Testing Cations & Anions (WJEC GCSE Chemistry: Combined Science)

Testing for Cations

• The flame test is used to identify the positive metals ion (cations) by the colour of the flame they produce
  • Ions from different metals produce different colours
• To carry out a flame test:
  • Dip the loop of an unreactive metal wire such as nichrome or platinum in dilute acid
  • Hold it in the blue flame of a Bunsen burner until there is no colour change
  • Dip the loop into the solid sample / solution and place it in the edge of the blue Bunsen flame

• It is important that place the wire into acid first to prevent contamination 
  • Not doing this might result in two or more ions being present on the wire meaning the colours will mix 
  • One colour could mask another colour and you will not be able to identify the ion

How to carry out a flame test

The blue flame must be used to carry out a flame test

• The colour of the flame is observed and used to identify the metal ion present:

Cation	Flame Colour
Li+	Red
Na+	Yellow-orange
K+	Lilac
Ca2+	Brick-red
Ba2+	Apple-green

Diagram showing the colours formed in the flame test for metal ions

• Negatively charged non- metal ions are known as anions
• You must be able to test for halide ions
  • These are the ions formed by the elements in Group 7 

How do I test for halide ions?

• Add silver nitrate solution, AgNO₃
• If a halide is present it forms a silver halide precipitate
• For example, the following reaction occurs between aqueous potassium chloride and and silver nitrate solution:

potassium chloride +  silver nitrate   →  potassium nitrate + silver chloride 

KCl (aq)   +     AgNO₃ (aq)   →  KNO₃ (aq)  +  AgCl (s)   

• The silver halide formed, in this case silver chloride forms a precipitate
  • This is represented using the state symbol, s
• The ionic equation for the precipitation reaction occurring in this example is:

Ag⁺ (aq) + Cl^– (aq) → AgCl (s)

• The general ionic equation for the precipitation reaction that occurs when a silver halide is formed is:

Ag⁺ (aq) + X^– (aq) → AgX (s)

• The potassium and nitrate ions do not participtate in the precipitate reaction so are known as spectator ions
• Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion
• Silver chloride forms a white precipitate
  • Ag⁺ (aq) + Cl^– (aq) → AgCl (s)
• Silver bromide forms a cream precipitate
  • Ag⁺ (aq) + Br^– (aq) → AgBr (s)
• Silver iodide forms a yellow precipitate
  • Ag⁺ (aq) + I^– (aq) → AgI (s)

Diagram to show the precipitates formed by different halide ions

Each silver halide produces a precipitate of a different colour