CIE IGCSE Chemistry

Revision Notes

13.1 Carbonates

Calcium Oxide & Calcium Carbonate

Manufacture of lime

  • Limestone consists mainly of calcium carbonate, CaCO3
  • Lime which is calcium oxide, is manufactured from calcium carbonate by thermal decomposition:
CaCO3 →  CaO + CO2
  • Slaked lime, calcium hydroxide, is made by adding a small amount of water slowly to calcium oxide:
CaO + H2O → Ca(OH)2
  • Limewater is a solution of calcium hydroxide in water, hence it is alkaline
  • The addition of carbon dioxide to calcium hydroxide produces the initial starting material, calcium carbonate:
CO2 + Ca(OH)2 → CaCO3 + H2O
  • This reaction is the basis of the standard chemical test for CO2


Using-Limewater-to-test-for-Carbon-Dioxide, IGCSE & GCSE Chemistry revision notesCa(OH)2 solution turns cloudy when in the presence of CO2 gas due to the formation of insoluble white calcium carbonate



  • The combination of these three reactions constitute the limestone cycle


The Limestone Cycle, IGCSE & GCSE Chemistry revision notesDiagram showing the stages in the limestone cycle


Uses of limestone and limestone products

  • Limestone (calcium carbonate) is used in the manufacture of iron and cement
  • In the production of iron, limestone is added to the blast furnace where it decomposes to form lime (CaO) and carbon dioxide
  • The lime reacts with silica impurities to form calcium silicate, which floats to the top of the molten iron and is removed:
CaO + SiO2 → CaSiO3
  • Cement is manufactured by heating a mixture of powdered limestone and clay in a rotary kiln
  • Once heated, calcium sulfate and water are added which produce cement
  • Cement is a hardened, interlocked structure of calcium aluminate (Ca(AlO2)2 and calcium silicate (CaSiO3)
  • CaCO3 is also used in treating excess acidity in soils and lakes where it is often preferred to lime because it does not make the water in the soil alkaline
  • Lime (calcium oxide) is used in lime mortar and in flue-gas desulfurization
  • Flue-gas desulfurization involves spraying acidic sulfur dioxide emissions with jets of slaked lime to reduce pollution by neutralising these gases before they leave the factory chimneys
  • It is also used in treating excess acidity in soils and lakes. If excess lime is used, however, the water in the soil may become too alkaline
  • Slaked lime (calcium hydroxide) is used in treating acidic soils and neutralising acidic industrial wasted products

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