Question

A silvery-white metal X reacts with water at room temperature to produce a water-soluble compound Y and a colourless gas Z. The reaction is highly exothermic and the Z catches fire immediately during the reaction. The solution of Y in the water on reacting with a stoichiometric amount of dilute solution of hydrochloric acid gives a solution of pH $\, = 7.0\,$
The compounds X, Y and Z respectively are:
A. $\,Al,Al{(OH)_{3,}}{H_2}\,$
B. $\,Ag,AgOH,{H_2}\,$
C. $\,K,KCl,{H_2}\,$
D. $\,Na,NaOH,{H_2}\,$

Answer 1

In thiS many the options given include many metal hydroxides. Almost all hydroxide ions form tight hydrogen bonds with water molecules in aqueous solutions with certain exceptions. As we have to find out the exothermic reaction here, we have to analyse the reaction in which heat is released and the energy is reduced.

Complete step by step answer:
Let us analyse each option one by one;
A thin film of aluminium oxide of a few millimetres is easily formed by aluminium alloy, which protects the alloy from reacting with water and $\,Al{(OH)_3}\,$. It is a type of floc and there is no exothermic reaction happening between aluminium with water. Hence, this is not the correct option.
Here, silver is the reactant. But, silver doesn’t react with pure water. Because it is placed in the lower section of the reactivity series and hence it doesn’t react with water. Hence, option B is also not correct.
Potassium reacts easily and intensively with water and, according to the following reaction mechanisms, forms a colourless simple potassium hydroxide solution and hydrogen gas:
$2K(s) + 2{H_2}O(l) \to 2KOH(aq) + {H_2}(g)$
This is an exothermic response and potassium is heated to such a degree that a purple flame is burned. Additionally, during the reaction, hydrogen released strongly reacts with oxygen and ignites. Therefore, if the second compound is $\,KOH\,$ in option C, then it may be the correct answer, but $\,KCl\,$ is treated as the second compound or Y.
Here, let us see the chemical reaction happening;
$2Na + {H_2}O \to 2NaOH + {H_2} + Heat$
Sodium hydroxide $\,(NaOH)\,$ is a water-soluble compound. Hydrogen gas is colourless and catches fire easily. This reaction is exothermic as well.
$NaOH + HCl \to NaCl + {H_2}O,$
pH of sodium chloride is $\,7.0\,$. Since Z suddenly catches fire during the reaction and the reaction with water is strongly exothermic. It should therefore be option D, as provided.

So, Option D is correct.

Note: When sodium hydroxide is reacted with water, dissolution is the process that is actually happening. The interaction between $\,N{a^ + }\,$and $\,O{H^ - }\,$ is ionic. Since the solute $\,NaOH\,$ with $\,N{a^ + }\,$ cations interact strongly with the oxygen of the water and the $\,O{H^ - }\,$ with the hydrogens of the water, this process is known as dissolution.