Colour of the bead in borax bead testis mainly due to the formation of
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Borax Bead test : On heating borax forms a colourless glassy bead of NaBO 2 and B 2 O 3 .
Na 2 B 4 O 7 .10H 2 O 
Na 2 B 4 O 7 
2NaBO 2 + B 2 O 3
On heating with a coloured salt , the glassy bead forms a coloured metaborate in oxidising flame.
For example , in oxidising flame copper salts give blue bead.
CuSO 4 
CuO + SO 3 ; CuO + B 2 O 3 
Cu(BO 2 ) 2 (blue bead)
However, in reducing flame the colours may be different due to different reactions.
2Cu(BO 2 ) 2 + C 
2CuBO 2 + B 2 O 3 + CO
2Cu(BO 2 ) 2 + 2C 
2Cu (brown red/red and opaque bead) + 2B 2 O 3 + 2CO.
Metal | Colour in oxidising flame | Colour in reducing flame | ||
When Hot | When Cold | When Hot | When Cold | |
Copper | Green | Blue | Colourless | Brown red |
Iron | Brown yellow | Pale yellow/Yellow | Bottle green | Bottle green |
Chromium | Yellow | Green | Green | Green |
Cobalt | Blue | Blue | Blue | Blue |
Manganese | Violet/Amethyst | Red/Amethyst | Grey/Colourless | Grey/Colourless |
Nickel | Violet | Brown/Reddish brown | Grey | Grey |
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