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

Explanation

Auto-protolysis is a reaction in which two same molecules react to give ions with proton transfer. For example, in water, a proton from one water molecule is transferred to another water molecule leaving behind `OH^-` ions and forming `H_3O^+` ion :
`H_2 O(l) + H_2 O(l) hArr H_3 O^(+)(aq) + OH^(-) (aq)`
Because of auto-protolysis , water is amphoteric in nature i.e., it reacts with acids as well as bases. It acts as a base towards acids stronger then itself and as an acid towards bases stronger than itself. For exmaple :
`underset("acid"_1)(H_2O(l)) + underset("base"_2)(NH_3)(aq) to underset("acid"_2)(NH_4^(+)(aq)) + underset("base"_1)(OH^(-) (aq))`
`underset("base"_1)(H_2 O(l)) + underset("acid"_2)(H_2 S(aq)) to underset("acid"_1)(H_3 O^(+)(aq)) + underset("base"_2)(HS^(-)(aq))`
It helps to undersetand Bronsted concept of conjugate acid-base pairs.