The chemical formula of plaster of Paris is CaSO4·½H2O, and its chemical name is calcium sulphate hemihydrate. It is made by heating gypsum (CaSO4·2H2O) until three quarters of the water of crystallisation is driven off, and it turns back into gypsum within minutes of being mixed with water, setting into a hard white solid. Its molar mass is 145.15 g/mol.
Plaster of Paris formula and chemical name
| Property | Value |
|---|---|
| Chemical formula | CaSO4·½H2O (also written (CaSO4)2·H2O) |
| Chemical name | Calcium sulphate hemihydrate |
| Other names | POP, plaster, calcined gypsum, hemihydrate plaster |
| Molar mass | 145.15 g/mol |
| CAS number | 10034-76-1 |
| Appearance | Fine white or off-white powder |
| Density | About 2.6 g/cm³ |
| Solubility in water | Sparingly soluble, roughly 0.3 g per 100 mL at 20 °C |
| Made from | Gypsum, CaSO4·2H2O, molar mass 172.17 g/mol |
The name comes from the large gypsum deposits at Montmartre in Paris, which were quarried and calcined on a commercial scale from the eighteenth century onwards.
Why is plaster of Paris written with half a water molecule?
A single molecule cannot carry half a water molecule, and this is the part of the formula that confuses most students. The half is an average.
In the crystal, one water molecule is shared between two formula units of calcium sulphate. Share one H2O among two CaSO4 and each one owns half, so the formula per unit of calcium sulphate is CaSO4·½H2O. If you prefer whole numbers, write the same compound as (CaSO4)2·H2O or 2CaSO4·H2O. The prefix “hemi” in hemihydrate means exactly this: half.
Compare the three members of the family and the arithmetic becomes obvious:
- Gypsum, CaSO4·2H2O: two water molecules per calcium sulphate (the dihydrate).
- Plaster of Paris, CaSO4·½H2O: half a water molecule per calcium sulphate (the hemihydrate).
- Anhydrite, CaSO4: no water at all.
Preparation of plaster of Paris from gypsum
Plaster of Paris is made by calcining gypsum, which means heating it gently so it loses most of its water of crystallisation without decomposing. The balanced equation is:
2(CaSO4·2H2O) → 2(CaSO4·½H2O) + 3H2O
Written per formula unit, the same reaction is CaSO4·2H2O → CaSO4·½H2O + 1½H2O. Each calcium sulphate unit starts with 2 water molecules and keeps 0.5, so it loses 1.5, and 75% of the original water leaves as steam. By mass, 172.17 g of gypsum yields 145.15 g of plaster of Paris and 27.02 g of water, a loss of about 15.7%.
What temperature? The 373 K and 393 K question
Textbooks quote different numbers and both are defensible, so learn them as a range with a ceiling rather than as one figure:
- NCERT Class 10 Science states the reaction as gypsum heated to 373 K (100 °C).
- NCERT Class 11 Chemistry, in the s-block unit, gives 393 K (120 °C).
- Industrial calcination of the ordinary beta form runs at roughly 120–160 °C, because a plant needs the reaction to finish in minutes rather than hours.
The safe way to answer an exam question is to give the figure your own textbook uses, then add the rule that actually matters: the temperature must not go far above 393 K (120 °C), or the plaster loses its setting property. Below that ceiling you get hemihydrate; above it you start making anhydrite. In a school laboratory the gypsum is heated in a china dish over a low flame, and the steam given off condenses on a cold watch glass held over the dish.
Dead burnt plaster: what overheating does
Keep heating past about 393 K and the last half water molecule goes too:
2(CaSO4·½H2O) → 2CaSO4 + H2O
The product is anhydrous calcium sulphate, called dead burnt plaster. Around 200 °C the conversion is complete and the material is dead in a practical sense: when you add water it takes up moisture so slowly that it will not set into a hard mass. The crystal structure has collapsed into an insoluble form that water can no longer rehydrate at a useful rate. A batch of dead burnt plaster cannot be rescued by adding more water; it is only fit for use as a filler.
So the process has a narrow working window. Too little heat leaves unconverted gypsum in the powder. Too much heat gives dead burnt plaster. Good plaster sits between the two.
The setting reaction: how plaster of Paris hardens
Mixing plaster of Paris with water simply reverses the preparation:
CaSO4·½H2O + 1½H2O → CaSO4·2H2O
The hemihydrate dissolves slightly, the solution becomes supersaturated with respect to gypsum, and long needle-shaped gypsum crystals grow out of it and interlock. That mesh of interlocking needles is what gives the set plaster its strength. Three details follow from the mechanism:
- It is exothermic. A bowl of setting plaster warms noticeably, which is why a fresh orthopaedic cast feels hot against the skin for a few minutes.
- It expands slightly on setting, by roughly 0.2 to 1% in volume, unlike cement and most mortars which shrink. The expansion presses the plaster into every detail of a mould, and it is the reason plaster is preferred for casts and impressions.
- It sets fast. Initial set typically comes in 5 to 15 minutes and the piece is hard in about 30 minutes, although full drying strength takes a day or more. Common sense on site: mix only what you can place before it stiffens, and never re-temper a stiffening mix with extra water, because that breaks the crystal mesh and leaves a weak, crumbly surface.
Note the nice symmetry of the chemistry. Gypsum loses 1½ water molecules on heating and takes the same 1½ back on setting. Plaster of Paris and gypsum are the same compound at two different levels of hydration.
Properties of plaster of Paris
- Fine white powder, odourless, with a slightly gritty feel.
- Sparingly soluble in water, and its solubility falls as the temperature rises.
- Sets into a hard, white, porous solid within about half an hour of mixing.
- Expands very slightly on setting, so cast detail stays sharp.
- Poor conductor of heat and non-combustible; the trapped water is released as steam in a fire, which is why gypsum plasterboard is used as a fire barrier.
- Light: set plaster has a bulk density of roughly 1.0 to 1.3 g/cm³, well under that of concrete.
- Weak in tension and softens if it stays wet, so it is an indoor material, not an exterior one.
- Non-toxic and skin-safe in normal handling, though the powder is a mild dust irritant and the setting heat can burn if a thick cast is applied directly to skin.
Uses of plaster of Paris
| Use | Why plaster of Paris suits it |
|---|---|
| Orthopaedic casts and plaster bandages | Sets in minutes, moulds itself to the limb, stays rigid and light while a fracture knits |
| Moulds for ceramics, metal casting and toys | Slight expansion on setting captures fine detail; the porous set plaster absorbs water from slip |
| False ceilings, cornices and wall finishing | Light, fire resistant, gives a smooth surface ready for paint; cast into boards and decorative sections |
| Blackboard chalk | Cheap, white, soft enough to leave a mark and hard enough to hold a stick shape |
| Statues, busts and ornamental work | Takes sharp impressions from a mould and carves easily after setting |
| Dentistry | Dental impressions and study models, where fast setting and dimensional accuracy matter |
| Fireproofing and sound insulation | Releases water as steam under fire; the porous structure damps sound |
| Repairing cracks in walls and filling gaps | Fills, expands slightly and sands flat quickly |
A point worth keeping straight for civil engineering students: plaster of Paris is a finishing material, not a structural one. Loads are carried by concrete, whose strength comes from the hydration of cement rather than of calcium sulphate. If you are studying mixes, read up on plain cement concrete (PCC) in foundation construction to see where each material belongs. Gypsum and cement do meet in one place: 2 to 5% gypsum is ground with clinker to control the setting time of ordinary Portland cement.
Storage precaution: why POP must be kept dry
Plaster of Paris must be stored in a moisture-proof, airtight container. The powder picks up water vapour straight from damp air, and the same setting reaction begins inside the bag. The result is a sack of hardened lumps of gypsum that will never set again, because the material has already used up its chemistry.
Practical storage rules: keep bags sealed and off the floor on a raised platform, store away from damp walls in a covered space, use older stock first, and treat a bag that has gone lumpy as scrap rather than trying to crush it back to powder. Shelf life is short in Indian coastal or monsoon conditions, often only a few months, so buy in quantities you will actually use.
References
- PubChem – Calcium Sulfate Hemihydrate, National Library of Medicine.
- NCERT Science, Class 10, Chapter 2: Acids, Bases and Salts (373 K figure).
- NCERT Chemistry, Class 11, The s-Block Elements: important compounds of calcium (393 K figure).
FAQs
What is the chemical formula of plaster of Paris?
The formula is CaSO4·½H2O, also written as (CaSO4)2·H2O. Its chemical name is calcium sulphate hemihydrate and its molar mass is 145.15 g/mol.
How is plaster of Paris prepared from gypsum?
By heating gypsum, CaSO4·2H2O, to about 373–393 K (100–120 °C). The equation is 2(CaSO4·2H2O) → 2(CaSO4·½H2O) + 3H2O. Each unit loses three quarters of its water of crystallisation as steam.
Why is plaster of Paris written as CaSO4·½H2O?
Because one water molecule is shared between two calcium sulphate units, so each unit holds half a molecule on average. The same compound can be written with whole numbers as (CaSO4)2·H2O.
What is dead burnt plaster?
Anhydrous calcium sulphate, CaSO4, formed when plaster of Paris is heated beyond about 393 K. It absorbs water so slowly that it no longer sets into a hard mass, which is why the overheated material is called dead.
Why should plaster of Paris be stored in a moisture-proof container?
It absorbs moisture from the air and sets back into gypsum inside the bag, turning into hard lumps. Once that happens the plaster cannot be used, so sealed containers kept off the floor are essential.