Calcium oxide



Name Calcium oxide
Chemical formula CaO
Appearance White solid


Formula weight 56.1 amu
Melting point 3200 K (2927 °C)
Boiling point 3773 K (3500 °C)
Density 3.3 ×103 kg/m3
Crystal structure NaCl
Solubility hydrolysed


ΔfH0gas 43.93 kJ/mol
ΔfH0liquid -557.33 kJ/mol
ΔfH0solid -635.09 kJ/mol
S0gas, 1 bar 219.71 J/mol·K
S0liquid, 1 bar 62.31 J/mol·K
S0solid 38.19 J/mol·K


Ingestion Dangerous—causes GI irritation, larger doses could be fatal.
Inhalation Dangerous - irritation; chemical bronchitis or even death for larger exposures.
Skin Irritation and possible burns.
Eyes May cause permanent damage.
More info Hazardous Chemical Database
SI units were used where possible. Unless otherwise stated, standard conditions were used.

Disclaimer and references

Calcium oxide (CaO), commonly known as lime or quicklime, is a widely used chemical compound. It is a white, caustic and alkaline crystalline solid. As a commercial product lime often also contains magnesium oxide, silicon oxide and smaller amounts of aluminium and iron oxides.

Calcium oxide is usually made by the thermal decomposition of CaCO3, heating materials such as limestone to around 500C and so removing the carbon dioxide in a reversible reaction. It is one of the first chemical reactions discovered by man and was known in prehistory: see limekiln.

As hydrated or slaked lime, Ca(OH)2, it was used in mortar and plaster to increase the rate of hardening. Hydrated lime is very simple to make as lime is a basic anhydride and reacts vigorously with water. Lime was also used in glass production and its ability to work with silicates is also used in modern metal production (steel, magnesium, aluminum and other non-ferrous metals) industries to remove impurities as slag.

It is also used in water and sewage treatment to reduce acidity, to soften, as a flocculant and to remove phosphates and other impurities; in paper making to dissolve lignin, as a coagulant and in bleaching; in agriculture to improve acid soils; and in pollution control - in gas scrubbers to desulphurize waste gases and to treat many liquid effluents. It is a refactory and a dehydrating agent and is used to purify citric acid, glucose, dyes and as a CO2 absorber. It is also used in pottery, concrete, paints and the food industry.

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