Qualitative analysis

• Flame test


  A test used to identify some cations, especially those in the s block of the Periodic Table
• Carrying out a flame test

  1. Clean a platinum or Nichrome wire by dipping it in concentrated hydrochloric acid

  2. Place a sample of the compound on the end of the wire

  3. Hold the wire on the edge of a non-luminous Bunsen flame

  4. Note any colour in the flame

  5. Observe the coloured flame through a diffraction grating or spectroscope
• Typical flame colours for selected metal cations


  • Sodium-orange-yellow
  • Potassium-lilac
  • Calcium-brick red (orange-red)
  • Strontium-crimson red
  • Barium-apple-green
  • Copper-greenish-blue
• When you look at the flame through a diffraction grating or spectroscope, you will see the coloured lines of the line emission spectrum in the visible region
• The colours seen in the flame test are due to the most obvious lines in the line emission spectrum
• Making a solution of the solid


  1. Attempt to dissolve the solid in a few cubic centimetres of water

  2. If it does not dissolve, warm gently

  3. If it is insoluble in water, try to dissolve it in nitric acid
• Solubility rules for common inorganic compounds

  • Common salts of Group I cations and NH4+ are soluble
  • All nitrates are soluble
  • Sulphates are soluble except those of Ca, Ba, Sr and Pb
  • Chlorides, bromides and iodides are soluble except those of Ag+ and Pb
  • Carbonates and sulphites are insoluble except those of Group I cations and NH4+
  • All hydroxides are insoluble except those of Group I cations, NH4+, Sr and Ba
• Using sodium carbonate to test for cations

  1. Aqueous sodium carbonate is sometimes used to confirm the presence or absence of a Group I cation or NH4+

  2. An aqueous solution of Group I cations or NH4+ will not form a precipitate on addition of sodium carbonate because these carbonates are soluble

  3. An aqueous solution containing other metal cations will give a precipitate because their carbonates are insoluble

  4. Group II carbonates give a white precipitate and transition element carbonates may give coloured precipitates

  5. Mg ions can be distinguished from other Group II ions by adding ammonium carbonate in the presence of ammonium chloride, no precipitate is formed
• A test for ammonium ions

  1. Many ammonium salts decompose on warming, ammonia gas is given off

  2. Ammonium nitrate is an exception, it decomposes to form nitrogen(1) oxide rather than ammonia

  3. All ammonium salts give off ammonia when warmed with dilute aqueous sodium hydroxide

  4. The ammonia can be detected by the fact that it turns damp red litmus paper blue
• When ammonium salts are heated in a test tube they almost always decompose, but when the vapours are cooled further up the tube, the solid salt is reformed
• Flame tests


  • When some elements are put in a non-luminous Bunsen flame, characteristic colours are seen, depending on the main colours in the line emission spectrum
• When a compound containing ammonium ions is warmed with aqueous sodium hydroxide, ammonia is given off