Intermediate Topic 630 of 1102 Inorganic Chemistry
Plaster of Paris
Calcium sulfate hemihydrate that sets to gypsum on adding water.
In-depth explanation of Plaster of Paris
Plaster of Paris is calcium sulfate hemihydrate, CaSO4 x 1/2 H2O, made by heating gypsum, CaSO4 x 2H2O, to about 120 to 150 degrees C to drive off part of its water. When mixed with water it rehydrates to gypsum, forming a paste that sets within minutes into an interlocking mass of tiny crystals. The mass expands very slightly as the crystals grow, which lets plaster fill moulds accurately. It is used for moulding, decorative ceilings and surgical casts. It sets without further heating, and overheating gives anhydrous calcium sulfate, which sets slowly.
Going deeper: Plaster of Paris
Key ideas: CaSO4.1/2H2O is made by heating gypsum CaSO4.2H2O at about 120 C; 2(CaSO4.2H2O) gives 2(CaSO4.1/2H2O) + 3H2O. Mixed with water, it sets within minutes into interlocking gypsum needles and expands slightly, which is why it fills molds. Heating above 200 C gives dead burnt plaster, which no longer sets.
Common mistakes and connections: Students write the hemihydrate as CaSO4.H2O or forget the fractional 1/2. Overheating ruins setting, since anhydrite hydrates very slowly. The setting is hydration, not drying; it needs water. Used in casts, sculpture and dentistry. Links to hydrates, stoichiometry of water of crystallization and calcium compounds.
Visualization of Plaster of Paris
Plaster sets as the hemihydrate rehydrates into interlocking gypsum crystals, a rigid ordered solid network that holds a mould shape. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A plaster cast for a broken arm is made by mixing plaster powder with water and letting it harden around the limb.
Frequently asked questions about Plaster of Paris
What is Plaster of Paris?
Calcium sulfate hemihydrate that sets to gypsum on adding water.
Is Plaster of Paris a beginner, intermediate or advanced topic?
Plaster of Paris is a intermediate topic in Inorganic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Plaster of Paris?
Study these earlier topics in Inorganic Chemistry first: Solvay Process, Alkaline Earth Metals, Calcium Compounds.
What comes after Plaster of Paris?
The next topics in Inorganic Chemistry are Cement and Magnesium Chemistry.
Can I practise Plaster of Paris with quizzes?
Yes. The interactive lesson includes a self-check question on Plaster of Paris, and the Inorganic Chemistry quiz draws on every topic in this area.
Where Plaster of Paris fits in Inorganic Chemistry
Plaster of Paris is topic 15 of 113 in Inorganic Chemistry.