In qualitative analysis, thermal decomposition is one of the important chemical reactions used to identify both unknown cations and anions.

In most tests, about 2 spatula full of the solid sample is placed in a dry, hard test tube. The sample is heated gently at first then strongly until no further change is observed. Click here to see heating procedure and precautions.

1. The colour and texture of the solid before heating gives some indication to the type of cation and anion in the sample.
2. The changes in the colour in the solid during heating and the colour of the residue formed provide a clue to the cation in the solid.
3. The identity of the gas liberated during heating provides a clue to the anion in the solid.