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How does water act as both an acid and a base in chemical reactions?

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water rapids, Niagara Falls, Canada — water Water is the most plentiful compound on Earth and is essential to life. Although water molecules are simple in structure (H₂O), the physical and chemical properties of water are extraordinarily complicated.

Water is an amphoteric substance, meaning it can act as both an acid (a proton donor) and a base (a proton acceptor) depending on the chemical environment.12

Mechanisms of Action

As a Base: When water reacts with an acid, such as hydrogen chloride or acetic acid, it accepts a proton to form a hydronium ion ( $H_{3} O^{+}$ ).13
As an Acid: When water reacts with a base, such as ammonia, it donates a proton to the base, resulting in the formation of a hydroxide ion ( $O H^{-}$ ).14

Autoionization

This dual nature is most clearly demonstrated through the autoionization of water, where water molecules react with one another:
$H_{2} O (l) + H_{2} O (l) ⇌ H_{3} O^{+} (a q) + O H^{-} (a q)$
In this equilibrium process, one water molecule acts as an acid by donating a proton, while the other acts as a base by accepting it.2