Question

why pure water cannot be called as insulator

Answer 1

Electricity flows only when there is a flow of electrons or ions present in a medium.

When you pass electricity in pure water, the oxygen doesn't have any d-orbitals or f-orbitals through which electrons may pass.

In pure water there are no electrolytic substance,so no ions present in the pure water. So electricity cannot pass through pure water.

Water cannot dissociate into OH- and H+ when electricity is passed through pure water. The ions you are talking about are the extra ions created by impurities which are acidic and basic in nature.

(The ions does not dissociate in H+ and O2-. They dissociate as OH- and H+)