A sulphate salt of Group 2 element of the Periodic Table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in the open for some time, it becomes a solid mass and cannot be used for moulding purposes. Identify the sulphate salt, and why does it show such behaviour? Give the reaction involved.


  • 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.

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