Testing for Cations and Anions - Triple Chemistry AQA

Learn how to test for Cations and Anions for the Triple Science Chemistry AQA exam. 

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Testing for Cations 1

REMEMBER: Cations are POSITIVE ions. They are attracted to the Cathode (Negative) 

Flame Tests

Lithium Li+ = Red flame                                                                                

 Sodium Na+ = Yellow/Orange flame                           

 Potassium K+ = Purple flame

 Calcium Ca 2+ = Brick-red flame

Barium Ba 2+ = Green Flame

Compounds of these metals burn with characteristic colours. They are used in fireworks. To test, heat the substance and see if it burns with a distinctive coloured flame.



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Testing for Cations 2

Coloured Precipitate

Calcium Ca 2+ : White precipitate 

Copper (II) Cu 2+ : Blue precipitate

 Iron (II) Fe 2+ : Sludgy green precipitate

Iron (III) Fe 3+ : Reddish Brown 

Aluminium Al 3+ : White but then dissolves to form a colourless solution 

Magnesium Mg 2+ : White


 

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Testing for Cations 3

Ammonium Compound

If there it is an AMMONIUM (NH 4+) there will be AMMONIA (NH 3) produced

Ammonia can be detected by:

  • Smell
  • Turns damp red litmus paper blue

Just add Sodium Hydroxide NaOH

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Testing for Anions 1

REMEMBER: Anions are NEGATIVE ions. They are attracted to the anode (POSITIVE) 

Carbonates

Check for CO2

  • Add dilute acid (HCl e.g.)
  • If a gas is produced, bubble it through limewater 
  • Does the limewater turn milky? It's a carbonate!

Or... Some carbonates change colour when they decompose

  • Copper carbonate turns from green to black then stays black when cool
  • Zinc Carbonate turns from white to yellow but turns white when cool 
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Testing for Anions 2

Sulphates and Halides produce PRECIPITATES 

Sulphate ions (SO4 2-)

Add HCl then Barium chloride solutions BaCl2 = White precipitate

Halide ions 

Add nitric acid and silver nitrate solution

Chloride Cl- : White precipitate of silver chlorine 

Bromide Br- : Cream precipitate of silver bromide

Iodide I- : Yellow precipitate of silver iodide 

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Testing for Anions 3

Nitrates 

  • Mix with aluminium powder 
  • Add Sodium hydroxide and heat
  • Smell ammonia or turns damp red litmus paper turns blue 
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Comments

Gaby Coghlan

This is Gaby's friend, I have noticed an error with this- the purple flame should be in purple font!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

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