An amphiprotic (amphoteric) species is a substance that can act either as a Brønsted acid by donating a proton or as a Brønsted base by accepting one, depending on the reaction partner. Water is the most important example: it donates a proton to bases and accepts one from acids. This dual behaviour underlies the self-ionisation of water.

两性物质是既能作为布朗斯特酸给出质子、又能作为布朗斯特碱接受质子的物质,视反应对象而定。水是最重要的例子:它向碱给出质子,又从酸接受质子。这种双重行为是水自电离的基础。