CBSE Class 10 Science 2020 · Paper 31/4/1 · Question 25(a)

Onion juice is an olfactory indicator (a substance whose odour changes in an acidic or a basic medium; the smell of onion is lost in a basic medium, while an acid leaves it unchanged). Since X turns blue litmus red, X is an acid (for example dilute $\text{HCl}$). (a) With zinc granules, bubbles of a colourless, odourless gas are evolved; the gas is hydrogen and burns with a pop sound: $\text{Zn}(s)+2\text{HCl}(aq)⟶{\text{ZnCl}}_2(aq)+{\text{H}}_2(g)$. (b) With solid sodium carbonate there is brisk effervescence; a colourless, odourless gas, carbon dioxide, is evolved and turns lime water milky: ${\text{Na}}_2{\text{CO}}_3(s)+2\text{HCl}(aq)⟶2\text{NaCl}(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$, and ${\text{Ca(OH)}}_2(aq)+{\text{CO}}_2(g)⟶{\text{CaCO}}_3(s)+{\text{H}}_2\text{O}(l)$.