Question

Water can also act as a proton donor and proton acceptor towards itself. This is referred to as the:
(A) condensation of water
(B) auto-ionization of water
(C) hydrolysis of water
(D) none of the above

Answer 1

Hint: We should know that acids are substances that can donate \[{{H}^{+}}\]ions to bases. We should know that a hydrogen atom is a proton and one electron, technically an \[{{H}^{+}}\]ion is just a proton. So an acid is a "proton donor", and a base is a "proton acceptor".

Complete step by step answer:
> We should know that a substance that can act as a proton donor or a proton acceptor is called amphiprotic. The most common amphiprotic substance we encounter in day to day life is water. We should know that acids are substances that can donate \[{{H}^{+}}\]ions to bases. Since a hydrogen atom is a proton and has one electron, technically an \[{{H}^{+}}\]ion is just a proton. So an acid is a "proton donor", and a base is a "proton acceptor". The reaction between an acid and base is essentially a proton transfer.
> We should check each and every option. First option is condensation of water. This is the wrong option because as we know that condensation is the process by which water vapour in the air is changed into liquid water.
> Now, we should check the second option that is auto-ionization of water. As we know from the above discussion that water is amphiprotic. It means that one water molecule can react with another to form an OH− ion and an H3O+ ion in an autoionization process.
\[2{{H}_{2}}O\left( l \right)\rightleftharpoons {{H}_{3}}{{O}^{+}}\left( aq \right)+O{{H}^{-}}\left( aq \right)\]
So, in the above reaction, water itself acts as a proton donor and proton acceptor. And this process is known as auto-ionization.
So, from the above discussion we can say that option B is correct. Water can also act as a proton donor and proton acceptor towards itself. This is referred to as the autoionization of water.

Note:
We should recall the Arrhenius definition of an acid. We should know that acid is a substance that dissociates in water to produce \[{{H}^{+}}\]ions (protons), and an Arrhenius base is a substance that dissociates in water to produce \[O{{H}^{-~}}\] (hydroxide) ions. If we follow these definitions an acid–base reaction involves the reaction of a proton with a hydroxide ion to form water.