Differences Between Hydrous and Anhydrous Calcium Sulphate
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 Explained: Structure, Forms & Applications
1. What is Calcium Sulphate and what is its chemical formula?
Calcium Sulphate is an inorganic compound composed of calcium and sulphate ions. It is a naturally occurring calcium salt with the chemical formula CaSO₄. It is most commonly found as the mineral gypsum, which is a soft, white or greyish solid. Calcium sulphate is known for its low solubility in water and is central to the production of many construction and industrial materials.
2. What are the different forms of Calcium Sulphate, and how do they differ?
Calcium Sulphate primarily exists in three main forms, distinguished by the amount of water of crystallisation they contain:
- Gypsum (CaSO₄·2H₂O): This is calcium sulphate dihydrate, containing two molecules of water. It is the most common natural form and is used in manufacturing cement and wallboard.
- Plaster of Paris (CaSO₄·½H₂O): This is calcium sulphate hemihydrate, containing half a molecule of water per molecule of CaSO₄. It is made by heating gypsum.
- Anhydrite (CaSO₄): This is the anhydrous form, containing no water molecules. It is often referred to as 'dead burnt plaster' when produced by overheating gypsum.
3. How is Plaster of Paris prepared from gypsum?
Plaster of Paris is prepared by carefully heating gypsum (Calcium Sulphate Dihydrate) to a temperature of about 393 K (120°C) in a kiln. During this process, gypsum loses a part of its water of crystallisation to form Calcium Sulphate Hemihydrate. The chemical reaction is: 2(CaSO₄·2H₂O) → 2(CaSO₄)·H₂O + 3H₂O. It is crucial not to overheat the gypsum, as this would remove all water and create anhydrous calcium sulphate, which does not set like plaster.
4. What are the main applications and uses of Calcium Sulphate?
Calcium Sulphate, in its various forms, has a wide range of important applications:
- Construction: Gypsum is a key component in Portland cement and is used to make plasterboards (drywall). Plaster of Paris is used for creating moulds, casts for statues, and decorative interior finishes.
- Medical: Plaster of Paris is widely used to make orthopaedic casts to immobilise broken bones.
- Agriculture: Gypsum is used as a soil conditioner to improve soil structure and provide calcium and sulphur nutrients for plant growth.
- Food Industry: Food-grade calcium sulphate is used as a coagulant in making tofu, a firming agent in canned vegetables, and a stabiliser in some dairy products.
5. Why does Plaster of Paris harden when mixed with water?
The hardening of Plaster of Paris is a chemical process involving rehydration. When water is added to Plaster of Paris (calcium sulphate hemihydrate), it transforms back into gypsum (calcium sulphate dihydrate). This reaction is exothermic and results in the formation of an interlocking mass of hard, needle-like gypsum crystals. This crystalline structure is responsible for the rigid, solid mass that sets firmly, a property that makes it useful for casts and moulds.
6. What is the fundamental difference between Calcium Sulphate and Calcium Sulphite?
The key difference between Calcium Sulphate and Calcium Sulphite lies in the anion bonded to the calcium ion (Ca²⁺).
- Calcium Sulphate contains the sulphate ion, which has the formula SO₄²⁻. Its chemical formula is CaSO₄.
- Calcium Sulphite contains the sulphite ion, which has the formula SO₃²⁻. Its chemical formula is CaSO₃.
7. What happens if gypsum is overheated during the preparation of Plaster of Paris?
If gypsum is heated above 393 K (approximately 120-130°C), it loses all its water of crystallisation. This results in the formation of anhydrous calcium sulphate (CaSO₄), a substance known as 'dead burnt plaster'. Unlike Plaster of Paris, dead burnt plaster loses the characteristic property of setting and hardening when mixed with water. Therefore, precise temperature control is essential for the successful industrial production of Plaster of Paris.
8. How does the ionic structure of Calcium Sulphate influence its physical properties?
Calcium Sulphate is an ionic compound consisting of calcium cations (Ca²⁺) and sulphate anions (SO₄²⁻) held together by strong electrostatic forces in a crystal lattice. This strong ionic bonding is responsible for its high melting point and its nature as a crystalline solid. Its limited solubility in water is due to the high lattice energy of the crystal, which is not easily overcome by the hydration energy of the ions. The ability of its lattice to incorporate water molecules (as in gypsum and Plaster of Paris) is fundamental to its most important uses.
