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On heating, hydrous crystals lose their water of crystallization and turn into a powder. They are then said to be anhydrous. Sometimes they may also lose their colour.
Take hydrated copper (II) sulphate crystals in a test tube and heat. The blue crystals in the test tube start turning white on heating. Water vapours released on heating condense on the cooler part of the test tube. Collect the water drops in a dish. The crystalline copper sulphate turns to a white amorphous form.
Allow the white amorphous powder in the test tube to cool down and then add the water collected in the dish to it; white amorphous powder turns back to blue crystalline form.
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