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Question

Question 47.
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 a behaviour ?
Write down the reaction involved.

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Solution

The sulphate salt which is used for making different shapes is plaster of Paris. Its chemical name is Calcium Sulphate hemihydrate (CaSO4.12H2O). The two formula units 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. So, gypsum sets as a hard solid mass and cannot be used for moulding purposes.

CaSO4.12H2O(Plaster of Paris) Sulphate salt(soft)+112H2OWaterCaSO4.2H2OGypsum (hard mass sets as)

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