Electrolytes and non-electrolytes
Substances whose solution or melt conducts electric current are electrolytes; those that do not are non-electrolytes. Electrolytes include water-soluble acids, alkalis and salts; sugar, alcohol, methane and distilled water are non-electrolytes. Solid NaCl does not conduct because its ions are locked in the crystal lattice. In water the polar water molecules attract the ions, pull them out of the crystal and surround them (hydration), giving freely moving ions — this is electrolytic dissociation. When current passes, cations move to the cathode and anions to the anode, so the solution conducts through the movement of ions. Acids ionise when their polar molecules interact with water; the hydrogen ion attaches to a water molecule to form H₃O⁺, usually written simply as H⁺. Atoms and ions differ sharply: unlike poisonous Cl₂, the Cl⁻ ion in table salt is harmless.
Teacher demonstration: the teacher dips the electrodes of a conductivity tester into dry salt, distilled water, salt solution and sugar solution in turn. Students watch the lamp and record “conducts / does not conduct” in a table. Only the teacher runs it; students must never experiment with electricity at home.