Q.What do you understand by the term 'auto-protolysis' of water? What is its significance?

Auto-protolysis (self-ionisation) of water refers to the reaction of water molecules with each other, in which one water molecule donates a proton (H⁺) to another:

Here, one water molecule acts as a Brønsted acid (proton donor, becoming OH⁻) and the other acts as a Brønsted base (proton acceptor, becoming H3O⁺). This is only possible because water is amphoteric — capable of acting as both acid and base.

Significance:

• It shows directly that pure water contains small, exactly equal concentrations of H3O⁺ and OH⁻ ions, even in the absence of any added acid or base.
• This equilibrium defines the ionic product of water, $K_w = [H_3O^+][OH^-]$, which has the value $1 \times 10^{-14}$ at 298 K.
• $K_w$ is the foundation of the pH scale: in pure water $[H_3O^+] = [OH^-] = 10^{-7}\ mol\,L^{-1}$, giving pH = 7 (neutral); it also underlies how acidic/basic solutions are quantitatively defined and related ($pH + pOH = 14$ at 298 K). …

Write the self-ionisation equilibrium, identify the proton-donor/acceptor roles (demonstrating amphoterism), then connect …

• Writing the equation without showing it as an equilibrium (⇌) — auto-protolysis is a dynamic, reversible process, not a one-way reaction. …