依数性（Colligative Property）是指溶液所具有的一类性质，这类性质只取决于溶质在溶剂中的「粒子」数量，而与溶质的本性无关。溶液的依数性包括：溶剂蒸气压的降低导致溶液凝固点下降、沸点上升和渗透压改变等性质。通过测量稀的非离子水溶液（例如尿素或葡萄糖的水溶液）中的依数性，可以求得溶质的相对分子质量的精确值。另外，测量离子溶液的依数性则可以估计溶质电离的百分比。

含非挥发性溶质的溶液的四种性质具有依数性：

就像溶剂的蒸气压下降符合拉乌尔定律者与溶质的摩尔数有关，溶液的凝固点下降及溶液溶液的沸点上升与溶质的重量摩尔浓度有关，溶液的渗透压与溶质的体积摩尔浓度有关一样。

In chemistry, colligative properties are properties of solutions that depend on the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the type of chemical species present. The number ratio can be related to the various units for concentration of solutions. The assumption that solution properties are independent of the nature of solute particles is only exact for ideal solutions, and is approximate for dilute real solutions. In other words, colligative properties are a set of solution properties that can be reasonably approximated by assuming that the solution is ideal.