What is calcium oxide?

The calcium oxide  (CaO) is an inorganic compound containing calcium and oxygen in ionic forms (not to be confused with calcium peroxide CaO 2 ). Worldwide it is known as lime, a word that designates all inorganic compounds that contain calcium carbonates, oxides and hydroxides, as well as other metals such as silicon, aluminum and iron.

This oxide (or lime) is also colloquially referred to as quicklime or slaked lime, depending on whether or not it is hydrated. Quicklime is calcium oxide, while slaked lime is its hydroxide. In turn, limestone (lime stone or hardened lime) is actually a sedimentary rock composed mainly of calcium carbonate (CaCO 3 ). 

It is one of the largest natural sources of calcium and constitutes the raw material for the production of calcium oxide. How is this rust produced? Carbonates are susceptible to thermal decomposition; heating calcium carbonates to temperatures higher than 825 ºC, lead to the formation of lime and carbon dioxide.

The above statement can be described as follows: CaCO 3 (s) → CaO (s) + CO 2 (g). Because the earth’s crust is rich in limestone and calcite, and seashells (raw materials for the production of calcium oxide) are abundant in the oceans and beaches, calcium oxide is a relatively inexpensive reagent.


The chemical formula of calcium oxide is CaO, in which the calcium ion is like acid (electron acceptor) Ca 2+ , and oxygen as the basic ion (electron donor) O 2 — .

Why is calcium charged +2? Because calcium belongs to group 2 of the periodic table (Mr. Becambara), and it only has two valence electrons available for the formation of bonds, which it gives up to the oxygen atom.

Structure of calcium oxide

Calcium oxide
Crystal structure of calcium oxide

In the upper image the crystalline structure (gem salt type) for calcium oxide is represented. The voluminous red spheres correspond to the Ca 2+ ions  and the white spheres to the O 2- ions .

In this cubic crystalline arrangement, each Ca 2+ ion is surrounded by six O 2- ions , occluded in the octahedral holes that leave the large ions between them.

This structure expresses the ionic character of this oxide to the maximum, although the notable difference in the radii (the red sphere is larger than the white one) gives it a weaker crystalline lattice energy when compared to MgO.

Properties of calcium oxide

Appearance of calcium oxide

Physically, it is a white crystalline, odorless solid with strong electrostatic interactions, which are responsible for its high melting points (2572 ºC) and boiling (2850 ºC). Furthermore, it has a molecular weight of 55.958 g / mol and the interesting property of being thermoluminescent.

This means that a piece of calcium oxide exposed to a flame can glow with an intense white light, known in English as limelight , or in Spanish, calcium light. Ca 2+ ions , in contact with fire, cause a reddish flame, as can be seen in the following image.

Calcium oxide
Limelight or limelight


CaO is a basic oxide that has a strong affinity for water, to such an extent that it absorbs moisture (it is a hygroscopic solid), reacting immediately to produce slaked lime or calcium hydroxide:

CaO (s) + H 2 O (l) => Ca (OH) 2 (s)

This reaction is exothermic (gives off heat) due to the formation of a solid with stronger interactions and a more stable crystal lattice. However, the reaction is reversible if the Ca (OH) 2 is heated , dehydrating it and igniting the slaked lime; then the lime is “reborn”.

The resulting solution is very basic, and if it is saturated with calcium oxide it reaches a pH of 12.8.

Likewise, it is soluble in glycerol and in acidic and sugary solutions. As it is a basic oxide, it naturally has effective interactions with acidic oxides (SiO 2 , Al 2 O 3  and Fe 2 O 3 , for example), being soluble in their liquid phases. On the other hand, it is insoluble in alcohols and organic solvents.

Uses / applications

CaO has a vast infinity of industrial uses, as well as in the synthesis of acetylene (CH≡CH), in the extraction of phosphates from wastewater and in the reaction with sulfur dioxide from gaseous waste.

Other uses for calcium oxide are described below:

As mortar 

If calcium oxide is mixed with sand (SiO 2 ) and water, it cakes with sand and reacts slowly with water to form slaked lime. In turn, the CO 2 in the air dissolves in the water and reacts with the slaked salt to form calcium carbonate:

Ca (OH) 2 (s) + CO 2 (g) => CaCO 3 (s) + H 2 O (l)

CaCO 3 is a more resistant and harder compound than CaO, causing the mortar (the previous mixture) to harden and fix the bricks, blocks or ceramics between them or to the desired surface.

In glass production

The essential raw material for the production of glasses is silicon oxides, which are mixed with lime, sodium carbonate (Na 2 CO 3 ) and other additives, to then be subjected to heating, resulting in a glassy solid. This solid is subsequently heated and blown into any figures.

In mining

Slaked lime occupies more volume than quicklime due to hydrogen bonding (OH — O) interactions. This property is used to break the rocks from within.

This is achieved by filling them with a compact mixture of lime and water, which is sealed to focus its heat and expansive power within the rock.

As a silicate removal agent

The CaO melts with the silicates to form a coalescing liquid, which is then extracted from the raw material of a certain product.

For example, iron ores are the raw material for the production of metallic iron and steel. These minerals contain silicates, which are undesirable impurities for the process and are removed by the method just described.

Calcium oxide nanoparticles

Calcium oxide can be synthesized as nanoparticles, varying the concentrations of calcium nitrate (Ca (NO 3 ) 2 ) and sodium hydroxide (NaOH) in solution.

These particles are spherical, basic (as is the macro-scale solid) and have a lot of surface area. Consequently, these properties benefit catalytic processes. Which? Research is currently answering that question.

These nanoparticles have been used to synthesize substituted organic compounds – such as pyridine derivatives – in the formulation of new drugs to carry out chemical transformations such as artificial photosynthesis , for the purification of water from heavy and harmful metals, and as photocatalytic agents.

The nanoparticles can be synthesized on a biological support, such as papaya and green tea leaves, to be used as an antibacterial agent.

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