$$ullet$$  Onion juice is an olfactory indicator. $$ullet$$  As liquid $$'X'$$ turns blue litmus red, and hence it is acidic in nature. (a) When liquid $$'X'$$(acid) reacts with zinc granules(metal), it produces salt with the evolution of colourless and odourless hydrogen gas. $$Acid + Zn\rightarrow Salt + H_2(g)$$ (b) When liquid $$'X'$$(acid) reacts with solid sodium carbonate, it produces salt and water along with the production of colourless and odourless carbon dioxide gas. $$Acid + Na_2CO_3(s)\rightarrow Salt + H_2O(l) + CO_2(g)$$ $$ ext{Additional information}$$ $$ullet$$  Olfactory indicators are those indicators that change their smell rather than their colour to help determine whether a given solution is acidic or basic. $$ullet$$  An acidic solution turns the blue litmus paper red  $$ullet$$  The basic solution turns the red litmus paper blue.