- The sulphate salt should be calcium sulphate which is white and soft substance.
- Calcium sulphate is popularly known as plaster of Paris.
- Plaster of Paris has half molecule of water of crystallization.
- When we leave plaster of Paris open for some time, it absorbs moisture to gain a number of molecules of crystallization.
- This newly formed compound is called Gypsum, which is hard to make moulds.