Summary

This chapter developed the Liquid State unit of the WBCHSE Class XII Chemistry syllabus in two connected stages. The first stage covered the colligative properties of true solutions — properties that depend only on the number of solute particles present, never on their chemical identity — beginning with the solubility of a gas in a liquid under Henry's law, and the closely related idea of solid solutions formed substitutionally or interstitially. Building on Raoult's law for volatile solutions and the distinction between ideal solutions and those showing positive or negative deviation, the chapter derived the relative lowering of vapour pressure as the foundational colligative relation, and from it the elevation of boiling point, the depression of freezing point, and osmotic pressure — each usable, in reverse, to determine an unknown solute's molar mass, with osmometry preferred for very large solutes such as proteins. Abnormal molar masses, arising when a solute dissociates into ions or associates into aggregates, were reconciled through the van't Hoff factor, which corrects every colligative-property formula for the true number of particles actually present in solution. …