$(a)$ $A$ substance $X$,an oxide of a metal,is used extensively in the cement industry. This element is found in our bones also. On treatment with water,it forms a solution which turns red litmus blue. Identify $X$ and also write the chemical reaction involved.
$(b)$ Choose a metal from the following metals which reacts only with hot water: Sodium,magnesium,iron. Mention the products formed during the reaction.

(N/A) The element found in bones is Calcium $(Ca)$. The oxide of this metal used in the cement industry is Calcium oxide $(CaO)$,also known as quicklime.
When $CaO$ reacts with water,it forms Calcium hydroxide $(Ca(OH)_2)$,which is basic in nature and turns red litmus blue.
The chemical reaction is: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$.
$(b)$ Among the given metals,Magnesium $(Mg)$ reacts only with hot water.
The products formed are Magnesium hydroxide $(Mg(OH)_2)$ and Hydrogen gas $(H_2)$.
The chemical reaction is: $Mg(s) + 2H_2O(l) \rightarrow Mg(OH)_2(aq) + H_2(g)$.