Britannica Sources used in this answer

How can water act as both an acid and a base?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

sodium sulfate — acid–base reaction Sodium sulfate, also called Glauber's salt, is, like other salts, the product of an acid-base reaction.

Water is considered an amphoteric substance because it can act as both an acid (a proton donor) and a base (a proton acceptor), depending on the other substances present in a reaction.12

According to the Brønsted-Lowry theory, this behavior is demonstrated in the following ways:

As a base: When water reacts with an acid like hydrogen chloride, it accepts a proton to form a hydronium ion ( $H_{3} O^{+}$ ):
$H_{2} O + H C l ⇌ H_{3} O^{+} + C l^{-}$ 1
As an acid: When water reacts with a base like ammonia, it donates a proton to form a hydroxide ion ( $O H^{-}$ ):
$H_{2} O + N H_{3} ⇌ N H_{4}^{+} + O H^{-}$ 13

This dual nature is also evident in the autoionization of water, where one water molecule acts as an acid and another as a base:
$H_{2} O (l) + H_{2} O (l) ⇌ H_{3} O^{+} (a q) + O H^{-} (a q)$ 2