(i) $$X$$ is sodium hydroxide ($$NaOH$$) When sodium chloride solution (brine solution) is electrolysed, sodium hydroxide solution is formed $$H_2$$ and $$Cl_2$$ gases are liberated. this is chlor-alkali process. $$2NaCl+2H_2Oxrightarrow{electrolysis}2NaOH+H_2+Cl_2$$ $$NaOH+HCl\rightarrow NaCl+H_2O$$     $$'X'$$ The above reaction is the neutralization reaction (ii) It is because the process of dilution is highly exothermic. If the water is added to the acid, the amount of heat released can cause a lot of damage.