Calcium sulphate hemihydrate is Plaster of Paris. The formula of Plaster of Paris is CaSO4.1/2H2O. Here, we will study about the preparation, properties and uses of Plaster of Paris, CaSO4.1/2H2O
Preparation of Plaster of Paris, CaSO4.1/2H2O
Heating Calcium sulphate hemihydrate to 393K forms Plaster of Paris.
2 CaSO4.H2O + 393 K ………………….> 2CaSO4.1/2H2O + 3H2O
Gypsum + 393 K …………………………..> Plaster of Paris + water
The essential conditions necessary are following :
(i) The temperature above 393K results in the loss of all water of crystallisation. Anhydrous CaSO4 formed is called dead burnt plaster.
2CaSO4.1/2 + Temperature (>393K). …………………..> 2 CaSO4 + H2O
Plaster of Paris Anhydrous Calcium Sulphate ( Dead burnt plaster )
Dead burnt plaster loses its properties of setting with water.
(ii) Gypsum may react with carbon containing fuel to form Calcium sulphite.
Properties of Plaster of Paris, CaSO4.1/2H2O
Plaster of Paris reacts with water in the presence of heat at 120 degree C to form Gypsum
A. Setting of Plaster of Paris, CaSO4.1/2H2O
- CaSO4.1/2H2O (s) + 3/2 H2O (l) + heat ……………………….> CaSO4.2H2O (s)
Plaster of Paris Gypsum
Setting of P.O.P. to a solid hard mass gypsum takes about 5 to 15 minutes.
2. There is a slight expansion in volume during setting of Plaster of Paris. Therefore, it takes the shape of the mould.
3. Common salt speeds up the rate of setting of Plaster of Paris. Small amount of borax or alum reduces the rate of setting.
B. Material of Plaster of Paris, CaSO4.1/2H2O
The material nature of Plaster of Paris is non combustible and non inflammable.
C. Plaster for fractured bones
Plaster of Paris helps in setting of fractured bones or strains in hospitals. This is because of its property of setting of P.O.P. to gypsum.
Uses of Plaster of Paris
- Plaster of Paris is used in building industry.
- It is used in dentistry.
- Plaster of Paris is used in making statues, models and other decorative materials
- It is used in making moulds for ceramics.
- P.O.P. is used in making surgical bandages and for plastering the broken bones.
- It is used in making blackboard chalks.
FAQs related to Plaster of Paris
- What happens when we add water to Plaster of Paris ?
Answer : Setting of Plaster of Paris to a solid hard mass gypsum takes place when we add water to P.O.P.
2. Is Plaster of Paris resistant to heat ?
Answer : POP has fire resistance quality and is ideal for heat insulation.
3. What is the formula of Plaster of Paris ?
Answer : Calcium sulphate hemihydrate CaSO4.1/2H2O is Plaster of Paris.
Check out the structure of NH3 here
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