Calcium oxide

Calcium oxide

Names
IUPAC name Calcium oxide
Other names Quicklime, burnt lime, unslaked lime, pebble lime
Identifiers
CAS Number	1305-78-8 N
ChemSpider	14095
Jmol 3D model	Interactive image
PubChem	14778
RTECS number	EW3100000
UNII	C7X2M0VVNH
UN number	1910
InChI InChI=1/Ca.O/rCaO/c1-2 Key: ODINCKMPIJJUCX-BFMVISLHAU
SMILES [Ca]=O
Properties
Chemical formula	CaO
Molar mass	56.0774 g/mol
Appearance	White to pale yellow/brown powder
Odor	Odorless
Density	3.34 g/cm3
Melting point	2,613 °C (4,735 °F; 2,886 K)
Boiling point	3,850 °C (6,960 °F; 4,120 K) (100 hPa)
Solubility in water	Reacts to form calcium hydroxide
Solubility in Methanol	Insoluble (also in diethyl ether, n-octanol)
Acidity (pKa)	12.8
Structure
Crystal structure	NaCl
Thermochemistry
Std molar entropy (So298)	40 J·mol−1·K−1
Std enthalpy of formation (ΔfHo298)	−635 kJ·mol−1
Pharmacology
ATCvet code	QP53AX18
Vapor pressure	{{{value}}}
Related compounds
Other anions	Calcium sulfide Calcium hydroxide
Other cations	Beryllium oxide Magnesium oxide Strontium oxide Barium 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 (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "lime" connotes calcium-containing inorganic materials, in which carbonates, oxides and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, "quicklime" specifically applies to the single chemical compound calcium oxide. Calcium oxide which survives processing without reacting in building products such as cement is called free lime.

Quicklime is relatively inexpensive. Both it and a chemical derivative (calcium hydroxide, of which quicklime is the base anhydride) are important commodity chemicals.

Preparation

Calcium oxide is usually made by the thermal decomposition of materials such as limestone, or seashells, that contain calcium carbonate (CaCO₃; mineral calcite) in a lime kiln. This is accomplished by heating the material to above 825 °C (1,517 °F), a process called calcination or lime-burning, to liberate a molecule of carbon dioxide (CO₂); leaving quicklime.

CaCO_3(s) → CaO_(s) + CO_2(g)

The quicklime is not stable and, when cooled, will spontaneously react with CO₂ from the air until, after enough time, it will be completely converted back to calcium carbonate unless slaked with water to set as lime plaster or lime mortar.

Annual worldwide production of quicklime is around 283 million metric tons. China is by far the world's largest producer, with a total of around 170 million tonnes per year. The United States is the next largest, with around 20 million tonnes per year.

Usage

• Heat: Quicklime releases Thermal energy by the formation of the hydrate, calcium hydroxide, by the following equation:

CaO (s) + H₂O (l) ⇌ Ca(OH)₂ (aq) (ΔH_r = −63.7 kJ/mol of CaO)

As it hydrates, an exothermic reaction results and the solid puffs up. The hydrate can be reconverted to quicklime by removing the water by heating it to redness to reverse the hydration reaction. One litre of water combines with approximately 3.1 kilograms (6.8 lb) of quicklime to give calcium hydroxide plus 3.54 MJ of energy. This process can be used to provide a convenient portable source of heat, as for on-the-spot food warming in a self-heating can.

• Light: When quicklime is heated to 2,400 °C (4,350 °F), it emits an intense glow. This form of illumination is known as a limelight, and was used broadly in theatrical productions prior to the invention of electric lighting.
• Cement: Calcium oxide is a key ingredient for the process of making cement.
• As an alkali in biodiesel production
• Petroleum industry: Water detection pastes contain a mix of calcium oxide and phenolphthalein. Should this paste come into contact with water in a fuel storage tank, the CaO reacts with the water to form calcium hydroxide. Calcium hydroxide has a high enough pH to turn the phenolphthalein a vivid purplish-pink color, thus indicating the presence of water.
• Paper: Calcium oxide is used to regenerate sodium hydroxide from sodium carbonate in the chemical recovery at Kraft pulp mills.
• Plaster: There is archeological evidence that Pre-Pottery Neolithic B humans used limestone-based plaster for flooring and other uses. Such Lime-ash floor remained in use until the late nineteenth century.
• Chemical or power production: Solid sprays or slurries of calcium oxide can be used to remove sulfur dioxide from exhaust streams in a process called flue-gas desulfurization.

Use as a weapon

Historian and philosopher David Hume, in his history of England, recounts that early in the reign of Henry III, the English Navy destroyed an invading French fleet by blinding the enemy fleet with quicklime:

D’Albiney employed a stratagem against them, which is said to have contributed to the victory: Having gained the wind of the French, he came down upon them with violence; and throwing in their faces a great quantity of quicklime, which he purposely carried on board, he so blinded them, that they were disabled from defending themselves.

Quicklime is also thought to have been a component of Greek fire. Upon contact with water, quicklime would increase its temperature above 150 °C and ignite the fuel.

Health issues

Because of vigorous reaction of quicklime with water, quicklime causes severe irritation when inhaled or placed in contact with moist skin or eyes. Inhalation may cause coughing, sneezing, labored breathing. It may then evolve into burns with perforation of the nasal septum, abdominal pain, nausea and vomiting. Although quicklime is not considered a fire hazard, its reaction with water can release enough heat to ignite combustible materials.

External links

• Lime Statistics & Information from the United States Geological Survey
• Factors Affecting the Quality of Quicklime
• American Scientist (discussion of ¹⁴C dating of mortar)
• Chemical of the Week – Lime
• Material Safety Data Sheet
• CDC - NIOSH Pocket Guide to Chemical Hazards