Question

Why do ions increases conductivity of substance?

Answer 1

because the symmetry of the atoms is broken when these previous neutral atoms become ions ("charged atoms"). in a neutral atom (= an atom in symmetry while an ion isn't symmetric - the ratio between electrons and protons isn't 1 as it is in the (uncharged) atom) a current flows only inside of it. in an ion this is not the case - the internal current can't circulate internally - either the ion tries to fill up an electron gap (positive) or it "tries" to get lost of an electron that's too much (negative ion) for the ion to become neutral, to become an atom again. therefore an interaction with other atoms or ions takes place - and this leads to a current that's not just circulating inside an atom. this increase of external interaction is equal to conductivity (which is inversely proportional to the resistance of a material).