Edexcel A Level Chemistry:复习笔记2.2.5 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

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

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