4.5 - Limestone & the Lime Cycle
Limestone, quicklime and slaked lime
- Limestone - A rock made mostly of calcium carbonate, CaCO3, formed from the compressed remains of sea creatures' shells and skeletons.
- Quicklime - Calcium oxide, CaO.
- Slaked lime - Calcium hydroxide, Ca(OH)2.
The limestone cycle
Limestone, quicklime, and slaked lime are linked by a cycle of three reactions.
Limestone → quicklime
Heating limestone strongly breaks it down into quicklime and carbon dioxide (thermal decomposition).
calcium carbonate → calcium oxide + carbon dioxide
CaCO3(s) → CaO(s) + CO2(g)
Quicklime → slaked lime
Adding water to quicklime produces slaked lime in an exothermic reaction.
calcium oxide + water → calcium hydroxide
CaO(s) + H2O(l) → Ca(OH)2(s)
Slaked lime → limestone
- Dissolving slaked lime in water gives limewater, a solution of calcium hydroxide.
- Passing carbon dioxide through limewater reforms calcium carbonate, completing the cycle.
- calcium hydroxide + carbon dioxide → calcium carbonate + water
- Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Uses of limestone and its products
Limestone
- Extracting iron - Removes acidic impurities in the blast furnace.
- Road-building - Crushed limestone is used as aggregate.
- Cement and concrete - Used to manufacture cement.
Neutralising acidity
Quicklime, slaked lime, and powdered limestone are all bases, so they can be:
- Spread on farmland to neutralise acidic soil and improve crop yields.
- Used to treat acidic lakes and rivers.
Quarrying limestone - Costs and benefits
| Problems | Benefits |
|---|---|
| Blasting, heavy machinery, and lorries create noise and dust. | Provides jobs in the quarry and local businesses. |
| Destroys plant and animal habitats. | Brings money into the local economy. |
| Scars the landscape and can lower nearby house prices. | Can fund improvements to local transport, roads, and infrastructure. |
TermDefinition
LimestoneA rock made mostly of calcium carbonate, formed from the compressed remains of sea creatures' shells and skeletons.
QuicklimeThe common name for calcium oxide.
Slaked limeThe common name for solid calcium hydroxide.
Thermal decompositionThe breaking down of a substance into two or more simpler substances by heating it.
LimewaterA solution of calcium hydroxide in water that turns milky or cloudy when carbon dioxide is bubbled through it.