Oxygen on the move
Hydrogen passes over hot copper oxide. Shiny copper appears.
- Copper oxide and hydrogen, side by side.
- An arrow shows the oxygen moving from one to the other.
- Copper and water form. Hydrogen is oxidised; copper oxide is reduced.
Follow the oxygen in each equation. Oxidised or reduced?
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The rule
Gaining oxygen is oxidation. Losing oxygen is reduction. Both happen together in one redox reaction.
Chemists also count loss of hydrogen as oxidation. Gain of hydrogen is reduction.
Hydrogen over hot copper oxide gives copper and water.
CuO + H₂ → Cu + H₂O
The oxygen moves from the copper oxide to the hydrogen.
Magnesium keeps burning in a jar of carbon dioxide.
2Mg + CO₂ → 2MgO + C
The oxygen moves from the carbon dioxide to the magnesium. Steam over hot zinc moves oxygen too. It goes from steam to zinc.
Zn + H₂O → ZnO + H₂
Carbon monoxide takes oxygen from iron oxide.
Fe₂O₃ + 3CO → 2Fe + 3CO₂
Sulphur dioxide can gain oxygen too.
2SO₂ + O₂ → 2SO₃
Here the oxygen comes from the O₂ gas.
Where one substance gains oxygen, another gives it up.
| Substance | Its oxygen | Its name |
|---|---|---|
| Hydrogen, in CuO + H₂ | gains | oxidised |
| Copper oxide | loses | reduced |
| Magnesium, in CO₂ | gains | oxidised |
| Carbon dioxide | loses | reduced |
| Zinc, in steam | gains | oxidised |
| Water, as steam | loses | reduced |
| Carbon monoxide, in Fe₂O₃ + 3CO | gains | oxidised |
| Iron oxide | loses | reduced |
| Sulphur dioxide, in 2SO₂ + O₂ | gains | oxidised |