Only coloured salts on heating with the glassy borax bead form coloured metaborates in oxidising flame. Na2B4O7.10H2O
Δ
–––>
Na2B4O7
Δ
–––>
2NaBO2+B2O3
Glassy bead
Coloured salt +B2O3→Coloured metaborate Sodium salts are colourless and hence do not give a positive test. Chromium - Green Iron - Green Cobalt - Blue