But ONLY in a mixture of two or more metallic ions.
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It might work as a beginning indicator, but it's not terrific. Suppose you had Lithium and Strontium. Both of them burn red. What could you conclude? Suppose as well that you had (say) sodium just by itself. It burns yellow the same color as most flames. Now what? It only works if you burn something like copper Chloride or Copper Sulphate which are by themselves. The former is green and the latter is blue. Any mixture will only serve to confuse you.