Tests for positive ions

Tests for positive ions -> flame tests and precipitate colour tests.

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  • Created by: Joanna XD
  • Created on: 05-04-11 19:33
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Tests for POSITIVE IONS
Flame tests:
Compounds of some metals give a characteristic colour when heated. This is the idea behind flame
tests.
Sodium, Na+ > orange/yellow flame
Potassium, K+ > lilac flame
Calcium, Ca++ > brick red flame
Copper, Cu++ > bluegreen flame
Add sodium hydroxide and look for a coloured precipitate:
This is also a test for positive ions. Many metal hydroxides are insoluble and precipitate out of the
solution when formed. Some of these hydroxides have a characteristic colour. So in this test you add a
few drops of sodium hydroxide solution to a solution of your mystery compound all in the hope of
forming an insoluble hydroxide. If you get a coloured insoluble hydroxide you can then tell which metal
was in the compound.
`'Metal'' Colour of precipitate Ionic reaction
Calcium, Ca++ White Ca2+(aq) + 2OH-(aq) -> Ca(OH)2(s)
Copper (II), Cu++ Blue Cu2+(aq) + 2OH-(aq) -> Cu(OH)2 (s)
Iron (II), Fe++ Sludgy green Fe2+(aq) + 2OH-(aq) -> Fe(OH)2 (s)
Iron (III), Fe+++ Reddish brown Fe3+(aq) + 3OH- (aq) -> Fe(OH)3 (s)
Aluminium, Al+++ White at first, but Al3+(aq) + 3OH-(aq) -> Al(OH)3 (s)
then redissolves in Then
excess NaOH to Al(OH)3(s) + OH- -> Al(OH)4- (aq)
form a colourless
solution.
The reactions in the table above are ionic equations. Ionic equations are 'half' a full equation, if you like.
They just show the bit of the equation you're interested in nothing else.

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