Description of experiment
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Needed compounds:
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chrome alum : KCr(SO4)2 . 12H2O
Class:
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elem=Cr
coordination
Summary:
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Chromium (III) builds a coordination complex with sulfate in the heat.
This coordination compound does not crystallize well.
Description:
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Add some solid chrome alum to cold water.
1) Heat the solid with the water until boiling: The solid quickly dissolves
and the liquid becomes green.
2) Dissolve in cold water: The solid slowly dissolves and the liquid becomes
violet/greyish blue.
Heat liquid (2): The liquid becomes green as well.
Mix both green liquids and let cool down: Resulting liquid remains green,
even when completely cooled down.
Heat again and evaporate a lot of water: A green syrup is formed, no crystals
separate from the liquid/syrup. When the syrup cools down, it solidifies to
a green shiny glas-like solid. The surface of the solid is highly reflective,
but it is wrinkled.
Add some cold water: The green syrup/solid slowly dissolves, the liquid
becomes clear and green.
Remark: Once the chrome alum is heated, it is converted to a green compound.
The green compound probably is a coordination complex between chromium (III)
and sulfate, which, once formed, is very stable.