Give reason for the following:
Metals other than zinc are not used in the laboratory preparation of hydrogen.

Explanation

Because (i) Sodium and potassium react with acid violently . (ii) Aluminium forms a protective coating of `Al_2O_3` due to its great affinity for oxygen, so it does not give hydrogen after the reaction has occurred for some time. (iii) Iron needs to be heated but on doing so it acquires impurities like `H_2S` and `SO_2`. (iv) Lead reacts with dil. `H_2SO_4` and HCl to form an insoluble coating of lead sulphate or lead chloride. Therefore, further reaction does not take place. (v) Metals below hydrogen in the activity series do not react with acids to produce hydrogen e.g. Cu + HCl (dil) `rarr` No reaction.