During the

(a) Silver is less reactive than Hydrogen and hence, cannot displace Hydrogen in a chemical reaction. ( Silver lies below Hydrogen in the activity series. )

(b) This reaction is exothermic in nature and hence the temperature of the mixture rises.

(c) Sodium is a highly reactive metal and combines with moisture to form Hydrogen gas. This reaction is highly exothermic and quick which leads to Sodium catching fire.

(d) The bubbles are observed due to the formation of Hydrogen gas. The reaction gradually becomes slow due to the formation of a coating of Lead chloride on Lead, which prevents further reaction.
Pb + 2HCl → PbCl₂ + H₂