Last Updated: April 2026
NEET Chemistry Solutions and Colligative Properties (Class 12, Chapter 2) consistently delivers 2-3 questions per exam. The chapter tests conceptual clarity on solution composition, Raoult’s law deviations, and colligative properties — all calculable with the right formula mastery.
Types of Solutions — Classification
| Solute State | Solvent State | Example |
|---|---|---|
| Gas | Liquid | CO₂ in water (aerated drinks) |
| Liquid | Liquid | Ethanol in water |
| Solid | Liquid | Salt in water |
| Gas | Gas | Air (O₂ + N₂) |
| Solid | Solid | Alloys (brass, bronze) |
Concentration Terms — Formulas
| Measure | Formula | Units | Temperature Dependent? |
|---|---|---|---|
| Molarity (M) | moles of solute / volume of solution (L) | mol/L | Yes |
| Molality (m) | moles of solute / mass of solvent (kg) | mol/kg | No |
| Mole Fraction (χ) | moles of component / total moles | Dimensionless | No |
| Mass % | (mass of solute / mass of solution) × 100 | % | No |
| ppm | (mass of solute / mass of solution) × 10⁶ | mg/kg | No |
Raoult’s Law
For a solution of volatile components: p₁ = p₁° × χ₁
where p₁ = vapour pressure of solvent in solution, p₁° = vapour pressure of pure solvent, χ₁ = mole fraction of solvent.
Deviations from Raoult’s Law
| Type | Condition | Examples | Interaction |
|---|---|---|---|
| Positive Deviation | p > Raoult value; ΔH_mix > 0 | Ethanol-acetone, Benzene-cyclohexane | A-B weaker than A-A or B-B |
| Negative Deviation | p < Raoult value; ΔH_mix < 0 | Chloroform-acetone, Nitric acid-water | A-B stronger than A-A or B-B |
| Ideal Solution | Follows Raoult’s law; ΔH_mix = 0 | Benzene-toluene, hexane-heptane | A-B = A-A = B-B |
Colligative Properties — All Four
1. Relative Lowering of Vapour Pressure (RLVP)
Formula: (p° – p) / p° = χ₂ (mole fraction of solute)
2. Elevation in Boiling Point (ΔTb)
Formula: ΔTb = Kb × m
where Kb = molal elevation constant (ebullioscopic constant), m = molality
Kb for water = 0.52 K·kg/mol
3. Depression in Freezing Point (ΔTf)
Formula: ΔTf = Kf × m
Kf for water = 1.86 K·kg/mol (most common NEET value)
Application: Antifreeze solutions use this principle
4. Osmotic Pressure (π)
Formula: π = iCRT = i(n/V)RT
where i = Van’t Hoff factor, C = molarity, R = 8.314 J/mol·K, T = temperature in Kelvin
Van’t Hoff Factor (i)
| Solute Type | Van’t Hoff Factor (i) | Example |
|---|---|---|
| Non-electrolyte | i = 1 | Glucose, Urea, Sugar |
| Strong electrolyte (1:1) | i = 2 | NaCl, KCl, HCl |
| Strong electrolyte (1:2) | i = 3 | CaCl₂, Na₂SO₄ |
| Strong electrolyte (1:3) | i = 4 | AlCl₃, FeCl₃ |
Henry’s Law
The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid: p = K_H × χ
Applications: Deep-sea divers, aerated drinks, scuba diving tanks
Henry’s law constant (K_H) increases with temperature → gases are less soluble in hot liquids
Isotonic, Hypotonic, Hypertonic Solutions
- Isotonic: Same osmotic pressure — no net movement across semi-permeable membrane
- Hypotonic: Lower osmotic pressure than cell → water enters cell → cell swells
- Hypertonic: Higher osmotic pressure → water leaves cell → cell shrinks (plasmolysis in plants)
FAQ
Why is molality preferred over molarity for colligative properties?
Molality is temperature-independent because it is based on mass of solvent (not volume of solution). Since colligative properties like boiling point elevation and freezing point depression are measured at different temperatures, molality gives consistent results. Molarity changes with temperature as volume changes.
What is abnormal molecular mass in colligative properties?
When a solute associates or dissociates in solution, the observed colligative property differs from the calculated value. Dissociation (like NaCl → Na⁺ + Cl⁻) increases particle count → i > 1. Association (like acetic acid forming dimers) decreases particle count → i < 1. Van't Hoff factor corrects for this.
Practice MCQs
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