What is Calcium Sulphate formula preparation reactions and uses
Structure of Calcium Sulphate
Calcium sulphate contains one atom of calcium and Sulphur and four atoms of oxygen. It is an ionic compound which constitutes one calcium cation and one sulphate anion (polyatomic ion). Calcium ion has +2 valency while sulphate polyatomic ion shows -2 valency. So, on combining they form a neutral compound calcium sulphate (CaSO4). Structure is given below-
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Hydrous and Anhydrous Forms of Calcium Sulphate
Plaster of Paris and Gypsum
Plaster of Paris and gypsum are hydrated forms of calcium sulphate. These two are well known hydrated forms of calcium sulphate as these are largely used in various fields especially medicinal and construction fields.
Plaster of Paris is a white powder which is hydrated calcium sulphate salt and on mixing with water changes into gypsum. Its chemical name is calcium sulphate hemihydrate. It is also known as POP (abbreviated form of Plaster of Paris). It is used by doctors as plaster to support the fractured bones in the right position. It is prepared by heating gypsum at 373K. reaction is given below –
CaSO4.2H2O heat (373K) → CaSO4.12 H2O + 1.5H2O
Gypsum Plaster of Paris
Chemical name of gypsum is calcium sulphate dihydrate. Its chemical formula is CaSO4.2H2O. It is used for protective and decorative coating of walls, ceilings and decorative elements. It cannot be molded into different shapes. It occurs naturally while Plaster of Paris is a man-made material. Although gypsum can also be prepared by plaster of Paris. When water is mixed with Plaster of Paris it becomes hard and forms gypsum. Reaction is given below –
CaSO4.12 H2O + 112H2O 🡪 CaSO4.2H2O
Plaster of Paris Gypsum
Uses of Calcium Sulphate
Calcium sulphate is used in various ways in numerous fields. Some of its uses are given below-
It is used in construction work.
It is used in decorative materials.
It is used as mortar.
It is used in surgical splints, casting, molds and modeling.
It is used in fertilizers and soil conditioners.
It is used as alabaster in sculptures.
It is a major source of dietary calcium.
It is used for adding hardness to water used for brewing.
It is used in baking.
It is a component of Portland cement.
It is a common ingredient in making mead.
It is used in foot creams, shampoos and other products.
It is largely used in dentistry.
It is used in mushroom cultivation.
Gypsum can be used to remove cancerous pollutants such as lead or arsenic from contaminated water.
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FAQs on Calcium Sulphate Structure Properties and Applications
1. What is calcium sulphate?
Calcium sulphate is an inorganic compound with the chemical formula CaSO4, composed of calcium ions and sulphate ions. It consists of Ca2+ and SO42- ions held together by ionic bonding. It occurs naturally as the mineral gypsum and is widely used in construction, medicine, and laboratory chemistry. Calcium sulphate exists in different hydrated forms depending on the amount of water present.
2. What is the chemical formula of calcium sulphate?
The chemical formula of calcium sulphate is CaSO4. It is formed by combining one Ca2+ ion with one SO42- ion to balance charges. Common hydrated forms include:
- CaSO4·2H2O (gypsum)
- CaSO4·½H2O (plaster of Paris)
3. What are the different forms of calcium sulphate?
Calcium sulphate exists in three main forms depending on hydration state. The major forms are:
- Anhydrous calcium sulphate: CaSO4
- Gypsum: CaSO4·2H2O
- Plaster of Paris: CaSO4·½H2O
4. How is plaster of Paris prepared from calcium sulphate?
Plaster of Paris is prepared by heating gypsum (CaSO4·2H2O) at about 100–120°C to partially remove water. The reaction is:
CaSO4·2H2O(s) → CaSO4·½H2O(s) + 3/2H2O(g)
This controlled dehydration forms calcium sulphate hemihydrate, commonly called plaster of Paris.
5. Is calcium sulphate soluble in water?
Calcium sulphate is sparingly soluble in water. At room temperature, only a small amount of CaSO4 dissolves to form Ca2+(aq) and SO42-(aq). Its low solubility distinguishes it from highly soluble sulphates like sodium sulphate (Na2SO4). The limited solubility is due to its relatively high lattice energy compared to hydration energy.
6. What happens when calcium sulphate reacts with water?
Anhydrous or hemihydrate calcium sulphate reacts with water to form gypsum (CaSO4·2H2O). For example, plaster of Paris reacts as follows:
2(CaSO4·½H2O)(s) + 3H2O(l) → 2CaSO4·2H2O(s)
This reaction is exothermic and causes the material to harden, which is why plaster of Paris is used for making casts and moulds.
7. What are the uses of calcium sulphate in everyday life?
Calcium sulphate is widely used in construction, medicine, and industry. Common uses include:
- Gypsum (CaSO4·2H2O) for drywall and cement
- Plaster of Paris for casts, moulds, and sculptures
- As a soil conditioner in agriculture
- As a drying agent in laboratories (anhydrous CaSO4)
8. How do you calculate the molar mass of calcium sulphate?
The molar mass of calcium sulphate (CaSO4) is approximately 136.14 g/mol. It is calculated as:
- Ca = 40.08 g/mol
- S = 32.06 g/mol
- O4 = 4 × 16.00 = 64.00 g/mol
9. What is the difference between gypsum and plaster of Paris?
The main difference is their water content: gypsum is CaSO4·2H2O, while plaster of Paris is CaSO4·½H2O. Key differences include:
- Gypsum contains two water molecules per formula unit.
- Plaster of Paris contains half a water molecule per formula unit.
- Plaster of Paris reacts with water to reform gypsum and harden.
10. How is calcium sulphate formed in a chemical reaction?
Calcium sulphate is commonly formed by reacting calcium compounds with sulphuric acid. For example:
CaCO3(s) + H2SO4(aq) → CaSO4(s) + CO2(g) + H2O(l)
This is an acid–carbonate reaction that produces solid CaSO4, carbon dioxide gas, and water. It is a typical laboratory and industrial method of preparing calcium sulphate.
