Question

WHY ONLY METAL CHLORIDES ARE PREFERRED FOR FLAME TEST?

Answer 1

Flame test is essentially done with chlorides of metals, because unlike  other compounds of the metals chlorides are volatile in nature. Thus, when metallic chlorides are introduced in non-luminous flame of a bunsen burner, they evaporate due to heat and cation (metallic ion) produces characteristic colour. Since most of compounds are non metal chlorides, therefore, the given compounds are heated with concentrated hydrochloric acid, in bunsen flame, so as to convert them into metallic chlorides.