Calcium Sulphate - Definition, Structure, Uses, FAQs

Q: Write the molecular formula of plaster of Paris?

The molecular formula of plaster of Paris is 2Caso 4 .H 2 O

Q: What is the chemical name of 2H2O(H2O2)?

The 2h 2 Ochemical name is hydrogen peroxide.

Q: What is the chemical name of CaSO4?

CaSO 4 chemical name is Calcium sulphate.

Calcium Sulphate - Definition, Structure, Uses, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:57 PM IST

Calcium sulphate is a naturally occurring inorganic calcium compound with the chemical formula CaSO₄ and its related hydrates. It is usually known in its dihydrate state, CaSO₄. 2H₂O. It is a white coloured powder which is barely soluble in water. Calcium sulphate consists of two well-known hydrate forms; one generally known as Plaster of Paris and the other is a naturally occurring mineral Gypsum. It has many industrial uses. They are largely used in many fields especially medical and construction. Calcium sulphate produces permanent hardness of water.

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Structure of calcium sulphate
Calcium sulphate Formula mass
Uses of Calcium Sulphate

Structure of calcium sulphate

Calcium sulphate consists of one calcium atom, one sulphur atom and four oxygen atoms. It is an ionic compound. It contains one cation of calcium and one anion of sulphate. Calcium ion shows +2 valency while polyatomic sulphate ion shows -2 valency. So combining these it forms a neutral compound of calcium sulphate, CaSO₄.

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Hydration states of Calcium Sulphate

Calcium sulphate exists in three hydration states.

i) Anhydrous state: CaSO₄

ii) Dihydrate state: CaSO₄ · 2 H₂O

It is also named as gypsum. CaSO₄ · 2 H₂O chemical name is calcium sulphate dihydrate.

iii) Hemihydrate state: CaSO₄ · 1⁄2 H₂O is the hemihydrate formula

The commercial name of calcium sulphate hemihydrate is Plaster of Paris. Sometimes calcium sulphate hemihydrates are distinguished as α- hemihydrate and β-hemihydrate.

Plaster of Paris (2CaSO₄.H₂O)

It is abbreviated as POP. It is a white powder. It is the hydrated calcium sulphate salt and on mixing with water it changes into gypsum. The chemical name is calcium sulphate hemihydrate. Heating at 393 K gives gypsum. If is heated in high temperature; higher than 393 K it will form anhydrite which is also known as ‘dead burnt plaster’ which on again mixing with water gives gypsum.

Plaster of Paris

Gypsum

Gypsum is a very common mineral composed of calcium sulphate dihydrate having chemical formula CaSO₄·2H₂O and molecular mass of 172.17. Pure gypsum is white in colour while other impurities might give a range of colours. It is used as a fertilizer, soil conditioner, gypsum blocks, and gypsum mortar and as the major component in different forms of plaster, blackboard, and sheet rock or drywall. Gypsum are used for centuries for sculpture, statuary, carvings, and other ornaments.

Solubility

Calcium sulphate solubility is least in water and after solidification it does not dissolve readily with water. In water different crystalline phases of calcium sulphate undergo dissolution and it is exothermic. In calcium sulphate its solubility decreases as temperature increases and solubility increases as temperature decreases.

It’s having retrograde solubility. Some calcium compound such as calcium hydroxide also undergo retrograde solubility, it’s exothermic and releases heat in dissolution reaction. So it is required to decrease temperature of the solution close to its freezing point to get maximum dissolution of calcium sulphate or calcium hydroxide.

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Calcium sulphate Formula mass

Molecular weight calculation;

40.078 + 32.065 + (15.9994*4)

Molecular weight= 136.14 g/mol

Preparation

Calcium sulphate prepared in laboratory by addition of dil. H₂S0₄ to CaCl₂ solution.

CaCl₂ + H₂SO₄ - CaSO₄ + 2HCl

Many chemical processes give calcium sulphate as by-product for example;

Gases from fossil fuel power plants, cement manufacture and other process contain sulphur dioxide. These exhaust gases are scrubbed to decrease the sulphur oxide content by using limestone or lime. In this flue-gas desulfurization process impure calcium sulphite is produced and then it oxidises on storage to give calcium sulphate, calcium sulphate precipitate are formed in phosphoric acid production reaction from phosphate rock, calcium fluoride reacted with sulphuric acid in hydrogen fluoride production also gives calcium sulphate precipitate and calcium sulphate by-products are also produced in the process zinc refining.

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Uses of Calcium Sulphate

Calcium sulphate is mainly used for plaster of Paris and stucco production.
In food industry calcium sulphate is used mainly in making of bread, cookies and brownies. Calcium sulphate serves several functions in baked goods. In baked goods calcium sulphate have different uses as anticaking agent, aid in the preservation of colour in coatings, texturizer, and flour bleaching agent, pH regulator, stabilizer and thickener. Calcium sulphate hydrate utilized as coagulant in some food products such as bean curd. It is permitted in some cheese Products, Bakery Products; Frozen Desserts, Artificial Sweeteners for Jelly and Preserves; Condiment Vegetables and some candies.
In dentistry calcium sulphate is used for the regeneration of bones as a graft material and extender and it act like barrier in particular guided tissue regeneration. It is an oddly biocompatible material and fully resorbed following implantation. It does not evoke a notable host response and make a calcium-rich milieu in particular implanted area. Calcium sulphate is a very common compound having application in several medical and dental procedures. It was used to help treat different types of osseous defects by acting as a bone substitute. It have different therapeutic applications because of its unique property. In dentistry, calcium sulphate gives good results in periodontal infrabony defects. Its restorable and biocompatible specialities utilized in ridge and sinus augmentations, furcation, periodontal intrabony and periapical defects, management of osteomyelitis and even during dental implant placement.
Drierite in its anhydrous state seems to be in blue or pink colour. It is because of the impregnation with cobalt (II) chloride that acts as the moisture indicator.
Calcium sulphate is an excellent soil conditioner and it utilized to enhance soil structure. It provides the needed calcium and sulphur. It has nutrient value. Calcium enhances soil structure and generate oxygen conditions needed for the soil and plants health. This improves the crop quality, hence, enhance yields. Plants are showing more sulphur deficiencies because of reduced levels of sulphur available in the atmosphere. Gypsum is an excellent source of sulphur for nutrition of plants. Calcium and sulphur useful for neutralizing soil activity.
Gypsum converted to calcium sulphate hemihydrate by heating. This has the molecular formula CaSO₄ .1/2 H₂O. 100-150 °C temperature are needed to eliminate the water from its structure. Temperatures of 170 °C are used for industrial calcination, but γ-anhydrite starts to generate at these temperature. The heat energy applied to the gypsum tends to eliminate water as vapour. Partial dehydration is take place here;

CaSO₄.2 H₂O → CaSO₄.1/2 H₂O + 1/2 H₂O↑

This reaction is endothermic, so it is relevant to the drywall performance, nonflammable to residential structures. In a fire, drywall preventing or substantially retarding damage to the framing and consequent structural collapse because part behind the drywall sheet remain cool as water eliminates from gypsum. But calcium sulphate release oxygen and act as oxidizing agent at higher temperature. Calcined gypsum rehydrated by mixing with water at ambient temperatures, it chemically revert to the dihydrate form very quickly, while physically "setting" to form a strong and rigid gypsum crystal lattice;

CaSO₄.1/2 H₂O + 1/2 H₂O↑ → CaSO₄ . 2 H₂O

This is an exothermic reaction and is responsible for the utilization of gypsum to make drywall, sticks for blackboard chalk and other different shapes. Also moulds for metal casting. The dehydration conditions are altered to obtain the α- and β-hemihydrate by adjusting the porosity of the hemihydrate. On heating about 180 °C temperature the nearly water-free form called γ-anhydrite is produced.

Dihydrate state reached when γ-Anhydrite reacts slowly with water. β-Anhydrite is generated on heating above temperature of 250 °C. Natural anhydrite are unreactive with water, besides fine ground. The similar crystal structures having "channels" that can accommodate different amounts of water or other smaller molecules such as methanol is the reason for variable composition of hemihydrate and γ-anhydrite and also their easiness for inter-conversion.

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NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. What are the hydration states of calcium sulphate?

Calcium sulphate are compounds which exists in three different hydration states that includes anhydrous state (CaSO₄), dihydrate state (CaSO₄ · 2 H₂O) it is also called gypsum and hemihydrate state (CaSO₄ · 1⁄2 H₂O) It is also known as Plaster of Paris. Sometimes hemihydrates are distinguished as α- hemihydrate and β-hemihydrate.

2. What is the real purpose of adding gypsum in cement manufacturing?

Gypsum is a compound which slows down the setting of cement.

3. Write the molecular formula of plaster of Paris?

The molecular formula of plaster of Paris is 2Caso₄.H₂O

4. Write laboratory preparation of calcium sulphate and its chemical reaction?

Calcium sulphate prepared in laboratory by addition of dilute sulphuric acid to CaCl₂ solution. Its chemical reaction is

CaCl₂ + H₂SO₄ - CaSO₄ + 2HCl

5. What is the chemical name of 2H2O(H2O2)?

The 2h₂Ochemical name is hydrogen peroxide.

6. What is the chemical name of CaSO4?

CaSO₄ chemical name is Calcium sulphate.

7. Write calcium sulphide formula.

Calcium sulphide formula is CaS.

8. How does calcium sulfate contribute to the hardness of water?

Calcium sulfate contributes to the permanent hardness of water. Unlike temporary hardness caused by calcium bicarbonate, which can be removed by boiling, the hardness from calcium sulfate remains even after boiling. This is because calcium sulfate doesn't decompose or precipitate out when water is heated, making it a persistent source of water hardness.

9. What is the relationship between calcium sulfate and acid rain?

Calcium sulfate can form as a result of acid rain reacting with limestone or marble (calcium carbonate). When sulfuric acid in acid rain reacts with calcium carbonate, it produces calcium sulfate. This reaction can lead to the deterioration of limestone buildings and monuments, a process known as stone leprosy.

10. How does the crystal structure of calcium sulfate change with hydration?

The crystal structure of calcium sulfate changes significantly with hydration. Anhydrous calcium sulfate has a compact orthorhombic structure. When hydrated to form gypsum (CaSO4·2H2O), it adopts a monoclinic structure with water molecules incorporated into the crystal lattice, increasing the spacing between calcium and sulfate ions.

11. How does calcium sulfate interact with water, and why is this important?

Calcium sulfate interacts with water through hydration reactions. Anhydrous calcium sulfate readily absorbs water to form gypsum, while gypsum can lose water when heated to form hemihydrate (plaster of Paris) or anhydrous forms. This reversible hydration process is crucial for applications in construction, molding, and medical casts.

12. How does calcium sulfate compare to calcium carbonate in terms of reactivity?

Calcium sulfate is less reactive than calcium carbonate. While calcium carbonate readily reacts with acids to produce carbon dioxide gas, calcium sulfate is more stable and doesn't effervesce with acids. This difference in reactivity affects their uses; for example, calcium carbonate is used as an antacid, while calcium sulfate is not.

13. How does the structure of calcium sulfate contribute to its properties?

The structure of calcium sulfate consists of Ca2+ ions and SO4^2- ions arranged in a crystal lattice. This ionic structure gives calcium sulfate its characteristic properties, such as high melting point, brittleness, and solubility in water. The strong ionic bonds between calcium and sulfate ions contribute to its stability and hardness.

14. Why does calcium sulfate have a high melting point?

Calcium sulfate has a high melting point (1460°C) due to its strong ionic bonds between Ca2+ and SO4^2- ions in its crystal lattice. These bonds require a significant amount of energy to break, resulting in a high melting point characteristic of many ionic compounds.

15. What is calcium sulfate and why is it considered an s-block compound?

Calcium sulfate is an inorganic compound with the chemical formula CaSO4. It's considered an s-block compound because calcium, its metal component, is found in the s-block of the periodic table. S-block elements have their outermost electrons in s orbitals, which influences their chemical properties and reactivity.

16. What is the difference between anhydrous calcium sulfate and gypsum?

Anhydrous calcium sulfate (CaSO4) contains no water molecules, while gypsum (CaSO4·2H2O) is the dihydrate form of calcium sulfate, containing two water molecules per formula unit. This difference in hydration affects their properties and uses. Anhydrous calcium sulfate is more hygroscopic and reacts more readily with water, while gypsum is more stable and commonly found in nature.

17. How does the solubility of calcium sulfate compare to other calcium salts?

Calcium sulfate is slightly soluble in water (about 2.4 g/L at 20°C), which is lower than many other calcium salts like calcium chloride or calcium nitrate. This relatively low solubility is due to the strong lattice energy of the calcium sulfate crystal, which must be overcome for dissolution to occur.

18. What role does calcium sulfate play in the setting of cement?

Calcium sulfate, typically in the form of gypsum, is added to cement clinker to control the setting time of cement. It reacts with tricalcium aluminate in the cement to form ettringite, which slows down the initial setting process, allowing for proper workability and preventing flash setting.

19. How does calcium sulfate contribute to soil amendment?

Calcium sulfate, often applied as gypsum, is used as a soil amendment to improve soil structure and provide calcium and sulfur nutrients. It helps to reduce soil compaction, improve water penetration, and can help remediate sodic soils by replacing excess sodium with calcium.

20. What is the environmental impact of calcium sulfate production and use?

The production of calcium sulfate through mining or industrial processes can have environmental impacts, including energy consumption and potential habitat disruption. However, calcium sulfate is generally considered non-toxic and can be recycled. Its use in construction can contribute to more sustainable building practices when used to replace more energy-intensive materials.

21. What is the significance of calcium sulfate in the food industry?

In the food industry, calcium sulfate is used as a coagulating agent, dough conditioner, and source of calcium fortification. It's commonly used in tofu production to help coagulate soy milk, and in baking to strengthen dough and provide calcium. Its low solubility makes it a stable additive in many food products.

22. How does plaster of Paris relate to calcium sulfate?

Plaster of Paris is a partially dehydrated form of calcium sulfate, with the formula CaSO4·½H2O. It's produced by heating gypsum to remove about 75% of its water content. When mixed with water, plaster of Paris rehydrates and sets, making it useful for creating molds, casts, and construction materials.

23. What is the environmental impact of gypsum mining for calcium sulfate production?

Gypsum mining for calcium sulfate production can have several environmental impacts:

24. How does calcium sulfate compare to other calcium salts in terms of bioavailability?

Calcium sulfate has moderate bioavailability compared to other calcium salts:

25. What are the health effects of calcium sulfate exposure?

Calcium sulfate is generally considered non-toxic and safe for most applications. However, inhalation of calcium sulfate dust can irritate the respiratory system. Long-term exposure to high levels of airborne calcium sulfate, particularly in industrial settings, may lead to respiratory issues. When used as a food additive or supplement, it's considered safe by regulatory agencies.

26. What is the role of calcium sulfate in art and sculpture?

Calcium sulfate, particularly in the form of plaster of Paris, is widely used in art and sculpture. Its ability to be easily molded when mixed with water and then harden into a solid form makes it ideal for creating casts, molds, and sculptural elements. It's also used in fresco painting as a smooth surface for applying pigments.

27. How does the presence of calcium sulfate affect the pH of soil or water?

Calcium sulfate is a neutral salt and doesn't significantly affect the pH of soil or water when dissolved. This neutrality makes it useful in agriculture for adding calcium to soils without altering pH, unlike lime (calcium carbonate) which increases pH. In water treatment, it can be used to add hardness without changing the pH balance.

28. What role does calcium sulfate play in the brewing industry?

In brewing, calcium sulfate (often referred to as gypsum) is used to adjust water chemistry, particularly in the production of certain beer styles. It increases the sulfate content of the water, which can enhance hop bitterness and create a drier finish in the beer. This process is known as "Burtonisation," named after the water profile of Burton-upon-Trent, England.

29. How does the thermal decomposition of calcium sulfate occur?

The thermal decomposition of calcium sulfate occurs in stages as it's heated:

30. What is the significance of calcium sulfate in the paper industry?

In the paper industry, calcium sulfate is used as a filler and coating pigment. It improves the brightness, opacity, and smoothness of paper. Its use can also enhance the printability of paper and reduce the amount of more expensive materials like cellulose fibers needed in paper production.

31. How does calcium sulfate affect the setting time of Portland cement?

Calcium sulfate, usually added as gypsum, is crucial in controlling the setting time of Portland cement. It reacts with tricalcium aluminate (C3A) in the cement to form ettringite, which coats the C3A particles and slows down their hydration. This prevents flash setting and allows for proper workability of the cement paste.

32. What is the role of calcium sulfate in the production of sulfuric acid?

Calcium sulfate is not directly used in the production of sulfuric acid, but it's a byproduct of some sulfuric acid production methods. In the lead chamber process, calcium sulfate can form when sulfuric acid reacts with limestone scrubbers used to neutralize excess sulfur dioxide. Understanding this helps in managing byproducts in industrial processes.

33. How does calcium sulfate contribute to the fireproofing of buildings?

Calcium sulfate, particularly in its hydrated form as gypsum, is an excellent fire-resistant material. When exposed to fire, the water molecules in gypsum are released as steam, absorbing heat and slowing the spread of fire. This property makes gypsum board (drywall) an important component in fireproofing buildings.

34. What is the difference between α-hemihydrate and β-hemihydrate forms of calcium sulfate?

Both α-hemihydrate and β-hemihydrate have the same chemical formula (CaSO4·½H2O), but differ in their crystal structure and properties:

35. How does calcium sulfate affect plant growth and agriculture?

Calcium sulfate (gypsum) benefits plant growth and agriculture in several ways:

36. What is the relationship between calcium sulfate and anhydrite?

Anhydrite is the anhydrous form of calcium sulfate (CaSO4), while gypsum is the dihydrate form (CaSO4·2H2O). Anhydrite can slowly hydrate to form gypsum when exposed to water, and conversely, gypsum can dehydrate to form anhydrite under certain conditions. This relationship is important in geological processes and industrial applications.

37. How does calcium sulfate interact with other ions in solution?

In solution, calcium sulfate dissociates into Ca2+ and SO4^2- ions. These ions can interact with other ions present:

38. What is the significance of calcium sulfate in dental materials?

Calcium sulfate, particularly in its hemihydrate form (plaster of Paris), is used in dentistry for:

39. How does the solubility of calcium sulfate change with temperature?

The solubility of calcium sulfate in water exhibits an unusual characteristic:

40. What is the role of calcium sulfate in the production of tofu?

In tofu production, calcium sulfate acts as a coagulant. When added to heated soy milk, it causes the proteins to coagulate and form curds. The calcium ions from calcium sulfate cross-link with the soy proteins, creating the characteristic texture of tofu. It also adds calcium to the final product, enhancing its nutritional value.

41. How does calcium sulfate contribute to the properties of drywall?

Calcium sulfate, in the form of gypsum, is the main component of drywall. It contributes to drywall's properties by:

42. How does calcium sulfate affect the taste and texture of foods when used as an additive?

As a food additive, calcium sulfate generally has minimal impact on taste due to its low solubility and neutral flavor. However, it can affect texture:

43. What is the role of calcium sulfate in the production of Portland cement?

In Portland cement production, calcium sulfate (usually added as gypsum) serves several important functions:

44. What are the crystalline forms of calcium sulfate and how do they differ?

Calcium sulfate exists in several crystalline forms:

45. How does calcium sulfate interact with proteins in food systems?

Calcium sulfate can interact with proteins in food systems in several ways:

46. What is the significance of calcium sulfate in the wine industry?

In the wine industry, calcium sulfate (often added as gypsum) is used for several purposes: