Question

why do impurities dissolved in water increase its conductivity?

Answer 1

Because the impurities contains more salts and acids.so when they dissolve in water the salts and acids will enter into water and the water can conduct electricity.

Answer 2

Pure water is not a good conductor of electricity.Ordinary distilled water is equilibrium with carbon dioxide . The electrical current is passed by the ions in the solution, the conductivity increases as the the concentration of ion increases, and the impurities also contains some amount of acid, base or salts... Which increases the conductivity of the water.
I hope this helps u..