CBSE Class 10 Science 2019 · Paper 31/2/3 · Question 9

(a) Silver lies below hydrogen in the reactivity series, so it cannot displace hydrogen from dilute hydrochloric acid; no reaction takes place, so no change is seen. (b) Lead lies just above hydrogen, so it displaces hydrogen from the acid, but slowly; the bubbles are of hydrogen gas: $\text{Pb}+2\text{HCl}\to {\text{PbCl}}_2+{\text{H}}_2$. (c) Sodium is a highly reactive metal: its reaction with the acid is very fast and highly exothermic, so the hydrogen gas evolved catches fire and the reaction is explosive: $2\text{Na}+2\text{HCl}\to 2\text{NaCl}+{\text{H}}_2$. (d) The reaction of aluminium with the acid is exothermic (it releases heat), so the temperature of the reaction mixture rises: $2\text{Al}+6\text{HCl}\to 2{\text{AlCl}}_3+3{\text{H}}_2$.