Solutions

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Solutions — Quick Facts

• Solution = solute (dissolved) + solvent (dissolves); e.g., salt in water
• Key concentration terms: Molarity (M = mol/L), Molality (m = mol/kg solvent), Mole fraction (χ), ppm
• Raoult’s Law: P(solution) = χ(solvent) × P°(solvent) — vapour pressure lowering
• Non-volatile solute lowers vapour pressure → boiling point elevation and freezing point depression

⚡ Exam tip: NEET frequently tests the relationship between osmotic pressure (π), molarity (M), and temperature. Remember π = nRT/V or π = CRT where R = 0.0821 L·atm/mol·K. This is the same formula as PV = nRT but used for osmosis.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Solutions — Study Guide

Overview: Solutions is a high-weightage Physical Chemistry topic in NEET, covering solution chemistry, colligative properties, and concentration terms. The importance of this topic lies in its direct applicability — colligative properties (boiling point elevation, freezing point depression, osmotic pressure) are tested almost every year and have strong conceptual overlap with osmosis in Biology. The mathematics is straightforward if you remember the formulas.

Key concepts:

Types of Solutions:

• Gaseous solutions: Gas in gas (e.g., air: O₂ in N₂), Gas in liquid (e.g., CO₂ in H₂O = soda water), Solid in gas (e.g., smoke = carbon particles in air)
• Liquid solutions: Gas in liquid, Liquid in liquid, Solid in liquid (most common in NEET)
• Solid solutions: Solid in solid (e.g., alloys like brass = Zn in Cu)

Concentration Terms (must know all):

Term	Definition	Unit
Molarity (M)	Moles of solute / Volume of solution (L)	mol/L
Molality (m)	Moles of solute / Mass of solvent (kg)	mol/kg
Mole fraction (χ)	Moles of component / Total moles	dimensionless
Normality (N)	Gram equivalents / Volume (L)	N
Mass %	(Mass of solute / Mass of solution) × 100	%
ppm	mg of solute / kg of solution	mg/kg

Raoult’s Law (for ideal solutions of non-volatile solutes):

• P(solution) = χ(solvent) × P°(solvent)
• P(solution) = P°(solvent) - χ(solute) × P°(solvent) [vapour pressure lowering]
• When a non-volatile solute is added, vapour pressure of solvent decreases proportionally to mole fraction of solute

Colligative Properties (key for NEET):

1. Boiling Point Elevation: ΔTb = Kb × m × i

   • Non-volatile solute raises boiling point
   • Kb = Ebullioscopic constant of solvent (water: Kb = 0.52°C kg/mol)

2. Freezing Point Depression: ΔTf = Kf × m × i

   • Solute lowers freezing point
   • Kf = Cryoscopic constant (water: Kf = 1.86°C kg/mol)
   • Application: Adding salt to ice → lowers freezing point → used for de-icing

3. Osmotic Pressure: π = nRT/V = CRT (for dilute solutions)

   • Most sensitive colligative property (detected even at very low concentrations)
   • π in atm, C in mol/L, R = 0.0821 L·atm/mol·K, T in Kelvin
   • Applications: Reverse osmosis (water purification), plant water uptake

4. Van’t Hoff Factor (i):

   • i = Actual number of particles / Number of formula units dissolved
   • For NaCl (full dissociation): i ≈ 2; For CaCl₂: i ≈ 3; For glucose (no dissociation): i = 1
   • Abnormal molecular weight: M(observed) = M(expected) / i

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Solutions — Comprehensive Notes

Full Coverage: The deeper aspects of solution chemistry include deviation from Raoult’s Law (positive and negative deviations), distillation of liquid mixtures (azeotropes), and solubility of gases (Henry’s Law). NEET tests these less frequently but they appear in competitive exams.

Deviation from Raoult’s Law:

• Positive deviation: A-B interactions weaker than A-A and B-B → vapour pressure higher than predicted → minimum boiling azeotrope (e.g., ethanol-water at 95.6% ethanol)
• Negative deviation: A-B interactions stronger than A-A and B-B → vapour pressure lower than predicted → maximum boiling azeotrope (e.g., nitric acid-water at 68% HNO₃)

Henry’s Law (Gas Solubility):

• At constant temperature, solubility of a gas in a liquid is directly proportional to its partial pressure above the liquid
• Formula: p = KH × x (p = partial pressure, KH = Henry’s constant, x = mole fraction)
• Application: CO₂ in soft drinks (under pressure → dissolves more CO₂); Decompression sickness in divers
• Temperature ↑ → gas solubility ↓ (why warm soda goes flat faster)

Calculation Examples (NEET Pattern):

• Given 5.85 g NaCl in 1 kg water: molality = 5.85/58.5 = 0.1 m; ΔTf = 1.86 × 0.1 × 2 = 0.372°C
• Given 0.1 M glucose solution at 300K: π = 0.1 × 0.0821 × 300 = 2.463 atm

Important Formulas:

• ΔTb = Kb × m × i
• ΔTf = Kf × m × i
• π = CRT (or πV = nRT)
• p = KH × x (Henry’s Law)
• Molarity ↔ Molality: M = m × d × 1000 / (1000 + m × MMsolute)

Common NEET Mistakes to Avoid:

• Confusing molality with molarity — molality uses kg of solvent, molarity uses L of solution
• Forgetting the van’t Hoff factor (i) in colligative property formulas — always check if solute dissociates
• Using wrong R value (0.0821 for π in atm, not 8.314 which is for other units)
• Mixing up boiling point elevation and freezing point depression — both are colligative but in opposite directions

Related Topics: pc-013 (Colloidal — related to osmotic pressure and surface phenomena), pc-008 (Redox — molarity calculations in titration), pc-005 (Electrochemistry — concentration cells)

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