Which is the most preferred metal for the laboratory preparation of hydrogen. Why is any other metal not used?

Zinc is the most preferred metal in the laboratory preparation of hydrogen.

Other metals are not used because of the following reasons:

1. Sodium and potassium react violently with acid.
2. Calcium and magnesium are expensive.
3. Aluminium forms a protective coating of Al₂O₃ due to its great affinity for oxygen. Due to the coating of Al₂O₃, aluminium does not give hydrogen with acid.
4. Iron has to be heated, but then the hydrogen thus produced contains impurities like hydrogen sulphide and sulphur dioxide.
5. Lead reacts with dil. sulphuric acid or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride. Therefore, further reaction is prevented.
6. Hydrogen cannot be prepared from metals that are below in the metal reactivity series such as copper and mercury, since only metals that are more reactive than hydrogen can displace it from acids.