When aqueous solution of borax is acidified with hydrochloric acid, a white crystalline solid is formed which is soapy to touch, is this solid acidic or basic in nature? Explain.

Explanation

When an aqueous solution of borax is acidified with HCl, boric acid is formed.
`underset("Borax")(Na_(2)B_(4)O_7)+2HCl + 5H_(2)O rarr 2NaCl + underset("Boric acid")(4H_(3)BO_(3))`.
Boric acid is a white crystalline solid. Due to its planar layered structure , it is soapy to touch . it is not a protonic acid since it does not ionize in aqueous solution to liberate `H^(+)` ions. However, due to small size of boron and presence of only six electrons in its valence shell, `B(OH)_(3)` accepts a pair of electrons from oxygen atom of a molecule of `H_(2)O` releasing a proton. Thus, it acts as Lewis acid.