When it is mixed with H2O it forms a solid plaster mass and heat is given out during this process and finally a porous mass results within 10-15 mins. H20 is absorbed to form a orthorhombie dehydrated and change to monoclinic gypsum(CaSO4), when the plaster hardens thus setting takes place.

**Common and Chemical Names of the Compound Formed**

When plaster of Paris is mixed with water, a compound called calcium sulfate hemihydrate is formed. This compound is commonly known as gypsum.

**Explanation**

**1. Plaster of Paris**
- Plaster of Paris is a white powdery substance that is derived from gypsum. It is made by heating gypsum at high temperatures to remove the water content.
- The chemical formula of plaster of Paris is CaSO4·1/2H2O. It contains one molecule of calcium sulfate (CaSO4) and half a molecule of water (H2O).

**2. Mixing with Water**
- When plaster of Paris is mixed with water, a chemical reaction occurs. The water molecules break the hemihydrate compound into its components, calcium sulfate and water.
- The water molecules combine with the hemihydrate to form dihydrate, which is a hydrated form of calcium sulfate.
- The chemical equation for this reaction is: CaSO4·1/2H2O + 3/2H2O → CaSO4·2H2O

**3. Compound Formed**
- The compound formed when plaster of Paris is mixed with water is calcium sulfate hemihydrate, which is commonly known as gypsum.
- The chemical formula of gypsum is CaSO4·2H2O. It contains one molecule of calcium sulfate (CaSO4) and two molecules of water (H2O).
- Gypsum is a white, crystalline solid that is commonly used in construction and as a building material. It is known for its ability to harden into a solid mass when mixed with water and then dried.
- Gypsum has various applications, including as a plaster for walls and ceilings, in dental materials, as a soil conditioner, and in the manufacture of cement and plasterboard.

In conclusion, when plaster of Paris is mixed with water, the compound formed is calcium sulfate hemihydrate, which is commonly known as gypsum. This compound has the chemical formula CaSO4·2H2O and is widely used in various industries for its properties and applications.