How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Why does dry HCl gas not change the colour of the dry litmus paper?

Why does an aqueous solution of an acid conduct electricity?

Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?