Qualitative analysis

• Salt


  An ionic compound that is formed from the replacement of one or more hydrogen ions of an acid by a metal ion or an ammonium ion
• Example of salt

  • Sodium Chloride
• Acid reacted

  • Hydrochloric acid
  • Sulfuric acid
  • Nitric acid
• Formula of salt formed

  • ... chloride
  • ... sulfate
  • ... nitrate
• Test for water

  1. When a hydrated salt is heated, water of crystallisation is given off

  2. In the presence of water, the white anhydrous copper(II) sulfate will change from white to blue to form hydrated copper(II) sulfate
• Soluble salts

  • All Group 1 salts
  • All ammonium salts
  • All nitrates
  • All chlorides (except silver chloride and lead(II) chloride)
  • All sulfates (except calcium sulfate, lead(II) sulfate and barium sulfate)
  • Only sodium carbonate, potassium carbonate and ammonium carbonate
• Insoluble salts

  • Silver chloride
  • Lead(II) chloride
  • Calcium sulfate
  • Lead(II) sulfate
  • Barium sulfate
  • All other carbonates
• Soluble bases (alkalis)

  • Only Group 1 oxides
  • Only SPA hydroxides
  • Calcium hydroxide (sparingly soluble)
• Insoluble substances

  • All metal hydroxides (except Group 1 and some Group 2)
  • All metal oxides (except Group 1)
• Qualitative analysis

  A process used by a chemist to identify the cations and anions in an unknown solution
• Cation


  Positive ion
• Anion


  Negative ion
• Steps to identify cations

  1. Add dropwise an alkali (sodium hydroxide or ammonium hydroxide/aqueous ammonia) to the unknown salt solution

  2. Continue to add an excess of the alkali

  3. Identify the cation based on the colour of the precipitate, its solubility in excess alkali, and whether ammonia gas is given off
• Cations tested


  • Aluminium
  • Ammonium
  • Calcium
  • Copper(II)
  • Iron(II)
  • Iron(III)
  • Zinc
• Precipitate formed


  The hydroxide of the metal ion (cation)
• Sodium ion, potassium ion and ammonium ion do not form precipitates with alkalis
• Zinc ion forms a white precipitate that is soluble in excess in both sodium hydroxide and aqueous ammonia
• Steps to identify anions

  1. Add dilute acid to the unknown salt solution and test the gas produced

  2. Add reagents (e.g. silver nitrate, barium nitrate) to the unknown salt solution and observe the precipitate formed
• Anions tested


  • Carbonate
  • Chloride
  • Nitrate
  • Sulfate
• Do not confuse the test for nitrate ions with the test for ammonium ions
• Iron(II) ions will be oxidised to iron(III) ions in the presence of oxygen or an oxidising agent
• Gas tests


  • Hydrogen
  • Oxygen
  • Carbon dioxide
  • Ammonia
• Identifying carbonate anion

  1. Add dilute acid

  2. Observe effervescence and carbon dioxide produced
• Identifying chloride anion

  1. Acidify with dilute nitric acid

  2. Add aqueous silver nitrate

  3. Observe white precipitate
• Identifying nitrate anion

  1. Add aqueous sodium hydroxide

  2. Add aluminium foil

  3. Warm carefully

  4. Observe ammonia produced
• Identifying sulfate anion

  1. Acidify with dilute nitric acid

  2. Add aqueous barium nitrate

  3. Observe white precipitate
• Tests for anions

  • Carbonate
  • Chloride
  • Nitrate
  • Sulfate
• Tests for cations

  • Aluminium
  • Ammonium
  • Calcium
  • Copper(II)
  • Iron(II)
  • Iron(III)
  • Zinc
• Effect of aqueous sodium hydroxide on cations

  • White precipitate, soluble in excess giving a colourless solution
  • Ammonia produced on warming
  • White precipitate, insoluble in excess
  • Light blue precipitate, insoluble in excess
  • Green precipitate, insoluble in excess
  • Red-brown precipitate, insoluble in excess
  • White precipitate, soluble in excess giving a colourless solution
• Effect of aqueous ammonia on cations

  • White precipitate, insoluble in excess
  • -
  • No precipitate
  • Light blue precipitate, soluble in excess giving a dark blue solution
  • Green precipitate, insoluble in excess
  • Red-brown precipitate, insoluble in excess
  • White precipitate, soluble in excess giving a colourless solution
• Tests for gases

  • Ammonia
  • Carbon dioxide
  • Chlorine
  • Hydrogen
  • Oxygen
  • Sulfur dioxide
• Test for ammonia

  Turns damp red litmus paper blue
• Test for carbon dioxide

  Gives white precipitate with limewater (precipitate dissolves with excess CO2)
• Test for chlorine

  Bleaches damp litmus paper
• Test for hydrogen

  'Pops' with a lighted splint
• Test for oxygen

  Relights a glowing splint
• Test for sulfur dioxide

  Turns aqueous acidified potassium manganate(VII) from purple to colourless
• Under high heat, some metal carbonates can breakdown to produce carbon dioxide and other products
• Reactivity series for metals

  • Potassium, Sodium (Stable, does not decompose on heating)
  • Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Copper (Decompose to form metal oxide and carbon dioxide on heating)
  • Silver (Decomposes into silver and carbon dioxide on heating)
• Usually, the heating of a powder will give you carbon dioxide. Hence, always test for carbon dioxide in your practical exam