Chemistry
Imaad took a few blue crystals of copper sulphate. On heating the crystals, he observed that the blue colour disappeared and the substance turned into a white powder. He learnt that this change occurred due to the loss of water of crystallisation on heating. He also learnt that plaster of Paris is prepared by heating gypsum (calcium sulphate dihydrate) at 100°C.
Answer the following questions:
(a) How many water molecules are present in one formula unit of hydrated copper sulphate?
(b) Name the white powder formed after heating copper sulphate crystals.
(c) How can Imaad obtain the blue colour again from the white powder? Write the chemical equation for the reaction.
(d) Write the chemical formula of gypsum. Name the product formed when gypsum is heated at 100°C.
Water
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Answer
(a) One formula unit of hydrated copper sulphate contains 5 water molecules of crystallisation.
(b) On heating, hydrated copper sulphate loses its water of crystallisation and forms anhydrous copper sulphate (white powder).
(c) The white anhydrous copper sulphate becomes blue again when water is added.
CuSO4 + 5H2O ⟶ CuSO4.5H2O
(d) The chemical formula of gypsum is :
CaSO4.2H2O
When gypsum is heated at 100°C, it loses water partially and forms plaster of Paris.
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