Calcium oxide

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Calcium oxide
Calcium oxide powder.jpg
CaO sample
Calcium oxide 3D.png
CaO molecular structure
IUPAC name
Calcium oxide
Other names
Molar mass 56.0774 g/mol
Appearance White solid
Density 3.34 g/cm3
Melting point 2,613 °C (4,735 °F; 2,886 K)
Boiling point 2,850 °C (5,160 °F; 3,120 K)
Solubility Reacts exothermically with all acids, ketones
Insoluble in alcohols, ethers, halocarbons, hydrocarbons
Vapor pressure ~0 mmHg
Acidity (pKa) 12.8
40 J·mol−1·K−1
−635 kJ/mol
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Related compounds
Related compounds
Magnesium oxide
Strontium oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Calcium oxide, also known as quicklime or burnt lime, is a white, caustic, alkaline chemical compound, mostly used in construction. It has chemical formula CaO.



Calcium oxide can be reacted, or slaked, with water, releasing large amounts of heat and producing calcium hydroxide. In open air, it reacts with water vapor and carbon dioxide, slowly converting to calcium carbonate over time. It is used as a component of many cement mixes, and can be reacted with acids to form calcium salts.


Calcium oxide is a fine white powder. It reacts violently with water and skin. It is insoluble in methanol.


Calcium oxide can be bought at the construction or home improvement stores as quicklime or burnt lime in sacks or buckets. The ambiguous term "lime" usually refers to a mixture of calcium and/or magnesium compounds, though sometimes may serve as a label for pure calcium oxide.

In many places, quicklime is no longer sold by most construction stores, mainly due to its hazards, and thus has become almost impossible to find.


Calcium oxide is prepared from the thermal decomposition of calcium carbonate (lime), at temperatures over 825 °C. If the quicklime is not stable and after it cools, it will spontaneously react with CO2 from the air, converting it back to calcium carbonate. It should also be performed in a dry environment to prevent hydrolysis back into calcium hydroxide.

Quicklime is best synthesized in bulk, in lime kilns. Limestone works better than powdered calcium carbonate, as heat is transferred better and little calcium oxide gets blown in the smoke.

Calcium hydroxide can also be dehydrated to calcium oxide by heating it at 580 °C for a few hours. This reaction is generally preferred, as it takes place at lower temperatures, which are much easier to attain.

In both cases, after the reaction has completed, the hot quicklime should be left in a container filled with a neutral gas, such as nitrogen, to prevent the oxide from reacting with water vapors or carbon dioxide. Oxygen can also be used. If synthesized in large amounts, this is not required, as only the surface oxide will convert to limestone, which acts as a protective layer for the inner quicklime.

If the temperature is high enough, the calcium oxide will harden into fragile chunks, which can be easily separated from the leftover calcium carbonate/hydroxide by sifting it through a sieve.




Calcium oxide is extremely corrosive to the human tissue as it reacts vigorously with the water from tissues, releasing lots of heat, which causes severe burns. Quicklime is not normally a fire hazard, but sometimes its reaction with water can release sufficient heat to ignite combustible materials. Neutralizing calcium oxide even with a weak acid will release copious amounts of heat. Intentionally mixing calcium oxide with water in a controlled and well-thought out setup can be used to produce the somewhat more stable calcium hydroxide. Gloves, face mask and goggles should be worn when handling the powder, as most the commercial powder is extremely fine and can easily become airborne.


Calcium oxide should be stored in closed or sealed bottles, in a dry place, to prevent it from absorbing water and carbon dioxide from air. Small amounts of CaO will rapidly degrade in contact with air, while larger amounts take much longer.


Calcium oxide should be neutralized before being discarded of. It doesn't pose any hazards to the environment after being neutralized and can be safely dumped in the ground, preferably construction sites. Avoid dumping it on grassland as it will burn it.


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