Calcium Carbonate and Calcium Sulphate(Plaster of Paris)

Imagine a world with no concrete structures, flowing fresh water, or any of the exquisite artworks. These seemingly very basic utilities all find their root in common, yet imperative, chemical compounds. Among the myriad vital roles that these compounds have played, the most striking have been those for Calcium Carbonate and Calcium Sulfate. Be it the limestone forming its very foundation, the chalk that aids us in our educational pursuits, or the hard, detailed plaster of Paris casts that seem to immortalize art, these appear as precious compounds.

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This Story also Contains

1. Understanding Calcium Carbonate and Calcium Sulphate
2. Some Solved Examples
3. Summary

Understanding Calcium Carbonate and Calcium Sulphate

Definitions and Explanations

Calcium carbonate, CaCO₃ is obtained in most rocks, primarily as the minerals calcite and aragonite. It forms the principal constituents of pearls, shells of marine organisms, and eggshells. Calcium sulphate, CaSO₄ occurs in a number of hydrated forms; the most well-known are gypsum, CaSO₄·2H₂O, and plaster of Paris, CaSO₄·0.5H₂O. Gypsum is converted to plaster of Paris on heating at about 150°C when it loses three-quarters of its water. Both find applications in various industrial and biological fields whereby their chemical composition and formation are core to their other applications more significantly.

Calcium Carbonate, CaCO3
In nature, it occurs as limestone, ice land spar, marble, and shells of sea animals.

Preparation
In the laboratory, it is prepared by passing CO₂ through lime water or by adding sodium carbonate solution into calcium chloride as follows:

$\begin{aligned} & \mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2 \rightarrow \mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O} \\ & \mathrm{CaCl}_2+\mathrm{Na}_2 \mathrm{CO}_3 \longrightarrow \mathrm{CaCO}_3+2 \mathrm{NaCl}\end{aligned}$

Uses

• It is used in the preparation of cement, and washing soda (NaHCO₃ by Solvay method).
• In the extraction of many metals like iron.
• Marble is used as a building material.
• Precipitated chalk is used in the manufacture of paints, medicines toothpaste, etc.

Calcium Sulphate (Plaster of Paris), CaSO4·1⁄2 H2O
It is known as calcium sulphate hemihydrate

Preparation
It is obtained when gypsum, CaSO_4.2H₂O, is heated to 393 K.

$2\left(\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}\right) \rightarrow 2\left(\mathrm{CaSO}_4\right) \cdot \mathrm{H}_2 \mathrm{O}+3 \mathrm{H}_2 \mathrm{O}$

Above 393 K, no water of crystallization is left and anhydrous calcium sulphate, CaSO₄ is formed. This is known as ‘dead burnt plaster’. It has a remarkable property of setting with water. On mixing with an adequate quantity of water it forms a plastic mass that gets into a hard solid in 5 to 15 minutes.

Uses

• Plaster of Paris is used for plaster of broken bones, making statues, toys, chalks, etc.
• It is also used as a building material.
• It is used for making molds for casting.

Real-Life Relevance and Applications

Calcium carbonate plays a very significant role in construction due to its inclusion in cement and concrete. It also acts as a dietary calcium supplement and an antacid. In the paper industry, it is used as a filler and coating material. This application provides more enhanced quality to the paper. Calcium sulphate, especially in the form of Plaster of Paris, is very important to medicine in making casts and splints.

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Some Solved Examples

Example 1

Question:

Which of the following options is true regarding the use of plaster of Paris?

1. It can be used in the building industry
2. Used in dentistry
3. Can be used for the immobility of the area of a fractured bone
4. All of these

Solution:

Plaster of Paris is widely used in various fields:

• It is used in the building industry for creating decorative elements.
• It is used in dentistry for making dental casts.
• It is used for immobilizing fractured bones by creating casts around the injured area.

Hence, the answer is option (4).

Example 2

Question:

Which of the following contains CaCO₃?

1. Chalk
2. Limestone
3. Marble
4. All of these

Solution:

Calcium Carbonate (CaCO₃) is found in all the given substances:

• Chalk is a form of CaCO₃.
• Limestone is primarily composed of CaCO₃.
• Marble is a metamorphic rock that also contains CaCO₃.

Hence, the answer is option (4).

Example 3

Question:

When CO₂ is passed through X, the solution turns milky white due to the formation of CaCO₃. What is X?

1. Ca(OH)₂
2. CaCO₃
3. CaO
4. None of these

Solution:

When CO₂ is passed through Ca(OH)2 (lime water), a milky white precipitate of CaCO₃ is formed as per the reaction:

Ca(OH)₂+CO₂→CaCO₃+H₂O

Hence, the answer is option (1).

Summary

Calcium carbonate and calcium sulphate represent those chemical compounds, which much more than existence, help at many levels in daily life and industries. From buildings and road construction to the execution of fine arts or devices used for medical purposes, they have a lot to do with everything. Thus, one who can know their properties, types, and uses enhances his respect toward such substances and, at the same time, points to the value of these materials as they are related to practical and academic aspects.