flame test

The terms "can give free atoms" is not correct. It should be "can give ions in vapour form".

The basic principle of flame test is the radiation of visible light when the excited electron of gaseous metal ion migrates from a higher energy level to the ground state (the lowest energy level). Therefore, the flame test would fail if the metal ions are not vaporized.

For example, flame test of CaSO4 in the absence of concentrated HCl always fails, because no vapour of calcium ion is formed as the Bunsen burner cannot provide such high temperature to vaporize CaSO4. However, in the presence of the concentrated HCl, calcium ions are vaporized in the form of calcium chloride (i.e, vapour containing calcium ions and chloride ions). Consequently, calcium ions in the flame give a characteristic brick red colour.
CaSO4(aq) + HCl(aq) → Ca^2+(g) + 2Cl^-(g) + other substances

2009-10-29 01:25:08 補充：
The equation should be:
CaSO4(aq) + 2HCl(aq) → Ca^2+(g) + 2Cl^-(g) + other substances

This is not a formal equation. However, It can only help you to understand the process.

2HCl+(l)　十　Cl2(s)　-->　H2(g)　十　2Cl2(s)