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Calcium Oxide

Calcium Oxide - Structure, Preparation, Physical and Chemical Properties of Calcium Oxide & Uses

Who doesn’t like to come into the limelight? Make a hilarious song or just be yourself during any vacation to some offbeat area- If you reel it, you are under the limelight! Fame is the limelight that everyone loves to come under, isn’t it? But hey! Are you really aware of what is the origin of this ‘limelight’? 

What if I say, ‘limelight’ is actually ‘calcium oxide’! Sounds incredible? Indeed, it is incredible. Once upon a time, before the advent of electricity, all the theatres, music halls and stage productions used to use a source of calcium oxide, which when heated up to its melting point, would give off a very bright light, known as the ‘limelight’. This intriguing limelight effect was first discovered in the 1820s by Goldsworthy Gurney.

A strong illumination is created when an oxyhydrogen flame is directed at a cylinder of CaO which can be heated to 2,572 °C before melting. Isn’t it amazing? So why wait? Let’s put some limelight into the “limelight’’ and dive straight into exploring more about this historically glorious chemical and understanding its chemistry.


  • What is Calcium Oxide?
  • Structure of Calcium Oxide
  • Preparation of Calcium Oxide
  • Physical Properties of Calcium Oxide
  • Chemical Properties of Calcium Oxide
  • Uses of Calcium Oxide
  • Practice Problems
  • Frequently Asked Questions-FAQs

What is Calcium Oxide?

Calcium oxide with the molecular formula CaO is referred to as lime, quicklime, baked lime and also burnt lime. It is a white, alkaline and crystalline chemical compound that is widely used in industries and pharmaceutical companies for its physical and chemical properties.

Calcium oxide is used for making various building products such as cement which is called free lime. It is an alkaline substance which has been used by human beings as far back as the medieval age and thus calcium oxide was dubbed as the oldest chemical used by human beings.

The history of calcium oxide goes long back to the ages of ancient times. The Roman writer Cato the Elder (234 - 149 BC) mentioned a method for making calcium oxide in 184 BC.

By the early 15th century, multiple European citizens were using calcium oxide (referred to as lime) in the construction of buildings. Scottish chemist Joseph Black (1728–1799) performed several early scientific studies of calcium oxide. He found that when the compound is exposed to air, it combines with carbon dioxide to produce calcium carbonate.

Structure of Calcium Oxide

Calcium oxide (CaO) molecules contain one calcium cation (which holds a charge of +2) and one oxygen anion (which holds a charge of -2).  calcium oxide is an ionic compound featuring an ionic bond between calcium and oxygen. The lewis structure of calcium oxide is illustrated below.


In the solid crystalline structure of CaO , all the O2- ions occupy corners and face centres in the FCC arrangement and Ca2+ ions occupy all the octahedral voids.


Preparation of Calcium Oxide 

Calcium oxide (CaO) is usually made by the phenomenon of thermal decomposition which occurs with materials such as seashells or limestone. These materials are rich in calcium carbonate and are converted into calcium oxide using a lime kiln or a rotary kiln. This is accomplished by heating the material to above 1070-1200K. 

This is a process called calcination or lime-burning. The heating is applied in order to liberate a molecule of carbon dioxide, leaving only quicklime as the product.

  • This process starts by the thermal decomposition of materials such as seashells or limestones that are rich in calcium carbonate at high temperatures but are maintained such that it does not reach its melting point. 
  • The reaction is an endothermic reaction, and thus carbon dioxide immediately begins to form in the lime kiln. However, it leaves back the white caustic substance called calcium oxide. 
  • It can be obtained by decomposing limestone at a high temperature (approximately 1200 K).
  • image
  • As this reaction is reversible, the carbon dioxide must be removed from the kiln immediately as per the Le-Chatelier’s principle so that reaction can proceed in the forward direction.

Physical properties of Calcium Oxide 

  • It is a white amorphous solid which is odourless.
  • The molar mass of CaO is 56.0774g mol-1.
  • It is soluble in water and glycerol (as it is ionic).
  • The boiling point of calcium oxide is 2870 °C
  • Melting point of calcium oxide is 2572 °C. Due to a high melting point, it is known as a refractory material, i.e., it does not melt on heating and is also hence used to produce light.
  • It is a stable compound and withstands high temperatures.
  • The standard molar entropy associated with calcium oxide corresponds to 40 kJ mol-1.
  • This compound is known to emit an intense glow when it is heated to temperatures above 2643 K.

Chemical properties of Calcium Oxide 

  • It absorbs moisture on exposure to the atmosphere and the product formed is calcium hydroxide known as slaked lime. This process is known as the slaking of lime.


Calcium oxide when absorbs CO2 on exposure to the atmosphere produces calcium carbonate.


  • It is a basic oxide, hence, combines with some acidic oxides at high temperatures and forms their respective salts.


  • It reacts with sulphuric acid to produce calcium sulphate. It's an acid-base reaction.


  • An important reaction sequence involving CaO is given as follows:


Uses of Calcium Oxide 

  • It is used in the manufacturing of cement.
  • It is used in the manufacturing of sodium carbonate from caustic soda.
  • It is used in the manufacturing of dyestuff.
  • It is used in the purification of sugar.
  • It is mostly used for medicinal purposes as well as for the development of insecticides and pesticides
  • It is used in industry to manufacture raw materials such as papers and steel. 
  • It is widely used in laboratories for the chemical processes that require subjecting substances to precipitation and dehydration. 
  • The calcium oxide upon further reactions provides a calcium compound.
  • It is used to determine the ages of various geological sediments as the accumulation of clay and lime in soil profiles shows the result of their translocation over time.

Practice Problem

Q1. Calcium oxide is:

A. Ionic Solid

B. Covalent Solid

C. Covalent Liquid

D. None of the above

Answer: (A)

Solution: Calcium oxide is made of calcium (Ca2+) and (Cl-) ions. So they form ionic bonds and hence it is solid in nature. So Option A is the correct answer.

Q2. The formula of Quicklime is:

A. CaO

B. Ca(OH)2

C. CaCl2

D. CaCO3

Answer: (A)

Solution: Quicklime is the common name for calcium oxide which is also called lime. Calcium hydroxide is called slaked lime in common terms. .

Q3. Why is calcium oxide used as refractory material?

Answer: The melting point of calcium oxide (CaO) is 2572 °C. Due to a high melting point, it is used as a refractory material, as it does not melt easily on heating and is also hence used to produce limelight.

Q4. What is the product obtained from the decomposition of calcium carbonate?

A. Soda-lime

B. Slaked lime

C. Calcium Oxide

D. No reaction

Answer: (C)

Solution: Calcium carbonate on decomposition at a temperature of around 1200 K produces calcium oxide and carbon dioxide.


Frequently Asked Questions-FAQs

Question 1. What is limewater? Is it drinkable?

Answer: Limewater is obtained when calcium oxide reacts with water and forms a calcium hydroxide solution. Yes, it is drinkable. In fact it aids digestion. It mimics the activity of hydrochloric acid in the stomach and helps in preventing indigestion, stomach upsets etc.

Question 2. Is Calcium oxide soluble in water?

Answer: Yes, it is soluble in water and it readily absorbs moisture from the atmosphere. It is also able to absorb carbon dioxide from the atmosphere and forms calcium carbonate.image

Question 3. What are the health-related risks related to calcium oxide?

Answer: Quicklime can cause severe irritation, especially when inhaled or if it comes in contact with wet skin or eyes.

  • Some of the effects of inhalation include sneezing, coughing, or laboured breathing.
  • Also, excess ingestion can cause abdominal pain, nausea burns with perforation of the nasal septum, and vomiting.
  • When quicklime reacts with water it can release enough heat to even ignite combustible materials.

Question 4. Give an example of unusual usage of quicklime?

Answer: They are used to make ‘’Bitan egg’’ or “Century egg’’, which is a Chinese culinary item, which is basically preserved duck, quail, and chicken eggs aged for weeks or even months by using a mixture of clay, ash, salt, quicklime and rice hulls. The yolk turns black and the shell turns brownish and jelly-like. 

They are also famously known as hundred-year eggs, thousand-year eggs.

Related Topics

Calcium Carbonate

Alkali metals

Sodium Chloride

Oxalic Acid

Sodium Carbonate


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