Explain the following :
(a) Sodium chloride is an ionic compound which does not conduct electricity in solid state whereas it does conduct electricity in molten state as well as in aqueous solution.
(b) Reactivity of aluminium decreases if it is dipped in nitric acid.
(c) Metals like calcium and magnesium are never found in their free state in nature.

Explanation

(a) Sodium chloride does not conduct electricity in solid state whereas it does conduct electricity in molten state as well as in aqueous solution. This is because in solid state movement of ions is not possible due to rigid structure. Movement of ions is what causes electricity.
In molten and aqueous state, ions are free to move and so they conduct electricity.
(b) Reactivity of aluminium decreases if it is dipped in nitric acid, because nitric acid is a strong oxidising agent. As a result, when aluminium is dipped in nitric acid, a layer of aluminium oxide is formed which prevents further reaction of aluminium.
(c) Metals like calcium and magnesium are never found in their free state in nature. This is because they are highly reactive metals and form compounds with almost all elements.