Testing for ions

Chemists are often asked to test a substance to find out if it contains specific ions or compounds.

Some cation in metal compounds can be identified using a flame test.

Most metal ions produce a strong colour when put into a blue Bunsen burner flame.

The colour of the flame can be used to identify which metal cation are present in an unknown sample.

Testing for cations (positive ions)

Flame tests are used to test for metal cations.

Flame tests

Most metal ions produce a strong colour when put into a blue Bunsen burner flame. The colour of the flame can be used to identify which metal cations are present in an unknown sample.

Method:

  • make a small loop in the end of a clean nichrome wire.
  • dip the loop into concentrated hydrochloric acid and then into the sample.
Flame test
  • hold the loop in a blue Bunsen flame and record the first colour you observe.
Flame test 2

Different metal ions produce different coloured flames.

Ion presentFlame colour
Lithium, Li+Crimson
Sodium, Na+Yellow / orange
Potassium , K+Lilac
Calcium, Ca2+Brick red
Copper, Cu2+Blue-green / green-blue

Prescribed Practical C2 - Identify the ions in an ionic compound using flame tests

Move on to Test
next