Describe the laboratory preparation of hydrogen with a labelled diagram? How are the impurities removed.

Reactants : Granulated zinc, dil. hydrochloric acid or dil. sulphuric acid.

Procedure : Place some pieces of granulated zinc in a flat bottom flask fitted with an air tight cork with two holes. Through one hole, pass a thistle funnel with a long stem provided with a stopper, and through the other, a long delivery tube.

Pour dilute hydrochloric acid (or dilute sulphuric acid) through the funnel.

Zn + 2HCl (dil.) ⟶ ZnCl₂ + H₂ ↑

Zn + H₂SO₄ (dil.) ⟶ ZnSO₄ + H₂ ↑

Observation : Reaction will gradually start in the form of effervescence and evolution of gas. When all the air from the apparatus has been expelled, collect the gas over water by downward displacement of water.

Impurities can be removed from hydrogen by passing it through :

1. Silver nitrate solution [to remove arsine and phosphine].
   AsH₃ + 6AgNO₃ ⟶ Ag₃As + 3AgNO₃ + 3HNO₃
   PH₃ + 6AgNO₃ ⟶ Ag₃P + 3AgNO₃ + 3HNO₃
2. Lead nitrate solution [to remove hydrogen sulphide].
   Pb(NO₃)₂ + H₂S ⟶ PbS + 2HNO₃
3. Caustic potash solution [to remove sulphur dioxide, carbon dioxide and oxides of nitrogen].
   SO₂ + 2KOH ⟶ K₂SO₃ + H₂O
   CO₂ + 2KOH ⟶ K₂CO₃ + H₂O
   2NO₂ + 2KOH ⟶ KNO₂ + KNO₃ + H₂O
4. A drying agent used to dry the gas. Common drying agents such as fused calcium chloride, caustic potash stick and phosphorus pentoxide remove water vapour.

Thus, the gas is purified and dried and then collected over mercury because mercury has no reaction with it.