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Water - Solubility - Ions

Positive Ions - Precipitate with Sodium Hydroxide.

Most hydroxides are insoluble.
A few drops of sodium hydroxide solution are added
to the solution containing the positive ions.

If no precipitate is formed, the metal ion is sodium or potassium.
These can be distinguished using a flame test.
Ammonium ions also do not give a precipitate with sodium hydroxide.
If a flame test on the solid shows no colour
and there is no precipitate with sodium hydroxide solution
then the positive ion is probably ammonium NH4+.
If the solution is heated, ammonia gas will be produced.

If a precipitate is formed, it may be white or coloured.
The table below shows how to identify the metal ion.

Metal Ion Precipitate
Aluminium   Al3+ White
Calcium   Ca2+ White
Magnesium   Mg2+ White
Copper   Cu2+ Blue
Iron   Fe2+ Green
Iron   Fe3+ Brown (rust)

If you get a white precipitate, add more sodium hydroxide solution
and the aluminium precipitate will dissolve.
If the white precipitate does not dissolve then the metal ion
is calcium or magnesium.
These can be distinguished using a flame test
(magnesium ions have no flame colour).

The next page shows the chemistry of these reactions.

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