Tests For Cations

  • Created by: India.02
  • Created on: 01-06-19 12:57

Flame Tests Identify Metal Ions

- Compounds of some metals produce a characteristic colour when heated in a flame

- You need to clean a nichrome or platinum wire loop by rubbing it with fine emery paper and holding it in a blue flame - when the flame remains blue, the loop is clean - dip the loop into the test sample and put it in the flame - record the colour

- Lithium ions produce a crimson flame - 

- Sodium ions produce a yellow flame - 

- Potassium ions produce a lilac flame - 

- Calcium ions produce an orange-red flame - 

- Copper ions produce a green flame - 

- The test only works for samples that contain a single metal ion -  if the sample contains a mixture of metal ions, the flame colours of some ions may be hidden by the colours of others

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Metals Form a Coloured Precipitate with NaOH

- Many metal hydroxides are insoluble and precipitate out of solution when formed - some have a characteristic colour

- Add a few drops of sodium hydroxide solution to a solution of the compound sample - all in the hope of forming an insoluble hydroxide

- Calcium ions produce a white precipitate:

- Copper(II) ions produce a blue precipitate:

- Iron(II) ions produce a green precipitate:

- Iron(III) ions produce a brown precipitate:

- Magnesium ions produce a white precipitate:

- Aluminium ions produce a precipitate that is white at first but then redissolves in excess NaOH and forms a colourless solution:

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