Edexcel A Level Chemistry:复习笔记2.2.5 Flame Tests

Flame Tests

 

  • Metal ions produce a colour if heated strongly in a flame
  • Ions from different metals produce different colours
  • The flame test is thus used to identify metal ions by the colour of the flame they produce
  • Dip the loop of an unreactive metal wire such as nichrome or platinum in concentrated acid, and then hold it in the blue flame of a Bunsen burner until there is no colour change
  • This cleans the wire loop and avoids contamination
    • This is an important step as the test will only work if there is just one type of ion present
    • Two or more ions means the colours will mix, making identification erroneous

     

  • Dip the loop into the solid sample and place it in the edge of the blue Bunsen flame
  • Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour

Identifying-Metal-Cations

Diagram showing the technique for carrying out a flame test

Explanation for the occurrence of the flame

  • In a flame test the heat causes the electron to move to a higher energy level
  • The electron is unstable at this energy level so falls back down
  • As it drops back down from the higher to a lower energy level, energy is emitted in the form of visible light energy with the wavelength of the observed light

 

Colours observed in flame tests

edexcel-2-2-5-flame-tests

  • Why does Mg2+ not have an observed colour?
    • The energy emitted during a flame test involving magnesium is outside the visible spectrum

转载自savemyexams

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