Description of experiment
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Needed compounds:
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ligroin : C7H16
acetone : CH3 CO CH3
cobalt chloride : CoCl2.6H2O
Class:
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elem=Co
precipitation
coordination
Summary:
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Cobalt chloride, both the anhydrous form and the hydrated form (with 6 H2O)
dissolve in acetone, giving a deep blue/cyan solution. When an apolar solvent
is added to this (e.g. ligroin), then the solution of anhydrous cobalt chloride
gives a fine light blue precipitate, the solution of the hydrated cobalt
chloride gives small droplets of deep cyan/blue solution, with a colorless
liquid above it.
Description:
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Dissolve some hydrated cobalt chloride in acetone: The solid dissolves and a
bright deep cyan/blue liquid is obtained.
Dissolve some anhydrous cobalt chloride in acetone: The solid dissolves and a
bright deep cyan/blue liquid is obtained, this looks precisely the same as with
the hydrated form.
(the anhydrous form of CoCl2 is obtained by carefully heating the hydrated
form, until it becomes light dull blue).
Add a small amount of ligroin to both liquids (half the volume of ligroin,
compared to the volume of acetone):
1) The liquid, containing the hydrated CoCl2 becomes colorless quickly and
small droplets of very dark blue liquid are separated. These droplets stick to
the glass and do not dissolve in the liquid. The liquid above the droplets is
colorless and clear.
2) The liquid containing the anhydrous CoCl2 becomes turbid and a little
lighter blue. With shaking, however, all solid dissolves again and a clear
cyan/blue liquid is obtained again. The anhydrous CoCl2 also dissolves in a 1:2
by volume mix of ligroin and acetone.
Add more ligroin to the liquids:
1) In the hydrated case, nothing changes anymore. The droplets remain the same,
the liquid remains colorless and clear.
2) The liquid becomes turbid again and now it remains turbid. The CoCl2 now
does not dissolve anymore in the liquid. There is too much apolar solvent and
the CoCl2 precipitates in the form of very small light blue solid particles.