Hydrogen may be prepared in the laboratory by the action of a metal with an acid.

(a) Which of the metals copper, zinc, magnesium or sodium would be the most suitable?

(b) Which of the acids dilute sulphuric, concentrated sulphuric, dilute nitric and concentrated nitric would you choose? Explain why you would not use the acids you rejected.

(c) How would you modify your apparatus to collect dry hydrogen? Which drying agent would you employ for this purpose?

(a) Zinc, because sodium reacts violently with acid, magnesium is expensive and copper does not displace hydrogen when reacted with acids as it lies below hydrogen in the metal activity series.

(b) Dilute sulphuric acid is preferred to prepare hydrogen in the laboratory because:

1. Nitric acid, even in its dilute form, is not used in the preparation of hydrogen from metals because it is a powerful oxidizing agent, and the oxygen formed due to its decomposition oxidizes the hydrogen to give water, thus defeating the purpose of the reaction.
   3Zn + 8HNO₃ ⟶ 3Zn(NO₃)₂ + 4H₂O + 2NO↑
2. The reason for not using concentrated sulphuric acid is that it is a strong oxidiser and will produce sulphur dioxide.
   Zn + 2H₂SO₄ ⟶ ZnSO₄ + SO₂ + 2H₂O

(c) The purified and dry hydrogen gas is collected over mercury because mercury has no reaction with it.
Common drying agents like fused calcium chloride, caustic potash stick and phosphorous pentoxide can be used to dry hydrogen gas.