Chapter 12: Solutions and Their Behaviour

The chapter then develops the concept of saturation as a dynamic equilibrium state in which dissolution and precipitation rates balance, while introducing solubility as a quantitative measure of this equilibrium at specific temperatures. Particularly important is the distinction between saturated and supersaturated solutions, the latter representing unstable conditions that spontaneously revert to equilibrium when disturbed. The energetics of dissolution are explored through the molar enthalpy of solution, which combines two competing processes: the endothermic separation of ions from solid lattices and the exothermic hydration of those ions in aqueous solution. Understanding whether dissolution is energy-absorbing or energy-releasing depends entirely on which process dominates. External factors such as pressure and temperature profoundly affect solubility in predictable ways; Henry's law quantifies the pressure dependence for gases while temperature effects differ markedly between gaseous and solid solutes. Beyond molarity, the chapter introduces alternative concentration units including molality, mole fraction, mass percent, and parts-per-million, each serving specific analytical purposes. The concept of colligative properties—those dependent solely on solute particle concentration rather than chemical identity—encompasses vapor pressure lowering described by Raoult's law, freezing point depression, and osmotic pressure. The chapter addresses how electrolytes complicate these relationships through the van't Hoff factor, accounting for partial ionization and ion pairing in solution. Finally, colloidal dispersions are distinguished from true solutions and suspensions as intermediate systems whose particles scatter light through the Tyndall effect, with particular attention to hydrophobic and hydrophilic colloids and the critical role of surfactants in emulsification and detergency.