(a) Sodium chloride is an ionic compound but it conducts electricity only in molten and aqueous state because in molten and aqueous form the compound liberate to give ions. These ions move freely and hence conduct electricity.
(b) Reactivity of aluminium decreases if it is dipped in nitric acid because it is a strong oxidising reagent. The layer of aluminium oxide (Al2O3) prevents further reaction of aluminium due to which its reactivity decreases.
(c) Metals like calcium and magnesium are never found in their free states in nature; they occur in the form of ores because these metals are present on the top of the reactivity series. They are so reactive that they react with gases and surrounding elements, form compounds and thus are not found in free state.