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 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 a behaviour? Give the reaction involved.
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The substance which is used for making different shapes is Plaster of Paris. Its chemical name is calcium sulphate hemihydrate (CaSO4 . ½H2O). The two formula unit of CaSO4 share one molecule of water. As a result, it is soft. When it is left open for some time, it absorbs moisture from the atmosphere and forms gypsum, which is a hard solid mass. 1 1 CaSO4 . H2O + 1 H2O → CaSO4. 2H2O 2 2 Plaster of Paris Gypsum (Soft) (Hard mass) (Sulphate salt)