We not prefer Iron for the preparation of hydrogen gas because impurities are present but zinc also contains impurities than why do we prefer that element?

We cannot use any other metal than zinc in the laboratory for preparation of hydrogen gas because of the following reasons:
(i) Sodium and potassium react with acid violently.
(ii) Calcium and Magnesium are cost effective.
(iii) Aluminium forms a protective coating of its oxide because of its high affinity for Oxygen.
(iv) Iron has to be heated. Due to this, impurities like hydrogen sulphide and sulphur dioxide gas are produced.
(v)  Lead cannot be used because it reacts with dilute sulphuric acid or hydrochloric acid to form salts.
(vi) Metals like copper, mercury, etc cannot be used as they come below the Hydrogen in the activity series. As, only the metals which are more reactive than the hydrogen itself can displace hydrogen from acids.

For example, Cu  +  HCl  →  No reaction

                            (dilute)