SC25

• Compounds of some metals produce a characteristic colour when heated in a flame test:

  • Lithium (Li+) is red
  • Sodium (Na+) is yellow
  • Potassium (K+) is lilac
  • Calcium (Ca+) is orange-red
  • Copper (Cu+) is blue-green
• Steps to do a flame test:

  1. Clean a nichrome wire loop by dipping it in hydrochloric acid then rinsing it in distilled water
  2. Dip the clean wire loop into the metal compound
  3. Put it in the hottest part of the Bunsen burner (clear blue flame)
  4. Record colour
• Many metal hydroxides are insoluble and precipitate out of solution when formed with a characteristic colour:

  • Iron III (Fe3+) is brown
  • Iron II (Fe2+) is green
  • Copper (Cu2+) is blue
  • Aluminium (Al3+) is white - redissolves in excess NaOH to form a colourless solution
  • Calcium (Ca2+) is white - precipitate remains in excess NaOH
• Hydroxide Test:

  1. Add a few drops of sodium hydroxide to an unknown solution
  2. Record the hydroxide precipitate colour
• Test for Ammonium Ions:

  1. Add sodium hydroxide solution to an unknown substance
  2. Gently heat the compound until gas is released
  3. Hold damp red litmus paper over the gas
  4. Paper will turn blue if ammonia gas is present
  5. Ammonia gas has a distinct strong smell (toxic at high concentrations/irritant)
• Flame photometry is an instrumental method that accurately identifies different metal ions and their concentrations:

  • each ion has a unique line spectrum
  • intensity of measured wavelength indicates concentration
  • calibration curve can work out concentration from intensity
  • identify different ions in mixtures unlike flame tests
• Advantages of Flame Photometry:

  • sensitive - can detect very tiny amounts
  • fast - can be automated
  • accurate - no human error
• Test for Halide Ions:

  1. Add dilute nitric acid to get rid of carbonate ions which may also produce a pale precipitate
  2. Add a few drops of silver nitrate solution

  Hydrochloric acid can't be used as chloride ions would be added
• Halide Ions Test Results:

  • chloride gives a white precipitate of silver chloride
  • bromide gives a cream precipitate of silver bromide
  • iodide gives a yellow precipitate of silver iodide
• Test for Carbonate Ions:

  1. Add dilute acid
  2. Mixture fizzes (effervescence) means a gas is produced
  3. Bubble the gas through limewater
  4. If it turns cloudy/milky then carbon dioxide is present
• Test for Sulfate Ions:

  1. Add dilute hydrochloric acid to stop any precipitation reactions not involving sulfate
  2. Add barium chloride solution
  3. A white precipitate of barium sulfate will form if sulfate ions are present
• Each ion test is unique because some ions may have the same result meaning we still wouldn't know that the unknown substance is