Question

Can ${H_2}O$ act as a Lewis acid, Lewis base or both?

Answer 1

Water is a compound composed of elements hydrogen and oxygen connected by covalent bonds. Water can exist in gaseous, liquid, and solid states. Water has the capability to dissolve a variety of molecules, hence water is also called a universal solvent. Moreover, water is a polar solvent and it has a high heat capacity and also has a high heat of vaporization.

Complete Step By Step Answer:
Species accepting an electron pair and will have vacant orbitals are Lewis acid and species donating an electron pair and will have lone pair electrons are Lewis acid.
Acids are electrophilic in nature i.e. they attract electrons and Bases are nucleophilic i.e. by their lone pair they attack the positive charge.
Water shows Amphoterism.
By donating its proton to a base to become its conjugate acid, water acts as an acid.
${H_2}O + N{H_3} \to NH_4^ + + O{H^ - }$
By accepting a proton from acid and thus becoming its conjugate base, water acts as a base.
${H_2}O + HCl \to C{l^ - } + {H_3}{O^ + }$
Therefore water can act as a Lewis acid as well as a Lewis base.
The environment in which the molecule has been placed plays an important effect in amphoteric properties like in an acidic medium, water does not act as acids and similarly, in a basic medium, water does not act as a base.

Note:
A simple way to identify Lewis acid and Lewis base are presence of a positive charge on the species signifying that it is electron-deficient and hence can act as Lewis acid similarly negative charge on the species signifies that it is electron-rich (because of lone pairs) and hence can act as Lewis base.