流平效应（英语：Leveling effect）指酸和碱的性质受到溶液种类影响的现象。例如在水溶液中，所有比水合氢（H3O+）酸度高的酸都要降到水合氢的酸度；同理，在水溶液中，所有碱性比（OH-）高的碱都会降到（OH-）的碱度。

水的流平效应是由于它具有高介电常数和强接收质子的倾向。不同溶液对酸和碱的流平作用不同；是因为它们和酸-碱的质子相互作用不同；在此基础上；溶液可分为四种类型：

对某一特定溶液所允许的PH范围，称为酸-碱区分窗口。

Leveling effect or solvent leveling refers to the effect of solvent on the properties of acids and bases. The strength of a strong acid is limited ("leveled") by the basicity of the solvent. Similarly the strength of a strong base is leveled by the acidity of the solvent. When a strong acid is dissolved in water, it reacts with it to form hydronium ion (H₃O). An example of this would be the following reaction, where "HA" is the strong acid: