During the practical in the lab when hydrogen sulphide gas having offensive odour is prepared for some test, we can smell the gas even 50 metres away. Explain phenomenon.

During the reaction of some metals with dilute hydrochloric acid, following observations were made. (a) Silver metal does not show any change (b) The temperature of the reaction mixture rises when aluminium (Al) is added (c) The reaction of sodium metal is found to be highly explosive (d) Some bubbles of a gas are seen when lead (Pb)is reacted with the acid Explain these observation giving suitable reasons.

Aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character. A piece of aluminium foil is treated with diluted hydrochloric acid or dilute sodium hydroxide solution in a test tube and on bringing a buring matchstick near the mouth of the test tube, a pop sound indicates the volution of hydrogen gas. The same activity when performed with concentrated nitric acid, reaction doesn't proceed. Explain the reason.