NaCl conducts when molten but not solid; silicon conducts better when heated. Explain both. Step 1: Solid NaCl: ions locked in lattice → no carriers → insulator. Step 2: Molten NaCl: ions free to migrate → conducts (electrolysis). Step 3: Silicon: at 0 K, valence band full, gap to conduction band → insulator-like. Step 4: Heating promotes electrons across the small gap → conductivity RISES with T (opposite of metals). Metals conduct worse when hot (vibrations scatter electrons).