Chapter 1 — Solutions

Chapter 1 of the Class 12 Chemistry NCERT textbook, "Solutions", covers homogeneous mixtures of two or more substances — including types of solutions, concentration units, Henry's law, Raoult's law, colligative properties (vapour pressure lowering, boiling point elevation, freezing point depression, osmotic pressure), and the van't Hoff factor for solutes that associate or dissociate.

• Expressing solution composition — Solutions are homogeneous mixtures whose largest component is the solvent, and the chapter builds a toolkit of concentration measures — molarity, molality, mole fraction, mass percentage and ppm — so composition can be quantified precisely across nine gaseous, liquid and solid types.
• Laws governing vapour pressure and solubility — Two guiding laws are developed: Henry's law links a gas's solubility to its partial pressure, while Raoult's law relates each volatile component's vapour pressure to its mole fraction, leading to the distinction between ideal solutions and non-ideal ones that deviate and form azeotropes.
• Colligative properties and molar mass — Four properties that depend only on the number of solute particles — not their nature — are used to determine molar masses, with the van't Hoff factor introduced to correct for solutes that dissociate into ions or associate into larger units in solution.

1. 01Solutions are homogeneous mixtures; the component in largest quantity is the solvent, and concentration units include molarity (mol/L), molality (mol/kg), mole fraction, mass percentage, and ppm
2. 02Henry's law states that at constant temperature the solubility of a gas in a liquid is directly proportional to its partial pressure above the liquid (p = KH x)
3. 03Raoult's law states that the partial vapour pressure of each volatile component equals its mole fraction multiplied by its vapour pressure in the pure state (p1 = x1 p1°)
4. 04Ideal solutions obey Raoult's law over the entire concentration range with zero enthalpy and volume of mixing; non-ideal solutions show positive or negative deviations and can form azeotropes
5. 05Colligative properties — relative lowering of vapour pressure, elevation of boiling point (DTb = Kb m), depression of freezing point (DTf = Kf m), and osmotic pressure (P = CRT) — depend only on the number of solute particles, not their identity
6. 06The van't Hoff factor i corrects colligative property equations for electrolytes that dissociate (i > 1) or molecules that associate (i < 1) in solution