kg = 0.2 m.
Example 2 — Freezing point depression: Find ΔTf for 0.2 m aqueous solution (Kf = 1.86 K·kg/mol) of a non-electrolyte.
ΔTf = Kf·m = 1.86 × 0.2 = 0.372 K, so the solution freezes at −0.372 °C.
Example 3 — With van't Hoff factor: Repeat for 0.2 m NaCl (i = 2).
ΔTf = i·Kf·m = 2 × 1.86 × 0.2 = 0.744 K — twice as much, since NaCl gives two ions.
Example 4 — Osmotic pressure: Find π for 0.1 M solution at 300 K.
π = CRT = 0.1 × 0.0821 × 300 = 2.46 atm.
Example 5 — Molar mass from depression: 1.0 g of a non-electrolyte dissolved in 50 g benzene lowers the freezing point by 0.40 K (Kf = 5.12 K·kg/mol). Find its molar mass.
molality m = ΔTf/Kf = 0.40/5.12 = 0.078 mol/kg. Moles of solute = 0.078 × 0.05 kg = 3.9 × 10⁻³.
M = 1.0/3.9 × 10⁻³ ≈ 256 g/mol. This is the standard "find the molar mass" pattern — get the molality from ΔT, convert to moles using the solvent mass, then divide the solute mass by the moles.
Important Question Patterns
1. Concentration (2 marks): convert between molarity, molality and mole fraction; why molality is temperature-independent.
2. Henry's law (2–3 marks): p = KH·x; effect of temperature and pressure; explain bends, soda bottles, anoxia.
3. Raoult's law & deviations (3 marks): compute total vapour pressure; classify positive/negative deviation with reasons and examples; link to azeotropes.
4. Colligative numerical (3 marks): ΔTb, ΔTf or π; find the molar mass of an unknown solute.
5. van't Hoff factor (2–3 marks): compute i; explain abnormal molar mass from dissociation or association.
⚡ Quick Revision
- Molarity = mol/L solution (temperature-dependent); molality = mol/kg solvent (temperature-independent, used in colligative formulas).
- Henry: p = KH·x; higher KH → lower solubility; gases less soluble on heating (bends, soda, anoxia).
- Raoult: p = pA°xA + pB°xB; non-volatile solute → (p° − p)/p° = xsolute.
- Ideal: ΔH = ΔV = 0 (benzene+toluene). Positive deviation → minimum-boiling azeotrope (ethanol+water). Negative deviation → maximum-boiling azeotrope (chloroform+acetone, HNO₃+water).
- Colligative: ΔTb = Kb·m ; ΔTf = Kf·m ; π = CRT. Osmotic pressure is the most accurate for large molecules.
- Reverse osmosis desalinates sea water. Isotonic = equal π.
- van't Hoff i: 1 dissociation (NaCl ≈ 2), < 1 association (acetic acid in benzene ≈ 0.5). Use i·Kf·m etc.
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