Introduction to Solutions

Overview

This lecture is a brief revision of the 12th-grade "Solutions" chapter, explaining concentration terms, solubility, vapor pressure, colligative properties, van't Hoff factor, etc., in simple language.

Definition and Types of Solution

• A solution is a homogeneous mixture of two or more components.
• The solute is the component present in a smaller amount, the solvent is the one present in a larger amount or whose phase is the same as the solution's phase.
• The phase of the solution is equal to the phase of the solvent.

Concentration Terms

• Mass percent = (Mass of component / Total mass of solution) × 100
• Volume percent = (Volume of component / Total volume) × 100
• Mass/volume percent = (Mass of component / Volume of solution in mL) × 100
• Parts per million (ppm) = (Weight of solute / Total weight) × 10⁶
• Mole fraction = (Moles of component / Total moles)
• Molarity = (Moles of solute / Volume of solution in liters), temperature dependent.
• Molality = (Moles of solute / Weight of solvent in kg), temperature independent.
• Normality = (Equivalents of solute / Volume of solution in liters)

Solubility

• Solubility is the maximum amount of a compound that can dissolve in a solvent at a given temperature.
• Solubility of solids generally increases with temperature, gases decrease.
• For gases, solubility increases with increasing pressure (Henry's law).

Vapor Pressure and Raoult's Law

• Vapor pressure of a pure liquid depends only on temperature.
• Total vapor pressure of solution = p1°x1 + p2°x2 (x = mole fraction)
• Ideal solutions follow Raoult's law; non-ideal solutions may show positive or negative deviation.
• Azeotropic mixture: a mixture with a specific composition that cannot be separated further by fractional distillation.

Colligative Properties

• Colligative properties depend on the number of solute particles, not their nature.
• Four main colligative properties:
  • Decrease in vapor pressure,
  • Increase in boiling point (ΔTb = Kb × m),
  • Decrease in freezing point (ΔTf = Kf × m),
  • Osmotic pressure (π = CRT).
• Van't Hoff factor (i): correction factor for changes in moles due to association or dissociation.

Key Terms & Definitions

• Solute — component in smaller amount.
• Solvent — component in larger amount or phase determining.
• Mole fraction (x) — ratio of moles of a component to total moles.
• Molarity (M) — moles of solute per volume of solution (liters).
• Molality (m) — moles of solute per weight of solvent (kg).
• Vapor pressure — pressure exerted by vapors of a liquid.
• Raoult's law — vapor pressure of solution depends on vapor pressures and mole fractions of components.
• Colligative property — property depending only on the number of solute particles.
• Van't Hoff factor (i) — correction of moles due to dissociation/association.
• Henry's law — solubility of gas in liquid proportional to pressure of gas above.
• Azeotrope — mixture that cannot be further purified by distillation.
• Osmosis — movement of solvent from less concentrated to more concentrated side.
• Osmotic pressure — pressure required to stop osmosis.

Action Items / Next Steps

• Memorize related colligative property formulas.
• Practice Raoult's law and Van't Hoff factor.
• Revise NCERT tables and definitions.
• Solve 5-10 practice questions for each topic.