Calcium Carbonate

Definition

Calcium Carbonate or CaCO₃ is a widely available natural compound. CaCO₃ is produced naturally by the sedimentation of the shells of fossilised creatures like snails or corals. The sedimented shells, after a long time, get transformed into lime limestone, marble or chalk.


As the name suggests, this compound is made up of Calcium and Carbonate ions. The surprising thing is that Calcium is not found in an isolated state in nature. It is found in the form of calcium carbonate and other compounds. Hence calcium carbonate is extremely important academically and medically.


Chemical Structure of Calcium Carbonate

Now, as Wayne Breslyn rightly says, if you want to draw the Lewis Dot Structure of calcium carbonate, you must first observe its name carefully and then look at the periodic table. What do you see? There are atoms of three different elements that make up calcium carbonate. So it is a Ternary ionic compound. Why is it ionic? Calcium is positively charged. It is 2+. On the other hand, the two out of three oxygen atoms have 8 valence electrons each instead of the usual 6. Hence it is 2- or in other words, negatively charged. Now that you know the background, here is the Lewis Dot structure of CaCO₃ -


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Sources of Calcium Carbonate

The primary natural source of calcium carbonate is limestone. Limestone is a sedimentary rock. As the name suggests, limestones form as a result of the sedimentation of seashells, bodies of coral organisms, algal debris etc. The great thing about limestone is, we do not need to search hard to get it. It is formed in shallow marine waters that have warm temperatures. We get calcium carbonate when the sediments of the bodies of marine creatures which have shells made of calcium carbonate remain accumulated for an extremely long time. This sedimentation results in the formation of the sedimentary rock - limestone.


Bear in mind, limestone is a rock. There are many minerals that are present in the limestone. However, calcite is its primary composition. And this calcite is a form of Calcium Carbonate.


You can also get Calcium Carbonate from Marble. Since Marble is made up of Limestone ( by metamorphosis), it has calcite in it. In fact, it is the calcite that gets recrystallised and turns into Marble. Chalk is also a source of Calcium carbonate.


Ground Calcium Carbonate Vs Precipitated Calcium Carbonate

There are two ways of obtaining Calcium Carbonate. Of course, we cannot directly use limestone, marble or chalk as calcium carbonate. The limestone is micronised by grinding it. The calcium carbonate that we get through grinding is called ground calcium carbonate. Since limestone has other materials in it, the ground powdery calcium carbonate can have many impurities. 


While ground calcium carbonate is quite granular, the end product is not that satisfiable and hence the applications of ground calcium carbonate are limited. Precipitated calcium carbonate is much more refined and free of impurities. At first, the calcium carbonate rock is ground and then the granular rock is heated. This results in the production of lime and carbon dioxide.


CaCO₃ → Heat = CaO + CO₂


The resulting lime is then mixed with water. This results in the production of slake - water mixed lime.


CaO + H₂O = Ca(OH)₂


This hydrated lime is then treated with carbon dioxide. The end product is CaCO₃ and H₂O.


Uses of Calcium Carbonate

  1. In the medical world, Doctors prescribe calcium carbonate as a dietary supplement to the patients who have less than required levels of calcium in their bodies. Calcium is extremely important to human beings. It strengthens the bones, teeth and even heart.

  2. Due to its white colour, Calcium carbonate is used in the paper industry, to give the papers a bright, polished look. It is used as a filler material.

  3. Just like we need calcium, plants too need calcium to a certain extent. Calcium carbonate is used to provide the required levels of calcium to the plants. CaCO₃ also maintains the proper pH level of the soil.

  4. Since Calcium Carbonate is non-toxic, it is added to food as an additive.

  5. CaCO₃ is also used as a cleaning agent in sewers.


Properties of Calcium Carbonate

  • Calcium carbonate is a naturally occurring compound found in limestones, marbles or chalks.

  • It can be in the form of calcite, aragonite or vaterite.

  • Ground calcium carbonate is white fluffy powder.

  • It does not dissolve in water.

  • If you heat Calcium carbonate, it will turn into CaO or Calcium oxide.

  • It will give out carbon dioxide if it reacts with dilute acids like H₂SO₄ or HCl.

  • Calcium carbonate is the one that gives hardness to the eggshells.


Thermal Decomposition

There are many metal carbonates that decompose and give out CO₂ when heated. This is called thermal decomposition. The same thing happens in the case of calcium carbonate. When heat is applied to CaCO₃, it breaks into CaO and CO₂.


CaCO₃ → Heat → CaO + CO₂


Limestone Cycle

Limestone cycle is nothing but the entire process of producing precipitated calcium carbonate from ground calcium carbonate. So the cycle begins when the ground CaCO₃ is thermally decomposed and we get CaO and H₂O. This calcium oxide is then made to react with water. This produces Ca(OH)2. Remember, we already have water. Ca(OH)2 dissolves in water. This mixture, when treated with CO₂, gives back CaCO₃. In this way, the cycle gets complete.


Here Is A Recap 

  • Calcium carbonate can be found in the form of calcite, aragonite and vaterite in rocks like limestone, marble etc.

  • Calcium Carbonate is made up of calcium cation and Carbonate anion.

  • There are two producing calcium carbonate. One is by grinding the calcite rich rocks like limestone and the other is by precipitation of calcium carbonate where we turn ground CaCO₃ into calcium oxide and then we treat it with water and carbon dioxide to get a more refined version of calcium carbonate.

  • Calcium carbonate is insoluble in water.

  • Calcium carbonate is mainly used as medicine, fillers and food additives.

  • Calcium carbonate is responsible for the hardness of eggshells.

Calcium carbonate is one of the most widely used inorganic substances. The beauty of CaCO₃ is it is not that hard to obtain from natural sources and at the same time, it is extremely useful. CaCO₃ is thus unique.

FAQ (Frequently Asked Questions)

1. What is Calcium Carbonate?

Ans. Calcium carbonate is a chemical compound found in the minerals like calcite or aragonite. It is inorganic. It occurs as calcite or aragonite in the rocks like limestone, marble etc.

2. How is Limestone Formed?

Ans. There are many sea organisms that have shells. For example snails, corals etc. When these sea organisms die, their bodies keep on getting accumulated in the sea bed. As a result of the accumulation of these organisms for a long period of time, they get converted into rocks. These rocks are called sedimentary rock. Limestone is a kind of sedimentary rock.

3. How is Marble Formed?

Ans. After the formation of limestone, heat and pressure can turn these limestones into marble. Marble is a kind of metamorphic rock that has the recrystallised form of calcite or dolomite.

4. What are the Three Main Forms of Calcium Carbonate?

Ans. The three main forms of calcium carbonate are-

  1. Calcite

  2. Aragonite

  3. Vaterite

5. What is Ground Calcium Carbonate?

Ans. The calcium carbonate that is obtained through grinding the limestones or marble to make them grainy is called ground calcium carbonate. Since limestone has other materials in it, the CaCO₃ obtained with the help of this method is not free of impurities.

6. What is Precipitated Calcium Carbonate?

Ans. Calcium carbonate can be obtained with the help of the precipitation method. The CaCO₃ obtained with this method is called precipitated CaCO₃. At first, the calcium carbonate rock is ground and then the granular rock-powder is heated. This results in the production of Calcium oxide and carbon dioxide.


CaCO₃ → Heat = CaO + CO₂ ( Thermal decomposition)


Then we produce lime water by adding H₂0 to CaO.


CaO + H₂O = Ca(OH)₂


This hydrated lime is then treated with carbon dioxide. The end product is CaCO₃ and H₂O.

7. What are the Properties of Calcium Carbonate?

Ans. The properties of CaCO₃:

  • Calcium carbonate is a naturally occurring compound found as calcite or aragonite or vaterite in rocks like limestone, marble etc. 

  • Ground calcium carbonate is white fluffy powder.

  • It is not soluble in water.

  • If you heat Calcium carbonate, it will result in thermal decomposition and you will get CaO and CO₂.

  • When CaCO₃ reacts with dilute acids like H₂SO₄ or HCl, it gives out CO₂

  • The hardness of eggshells comes from CaCO₃.

8. What are the Primary Uses of CaCO₃?

Ans. CaCO₃ is primarily used as filler in the paper industry, as medicine and as food additives. It is also used as a cleaning agent.