Guides
Lab Preparation of O₂, CO₂ and H₂ — Equations & Methods
Oxygen, carbon dioxide and hydrogen are the three gases most often prepared in school labs. Below are the common reactions, equations and collection methods. The apparatus and collection method depend on whether the reaction needs heat and on the gas's density and solubility.
Preparing oxygen O₂
- Catalytic decomposition of H₂O₂ (room temp) 2H₂O₂ --MnO₂--> 2H₂O + O₂↑. Manganese dioxide is the catalyst; no heating, simple setup.
- Heating potassium permanganate 2KMnO₄ --Δ--> K₂MnO₄ + MnO₂ + O₂↑. Tilt the tube mouth slightly down and plug a little cotton.
- Collection Oxygen is only slightly soluble, so water displacement gives a purer sample; being denser than air, upward air displacement also works.
Preparing carbon dioxide CO₂
- Reaction CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ (limestone/marble with dilute HCl, room temp).
- Why not dilute H₂SO₄ or concentrated HCl Dilute sulfuric acid forms slightly soluble CaSO₄ that coats the limestone and stops the reaction; concentrated HCl is volatile and contaminates the gas.
- Collection CO₂ is denser than air and dissolves in water, so use upward air displacement only — not water displacement.
Preparing hydrogen H₂
- Reaction Zn + dilute H₂SO₄ → ZnSO₄ + H₂↑ (or Zn + 2HCl → ZnCl₂ + H₂↑).
- Choice of metal Use zinc granules (moderate rate); avoid very reactive K/Na (too violent) or unreactive copper (no reaction).
- Collection H₂ is nearly insoluble, so water displacement works; being lighter than air, downward air displacement works too. Always test for purity before igniting.
Safety notes
- Point a heated tube mouth away from people; remove the delivery tube before the burner to avoid suck-back.
- Hydrogen is flammable — always test its purity before lighting to prevent an explosion.
For more, see common equations and the reaction types.