Topic 6 of 7
Use reactivity to explain extraction and heating
Strongly bound compounds of reactive metals are harder to break down.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Use reactivity to explain extraction and heating
Strongly bound compounds of reactive metals are harder to break down.
An ore contains a metal compound in a form worth extracting. More reactive metals generally need more demanding extraction. Carbon can reduce oxides of metals below it in the relevant series, such as zinc, iron, lead and copper, when heated appropriately. Very reactive metals such as potassium, sodium, calcium and magnesium require other methods, commonly electrolysis of suitable molten compounds. This is a principle, not a requirement to memorise every industrial furnace.
Pure: reduction experiments provide evidence for the series. Carbon reduces copper(II) oxide: 2CuO(s) + C(s) -> 2Cu(s) + CO2(g). Hydrogen also reduces suitable oxides: CuO(s) + H2(g) -> Cu(s) + H2O(g). Oxides resisting a given reducing agent under comparable conditions point to more reactive metals. Use supplied results and conditions; the hydrogen reference for dilute-acid reactions is not a universal oxide-reduction cutoff.
| Metal in the oxide | Heated with carbon | Heated in hydrogen |
|---|---|---|
| K, Na, Ca, Mg | Not readily reduced under these conditions | Not readily reduced under these conditions |
| Zn | Can be reduced | Not readily reduced in the usual school comparison |
| Fe, Pb, Cu | Can be reduced | Can be reduced with suitable heating |
| Ag | Silver oxide decomposes on heating alone | A heated-silver result alone cannot prove reduction by hydrogen |
Worked example
Combine reaction evidence rather than over-reading one test
Carbon reduces ZnO and CuO under suitable heating but not MgO. Zinc displaces copper from aqueous copper(II) sulfate. What relative order follows?
- MgO resisting carbon while ZnO is reduced places magnesium above zinc in this comparison.
- Both ZnO and CuO being reduced by carbon does not distinguish zinc from copper.
- Zinc displacing Cu2+ supplies that missing comparison: zinc forms Zn2+ more readily than copper forms Cu2+.
Mg > Zn > Cu. A successful reduction test places a metal relative to the reducing agent; combine it with other tests to resolve the full order. Heating conditions matter, and hydrogen reducing iron oxide does not put iron below hydrogen in the dilute-acid series.
| Carbonates | Expected pattern |
|---|---|
| Potassium and sodium | Stable under ordinary laboratory heating |
| Calcium, magnesium, zinc, iron(II), lead(II), copper(II) | Generally decompose on sufficient heating to metal oxide and CO2; more reactive metal usually means greater thermal stability |
| Silver | Low thermal stability; carbonate first loses carbon dioxide and its unstable oxide then gives silver and oxygen. Overall: 2Ag2CO3(s) → 4Ag(s) + 2CO2(g) + O2(g). |
For example, CuCO3(s) -> CuO(s) + CO2(g): green carbonate becomes black oxide, and the gas turns limewater milky. Calcium carbonate needs stronger heating than copper carbonate. Compare onset temperatures under controlled conditions rather than just how soon bubbles were noticed.