question_answer 80)

  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.

Answer:

  The sulphate salt which is used for making different shapes is plaster of Paris. Its chemical name is calcium sulphate hemihydrate . 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. .