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  • 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,

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 behaviour? Give the reaction involved.

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Answer:

Calcium belongs to group 2. Calcium sulphate is a white, soft substance.

The sulphate salt which is used for making different shapes is plaster of Paris. Its chemical name is calcium sulphate hemihydrate \left ( CaSO_{4}.\frac{1}{2}H_{2}O \right ) . The two formula units of CaSO_{4} 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.

CaSO_{4}.\frac{1}{2}H_{2}O+1\frac{1}{2}H_{2}O\rightarrow CaSO_{4}.2H_{2}O

plaster of paris                                         Gypsum

Plaster of Paris shows such behaviour because of presence of half water molecule as water during crystallisation. 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.

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