Henry's law (p = K_H × x) describes gas solubility, with K_H increasing at higher temperatures so gases become less soluble in warm liquids. Raoult's law states that the partial vapour pressure of each component of an ideal binary solution equals its mole fraction times its pure-component vapour pressure. Ideal solutions (benzene + toluene) show $\Delta H$_mix = 0 and $\Delta V$_mix = 0, while positive deviation solutions (ethanol + water) have higher vapour pressures than predicted and form minimum boiling azeotropes, and negative deviation solutions ($CHCl_{3}$ + acetone) have lower vapour pressures and form maximum boiling azeotropes. The four colligative properties — relative lowering of vapour pressure, boiling point elevation, freezing point depression, and osmotic pressure — all depend only on the number of solute particles, not their chemical nature. Boiling point elevation uses $\Delta Tb$ = iKbm (Kb water = 0.52 K·kg/mol) and freezing point depression uses $\Delta Tf$ = iKfm (Kf water = 1.86 K·kg/mol). Osmotic pressure π = iCRT uses molar concentration and temperature in Kelvin, and is the most sensitive property for macromolecule molar mass determination. The van't Hoff factor i corrects for electrolyte behaviour: i > 1 for dissociation (NaCl → i ≈ 2) and i < 1 for association (acetic acid in benzene → i ≈ 0.5). For associating solutes, the apparent molar mass calculated from colligative data is greater than the true molar mass. Reverse osmosis — applying pressure exceeding osmotic pressure — forces solvent through a semipermeable membrane and is used in desalination and water purification. These concepts are essential for NEET, appearing as calculation questions, assertion-reason pairs, and solution identification problems almost every year.