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Solutions in Chemistry

Sep 10, 2025

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Overview

This lecture covers Chapter: Solutions (Class 12 Chemistry), focusing on types of solutions, methods of expressing concentration, solubility, Henry's Law, Raoult's Law, ideal and non-ideal solutions, and azeotropes.

Types of Mixtures and Solutions

  • Most substances in daily life are mixtures, not pure substances.
  • A homogeneous mixture (solution) has uniform composition and properties throughout.
  • Solutions can be binary (two components: solvent in excess, solute in smaller quantity).
  • State of solution depends on the solvent (can be gaseous, liquid, or solid solutions).
  • Physical state is determined by the solvent's state.

Expressing Concentration of Solutions

  • Mass Percentage: (Mass of solute / Mass of solution) × 100.
  • Volume Percentage: (Volume of solute / Volume of solution) × 100.
  • Mass by Volume Percentage: (Mass of solute / Volume of solution) × 100, commonly used in pharmacy.
  • Parts Per Million (ppm): (Parts of solute / Total solution parts) × 10^6, used for pollutants in trace amounts.
  • Mole Fraction (X): (Moles of component / Total moles in solution); sum of mole fractions in binary solutions equals 1.
  • Molarity (M): (Moles of solute / Volume of solution in liters); temperature dependent.
  • Molality (m): (Moles of solute / Mass of solvent in kg); temperature independent.

Solubility and Factors Affecting Solubility

  • Solubility: Maximum amount of solute that can dissolve in a specific amount of solvent at a given temperature and pressure (saturated solution).
  • "Like dissolves like": Polar solvents dissolve polar solutes; non-polar solvents dissolve non-polar solutes.
  • Solubility depends on nature of solute & solvent, temperature (increases for endothermic, decreases for exothermic dissolution), and pressure (significant for gases).

Henry's Law – Gas Solubility in Liquids

  • Henry's Law: Solubility of a gas in liquid is directly proportional to the partial pressure of the gas.
  • Formula: p = KH × X (p = partial pressure, KH = Henry's constant, X = mole fraction of gas).
  • KH increases with temperature, leading to decreased gas solubility (important for aquatic life and soft drinks).
  • Applications: scuba diving (to prevent bends), high altitudes (anoxia), and carbonated beverages.

Raoult's Law – Vapor Pressure of Solutions

  • Raoult’s Law (Non-volatile solute): Vapor pressure of solution = mole fraction of solvent × vapor pressure of pure solvent.
  • Adding non-volatile solute to solvent lowers vapor pressure, proportional to solute amount.
  • Two Volatile Liquids: Total vapor pressure is the sum of each component’s partial vapor pressure (both depend on respective mole fractions).
  • Graphical representation: partial and total vapor pressures vs. mole fraction.

Ideal and Non-Ideal Solutions

  • Ideal Solutions: Obey Raoult’s Law throughout; ΔH (enthalpy) and ΔV (volume) of mixing = 0.
    • Examples: Benzene-toluene, hexane-heptane.
  • Non-Ideal Solutions: Deviate from Raoult’s Law; ΔH and ΔV ≠ 0.
    • Positive deviation: Weaker AB interactions, increased vapor pressure, ΔH and ΔV positive (e.g., acetone–ethanol).
    • Negative deviation: Stronger AB interactions, decreased vapor pressure, ΔH and ΔV negative (e.g., chloroform–acetone).

Azeotropes

  • Azeotropes: Mixtures boiling at a constant temperature with unchanged composition.
  • Minimum boiling azeotropes: Boil at lower temps than either component; formed by positive deviation.
  • Maximum boiling azeotropes: Boil at higher temps than either component; formed by negative deviation.

Key Terms & Definitions

  • Homogeneous mixture — Uniform composition throughout.
  • Solvent — The component present in larger amount in a solution.
  • Solute — The component present in a smaller amount in a solution.
  • Mole Fraction (X) — Ratio of moles of a component to total moles in the solution.
  • Molarity (M) — Moles of solute per liter of solution.
  • Molality (m) — Moles of solute per kg of solvent.
  • Henry’s Law — Solubility of a gas in a liquid is proportional to the partial pressure of the gas above the solution.
  • Raoult’s Law — Vapor pressure of a solution is proportional to the mole fraction of the solvent.
  • Ideal Solution — Follows Raoult’s Law exactly, with no enthalpy or volume change on mixing.
  • Non-Ideal Solution — Deviates from Raoult’s Law; enthalpy and/or volume change on mixing.
  • Azeotrope — A mixture with a constant boiling point and fixed composition.

Action Items / Next Steps

  • Review NCERT textbook examples and diagrams for all types of solutions and deviations.
  • Prepare for problem-solving and previous year questions as discussed in the upcoming sessions.
  • Revise key formulas and definitions for use in numerical problems.
  • Read remaining topics: colligative properties, van’t Hoff factor, abnormal molar masses (upcoming session).

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