Gypsum and plaster of Paris are both related materials used in construction and art, but they differ in terms of composition and how they are used. Gypsum is a naturally occurring mineral that is mined from the earth, while plaster of Paris is a fine powder made from heating gypsum.
What is the difference between gypsum and plaster of Paris? Gypsum has two molecules of water bonded to each calcium sulfate molecule, whereas Plaster of Paris has one-half molecule of water bonded to each calcium sulfate molecule.
Key Areas Covered
1. What is Gypsum
– Definition, Features
2. What is Plaster of Paris
– Definition, Features
3. Similarities Between Gypsum and Plaster of Paris
– Outline of Common Features
4. Difference Between Gypsum and Plaster of Paris
– Comparison of Key Differences
5. FAQ: Gypsum and Plaster of Paris
– Answers to Frequently Asked Questions
Key Terms
Gypsum, Plaster of Paris
What is Gypsum
Gypsum is a naturally occurring mineral found in abundance as sedimentary rock deposits. Gypsum has the chemical formula CaSO4.2H2O, indicating its composition of calcium sulfate with two water molecules clinging to it. It comes in various forms. Massive gypsum forms white chalky deposits, while there are also beautiful translucent crystals called selenite. Alabaster, a fine-grained variety of gypsum used in sculptures and ornaments.
When crushed and heated to remove some water content, it transforms into plaster of Paris, a fast-setting material perfect for creating molds, casts, and wall plasters. Its fire-resistant properties and ability to take different shapes make it a valuable building material.
Apart from the construction industry, gypsum has applications in agriculture, too. When crushed and spread on the soil, it acts as a soil conditioner, improving drainage and adding valuable calcium and sulfur for plant growth. It also has uses in the production of paper and textiles and even as an ingredient in some fertilizers.
What is Plaster of Paris
Plaster of Paris originates from gypsum in a process called calcination. When gypsum is heated to around 150-200°C, it loses three-fourths of its water content. This dehydration reaction is as follows:
CaSO4 • 2H2O (gypsum) → CaSO4 • ½ H2O (plaster of Paris) + 1.5 H2O (water vapor)
The resulting product is plaster of Paris, with the chemical formula CaSO4 • ½ H2O (calcium sulfate hemihydrate). This “hemihydrate” part indicates it only has half the original water content compared to gypsum.
When plaster of Paris is mixed with water, the remaining water molecules begin to reattach to the calcium sulfate molecules. This rehydration process is essentially the reverse of the calcination reaction. However, it doesn’t simply revert back to gypsum. As the water reattaches, the plaster of Paris particles interlock with each other, forming a rigid crystal lattice. This interlocking structure traps the remaining water molecules within the solid matrix, leading to the hardening of the plaster.
The rehydration process isn’t entirely complete. Even after hardening, a small amount of water remains trapped within the plaster. This trapped water contributes to the limited water resistance of the plaster of Paris.
Plaster of Paris has many uses. In earlier days, casts were used to create casts in the medical field because of their molding ability. Its ability to fill gaps and smooth surfaces makes it a valuable tool for repairing cracks in walls, ceilings, and other building elements.
Similarities Between Gypsum and Plaster of Paris
- Both are primarily composed of calcium sulfate (CaSO4).
- Both gypsum and plaster of Paris have the ability to harden when mixed with water.
Difference Between Gypsum and Plaster of Paris
Definition
- Gypsum is a naturally occurring mineral that is mined from the earth, while plaster of Paris is a fine powder made from heating gypsum.
Water Content
- Gypsum has two molecules of water for every calcium sulfate molecule. Plaster of Paris, on the other hand, is a dehydrated form of gypsum with only half a molecule of water.
Nature
- Gypsum is a soft sulfate mineral found naturally in rock formations. Plaster of Paris is man-made, created by heating gypsum to remove some of its water content.
Casting
- Gypsum sets very slowly and doesn’t have many casting applications. Plaster of Paris sets much faster due to its lower water content, making it ideal for casts, molds, and creating detailed forms.
Uses
- Gypsum is used for manufacturing plaster of Paris, drywall, cement (as a setting agent), and fertilizer, whereas plaster of Paris is used for casting molds, sculptures, casts for broken bones, and wall repairs.
Conclusion
Gypsum and plaster of Paris are calcium sulfate compounds capable of hardening with water. However, the difference between Gypsum and plaster of Paris lies in water content and origin. Gypsum, a naturally occurring mineral, contains two water molecules per calcium sulfate molecule, whereas plaster of Paris is the dehydrated form with only half a water molecule.
FAQ: Gypsum and Plaster of Paris
1. Which is better, Plaster of Paris or gypsum?
Plaster of Paris sets quickly and can be easily molded, making it ideal for artistic and medical casting, as well as for creating decorative elements. Gypsum, however, is useful in construction for drywall and plastering due to its strength, fire resistance, and ability to maintain a stable environment. The choice between plaster of Paris and gypsum will depend on your specific requirements.
2. What is the difference between gypsum and Plaster of Paris formula?
Gypsum has the formula CaSO4·2H2O, which includes two water molecules in its structure. Plaster of Paris has the formula CaSO4·0.5H2O. There are two water molecules attached to calcium sulphate in gypsum whereas there is half a water molecule attached to calcium sulphate in plaster of Paris.
3. Is plaster of Paris waterproof?
No, plaster of Paris is not a waterproof substance. It is a porous material that readily absorbs water, which can lead to structural damage over time, such as cracking and crumbling when exposed to moisture.
4. How can plaster of Paris be converted into gypsum?
To convert plaster of Paris back into gypsum, you simply need to add water. When plaster of Paris is mixed with water, it reacts and forms gypsum as it rehydrates and hardens. This process is reversible and is commonly used in construction and casting applications.
Reference:
1. “Plaster of Paris.” Britannica Encyclopedia.
2. “Gypsum.” Britannica Encyclopedia.
Image Courtesy:
1. “Calcium sulfate hemihydrate” By Martin Walker – Picture taken by Martin Walker in June 2005. (Public Domain) via Commons Wikimedia
2. “Gypsum (31863387400)” By James St. John – Gypsum (CC BY 2.0) via Commons Wikimedia
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