question_answer

  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 state why does it show such a behaviour. Give the reaction involved.                              [NCERT Exemplar]  

Answer:

             The substance which is used for making different shapes is Plaster of Paris. Its chemical name is calcium sulphate hemihydrate  the two formula unit of CaS04 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.