Calcium Oxide (CaO) Properties (25 Facts You Should Know)

Calcium oxide is a white or grayish-white solid compound that is also known as lime or quicklime. Let us explain the properties (physical and chemical) of calcium oxide.

Calcium oxide is mostly prepared from the thermal decomposition (heating above 8250 C) of calcium carbonate. This quicklime is not stable because it further reacts with CO2 to form CaCO3. CaO is not a volatile compound and it has high to intermediate expansion and contraction rates.

Let us clarify the IUPAC name, magnetism, polarity, conductivity, melting as well as boiling point, reactions, and many more relevant topics on calcium oxide in detail.

Calcium Oxide IUPAC Name

The IUPAC (International Union of Pure and Applied Chemistry) name of calcium oxide is calcium oxide itself. Calcium is the metal atom and oxygen is in a -2-oxidation state. Therefore, oxygen is written as oxide.

Calcium Oxide Chemical Formula

Calcium oxide has the chemical formula CaO.  There is an ionic interaction between calcium (Ca2+) and oxide (O2-) ions.

Calcium Oxide CAS Number

Calcium oxide has the CAS registry number (authentic numeric identifier which can contain up to 10 digits) 1305-78-8.

Calcium Oxide Chem Spider ID

Sulfuric acid has the ChemSpider (ChemSpider is a free chemical structure database) ID 14095.

Calcium Oxide Chemical Classification

  • Calcium oxide can be chemically classified as inorganic crystal compound.
  • Calcium oxide can be chemically classified as alkaline or basic (lewis base).
  • Calcium oxide can be chemically classified as ionic compound.

Calcium Oxide Molar Mass

The molar mass of calcium oxide is 56.0774 g. This molar mass is obtained from the summation of the molar mass of calcium and oxygen which are 40.078 g/mole and 15.999 g/mole respectively.

Calcium Oxide Color

Pure calcium oxide is a white or grayish-white solid crystalline or powdered compound. Otherwise, it is yellow or brownish due to presence of the impurities like iron, magnesia, silica, and alumina.

Calcium Oxide Viscosity

Calcium oxide is a medium viscous compound and it is denser than water.

Calcium Oxide Molar Density

The molar density of calcium oxide is 0.0595 mol/cm3 because it has a density of 3.34 g/cm3 and the molar mass of 56.0774 g/mol.

Calcium Oxide Melting Point

Calcium oxide has a melting point of 26130 C or 47350 F. It shows a high melting point because the CaO molecules are attached to the crystal due to strong ionic force.

Calcium Oxide Boiling Point

The boiling point of calcium oxide is 28500 C or 51600 F.

Calcium Oxide state at Room Temperature

At room temperature calcium oxide is an ionic crystalline solid due to its high boiling and melting point.

Calcium Oxide Ionic/Covalent Bond

There is an ionic bond between metal atoms, calcium ions (Ca2+), and nonmetal oxide ions (O2-). The neutral calcium ion donates its two valence electrons to oxygen, and it becomes Ca2+ and oxygen becomes O2-.

calcium oxide properties
Ionic Interaction in Calcium Oxide

Calcium Oxide Ionic/Covalent Radius

The radius or the internuclear distance between Ca2+ and O2- is 240 pm. This value is obtained from the ionic radius of Ca2+ and O2- which are 114 pm and 146 pm respectively.

Calcium Oxide Electron Configurations

Electron configuration is the relative arrangement of the revolving electrons in different orbitals or shells. Let us explain it elaborately.

Calcium oxide has the electron configuration of calcium and oxygen. Calcium is an s-block element and oxygen is a p-block element having the electron configuration of [Ar] 4s2 and [He] 2s2 2p4.

Calcium Oxide Oxidation State

Calcium oxide has a zero oxidation state because it is a neutral compound. In CaO calcium is in +2 and oxygen is in a -2 oxidation state which makes CaO a neutral compound.

Calcium Oxide Acidity/Alkaline

Calcium oxide is a lewis base because it can donate electron pairs of oxygen atoms. It has a pka value of 12.8.

Is Calcium Oxide odorless?

Calcium oxide has no characteristic odor. Therefore, it is an odorless compound.

Is calcium oxide paramagnetic?

Paramagnetic compounds are weakly attracted by the external magnetic field due to the presence of unpaired electrons. Let us check whether CaO is paramagnetic or not.

Calcium oxide is not a paramagnetic compound. The reason behind this diamagnetism is that all the electrons in Ca2+ are paired up ([Ar]) and from the molecular orbital theory, it can be concluded that there are also no unpaired electrons in O2-. Therefore, it is a diamagnetic compound.

Calcium Oxide Hydrates

The hydration reaction of calcium oxide is an example of an exothermic process that releases heat in the surroundings. When CaO comes to contact with water, crystalline or amorphous white calcium hydroxide is formed. CaO (s) + H2O (l) = Ca(OH)2 (s).

Calcium Oxide Crystal Structure

Solid calcium oxide possesses a halite, rock salt cubic crystal structure having the space group cF8. In this crystal each Ca2+ ion is attached to six O2- and each O2- ion is bonded with six Ca2+ ions. The lattice parameters are a = b = c = 4.84 A0 and α = β = γ = 900.

Calcium Oxide Polarity and Conductivity

  • Calcium oxide is neither a polar nor nonpolar compound. It is an ionic substance. But there is a distinct charge separation between electropositive calcium and electronegative oxygen atoms.
  • CaO shows an electrical conductivity of 19.50 W mK-1 because it is an ionic compound.

Calcium Oxide Reaction with Acid

Calcium oxide is a lewis base. Therefore, it spontaneously reacts with an acid to form salt and water which is known as the neutralization reaction.

  • CaO + 2HCl = CaCl2 + H2O
  • CaO + 2HNO3 = Ca(NO3)2 + H2O
  • CaO + H2SO4 = CaSO4 +H2O

Calcium Oxide Reaction with Base

Calcium oxide does not react with any basic or alkaline compound because it itself shows its basic nature.

Calcium Oxide Reaction with Metal

Calcium oxide does not react with metal because none of the metals can displace calcium from calcium oxide.

Calcium Oxide Reaction with Oxide

Calcium oxide generally reacts with the oxide of nonmetals. The examples are listed below-

  • CaO + P2O5 (phosphorus pentoxide) = Ca(PO3)2
  • CaO + CO2 (carbon dioxide) = CaCO3
  • CaO + H2O = Ca(OH)2


A maximum amount of calcium oxide is used in the basic oxygen steelmaking which is known as the BOS process. Besides that, it is applied to enhance the load-carrying capacity of clay-containing soils. A small amount of CaO is also used in the production of glass, calcium aluminate cement, and various chemicals.