Chemistry GCSE C3

The different tests with their methods to find the cations and anions.

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C3 - Flame Tests Identify Metal Ions

Compounds of some metals burn with a characteristic colour. (used in fireworks)

Lithium, Li+, burns with a crimson-red flame.

Potassium, K+, burns with a lilac flame.

Barium, Ba2+, Burns with a green flame.

Sodium, Na+, burns with a yellow-orange flame.

Calcium, Ca2+, burns with a brick-red flame. 

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C3 - Coloured Precipitates with NaOH.

Many metal hydroxides are insoluble and precipitate out of solution when formed, some of theses hydroxides have a characteristic colour.

Method: add a few drops of sodium hydroxide solution to a solution of your mystery compound.                                                                                          

Metal

Colour of  precipitate

Ionic  reaction

Calcium, Ca2+

White

Ca2+  +  2OH- à Ca(OH)2

Copper(II), Cu2+

Blue

Cu2+  + 2OH- à Cu(OH)2

Iron(II), Fe2+

Sludgy Green

Fe2+  + 2OH- à Fe(OH)2

Iron(III), Fe3+

Reddish Brown

Fe3+  +3OH- à Fe(OH)3

Aluminium, Al3+

White at first, then redissolves in excess NaOH to form a colourless solution.

Al3+  +3OH- à Al(OH)3  then
Al(OH)3  + OH-
à Al(OH)4-

Magnesium, Mg2+

White

Mg2+  +  OH- à Mg(OH)2

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C3 - Ammonium compound + NaOH.

You should put some of your compound in a boiling tube along with sodium hydroxide.

When it is heated if ammonia is produced you will smell it so that is number one clue that ammonia is present in your substance.

Another way is to take damp red litmus paper and if ammonia is present it turns blue.

You can use this for testing for ammonium ions (NH4+) using sodium hydroxide. Add some sodium hydroxide solution to your substance - if ammonia is given off then there are ammonium ions present in your mystery substance.

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C3 - Testing for Carbonates

Check for CO2

Step 1.You can test to see if a gas is carbon dioxide by bubbling it through limewater. 
 If the water goes cloudy then carbon dioxide is present.

Step 2. You can use this to test for carbonates, as carbonates react with dilute acids to form carbon dioxide.

 Acid + Carbonate --> Slat + Water + Carbon Dioxide

Method: put one spatula of carbonate into a test tube and heat strongly, then allow it to cool.
Copper carbonate turns from green to black, and stays black when cool. 
Zinc carbonate turns from white to yellow, but when it cools its turns back white again. 

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C3 - Tests for Sulphates and Halides

You can test for certain ions by seeing if a precipitate is formed after these reactions.....

Sulphate ions, SO42-
1) Add dilute HCl, followed by barium chloride solution, BaCl2
2) A white precipitate of barium sulphate means the original compound was a sulphate.

Chloride, Bromide or Iodine ions, Cl-, Br-, I-
1) Add dilute nitric acid
2) Then add some silver nitrate.

A chloride gives off a white precipitate of silver chloride
A bromide gives off a cream precipitate of silver bromide
A iodide gives off a yellow precipitate of silver iodide. 

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C3 - Tests for Nitrates

The test for nitrates produces ammonia.

1) MIx some of your mystery compound with a little aluminium powder.
2) Then add a few drops of sodium hydroxide solution and heat it, if you started off with a nitrate it'll be reduced to ammonia.
3) As always, you will either smell it or use damp litmus paper. 

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