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Question
The metal salt A is blue in colour. When salt A is heated strongly over a burner, then a substance B is eliminated and a white powder C is left behind. When a few drops of a liquid D are added to powder C, it becomes blue again. What could be A, B, C and D?
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Solution
A is copper sulphate pentahydrate, CuSO4.5H2O; B is water of crystallisation, 5H2O; C is anhydrous copper sulphate, CuSO4; D is water, H2O.
The blue colour of copper sulphate is due to the presence of 5 water molecules. When copper sulphate crystals are heated, they lose their water molecules and turn into a grey-white powder known as anhydrous copper sulphate. The chemical équation for this process is:
\[\ce{CuSO4 * 5H2O -> CuSO4 + 5H2O}\]
After adding a few drops of water (D), the anhydrous copper sulphate rehydrates into copper sulphate pentahydrate, and due to the re-entry of water molecules into its crystal lattice, it becomes blue again. The chemical equation for this process is:
\[\ce{CuSO4 + 5H2O -> CuSO4 * 5H2O}\]
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The Salt Story
From: The New Indian Express 9 March 2021
The salt pans in Marakkanam, a port town about 120 km from Chennai are the third largest producer of salt in Tamil Nadu. Separation of salt from water is a laborious process and the salt obtained is used as raw materials for manufacture of various sodium compounds.
One such compound is Sodium hydrogen carbonate, used in baking, as an antacid and in soda acid fire extinguishers.
The table shows the mass of various compounds obtained when 1litre of sea water is evaporated.
| COMPOUND | FORMULA | MASS OF SOLID PRESENT /g |
| Sodium Chloride | NaCl | 28.0 |
| Magnesium Chloride | MgCl2 | 8.0 |
| Magnesium Sulphate | MgSO4 | 6.0 |
| Calcium Sulphate | CaSO4 | 2.0 |
| Calcium Carbonate | CaCO3 | 1.0 |
| TOTAL AMOUNT OF SALT OBTAINED | 45.0 | |
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