The precipitate test tests for positive ions.
Many metal hydroxides are insoluble and precipitate out of a solution when formed.
Some of these hydroxides have a characteristic colour.
Add a few drops of sodium hydroxide solution to a solution of unknown compound. This should form an insoluble hydoxide.
If you get a coloured insoluble hydoxide you can detect which ions are present.
Calcium ( Ca2+) > white > Ca2+ (aq) + 2OH- (aq) --> Ca(OH)2 (s)
Copper (II) (Cu2+) > blue > Cu2+ (aq) + 2OH- (aq) --> Cu(OH)2 (s)
Iron (II) (Fe2+) > green> Fe2+ (aq) + 2OH- (aq) --> Fe(OH)2 (s)
Iron (III) (Fe3+) > brown > Fe3+ (aq) + 2OH- (aq) --> Fe(OH)3 (s)
Aluminium (Al3+) > white, then redissolves in excess NaOH to form colourless solution > Al3+ (aq) + 2OH- (aq) --> Al(OH)3 (s) THEN Al(OH)3 +OH- (aq) --> Al(OH)4-
Magnesium (Mg2+) > white > Mg2+ (aq) + 2OH- (aq) --> Mg(OH)2 (s)
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