Colligative property

A colligative property is a solution property whose dilute ideal limit depends on the number of dissolved particles relative to the solvent, rather than on their detailed identity. Examples include freezing-point depression, boiling-point elevation and osmotic pressure. Dissociation matters because one formula unit of dissolved sodium chloride can supply two ions.^[ref]

For dilute aqueous chemical solutions, the freezing-point change is approximately $\Delta T_f=i{K}_{f}m$, where $m$ is molality, $K_f$ depends on the solvent, and $i$ is the effective particle-number factor. Taking $K_f=1.86\mathrm{K}\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$, $0.10\mathrm{mol}\mathrm{k}{\mathrm{g}}^{-1}$ glucose gives about $0.186\mathrm{K}$ depression, while ideally dissociated sodium chloride at the same molality gives about $0.372\mathrm{K}$. Interactions make real concentrated solutions depart from this ideal counting rule.^[ref]