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2. The Colour of Aqueous Solutions

Apart from compounds containing ions giving characteristic flame colours, aqueous solutions of these compounds also give characteristic colours.

 

Solution

Colour

Ion responsible for the colour

Copper II sulfate

Blue

Cu2+

Iron II sulfate

Green

Fe2+

Iron III sulfate

Brown

Fe3+

Copper II nitrate

Blue

Cu2+

Potassium manganate VII

Purple

MnO42-

Potassium chromate

Yellow

CrO42-

 

 

Testing for Anions

To test for anions we use a variety of different reagents depending on the anion you are testing for:

 

Type of Anion

Method

Halide ions

Into a test tube put 5 cm3 of the halide ion you are testing.

Add 2 drops of nitric acid and then a few drops of silver nitrate solution (AgNO3)

Cl - = white precipitate,
Br - = pale cream precipitate,
I - = pale yellow precipitate
F - = no precipitate

Sulfate ion

To 5cm3 of an aqueous solution of the sulfate ion add 2-3 drops of Barium chloride solution (BaCl2)

If the sulfate ion is present a white precipitate will form

Carbonate ion

To 5cm3 of an aqueous solution of the carbonate ion add drops of dilute hydrochloric acid

If the carbonate ion is present in the solution carbon dioxide gas will be given off (can be tested with lime water).

 

 

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Atoms and Ions

Testing for Ions

1. Flame Tests

The wire loop must be thoroughly cleaned before each test so that no colour can be seen when the loop is placed in a roaring Bunsen flame. The loop is dipped in concentrated sulfuric acid and placed in the Bunsen flame, just above the blue cone (the hottest part of the flame). This is repeated until the Bunsen flame remains unchanged.

wire loop

The loop is then dipped into a solution of the cation to be tested before placing it close to the hottest part of the Bunsen flame. If the flame changes colour then this will indicate the presence of specific cations (metal ions).

 

flame test

 

Testing for Cations

Each solution containing one of the cations listed below is tested by adding to separate samples

  • a few drops of sodium hydroxide solution, followed by excess sodium hydroxide solution

  • a few drops of ammonium hydroxide solution, followed by excess ammonium hydroxide solution

 

Cation

The images are not to scale!

Effect of adding

Sodium Hydroxide solution

Ammonium Hydroxide solution

Al 3+

aluminium ion

White precipitate

the precipitate will dissolve in excess NaOH solution.

White precipitate

the precipitate does NOT dissolve in excess NH4OH solution.

Ca 2+

calcium ion

White precipitate

the precipitate will NOT dissolve in excess NaOH solution.

No precipitate

Mg 2+

magnesium ion

White precipitate

the precipitate will NOT dissolve in excess NaOH solution.

White precipitate

the precipitate does NOT dissolve in excess NH4OH solution.

Cu 2+

copper II ion

Blue precipitate

the blue precipitate will NOT dissolve in excess NaOH solution.

Blue precipitate

the blue precipitate dissolves in excess NH4OH solution to give a deep blue solution..

Fe 2+

iron II ion

Green precipitate

the green precipitate will NOT dissolve in excess NaOH solution.

Green precipitate

the green precipitate does NOT dissolve in excess NH4OH solution.

Fe 3+

iron III ion

Brown precipitate

the brown precipitate will NOT dissolve in excess NaOH solution.

 

Brown precipitate

the brown precipitate does NOT dissolve in excess NH4OH solution.

Zn 2+

zinc ion

White precipitate

the precipitate will dissolve in excess NaOH solution.

White precipitate

the precipitate will dissolve in excess NH4OH solution.

 

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