(Borax Bead Test)

On treatment with metal salt, boric anhydride forms metaborate of the metal which gives different colours in oxidising and reducing flame.

For example, in the case of copper sulphate, following reactions occur.

$CuSO _4+ B _2 O _3 \longrightarrow Cu \left( BO _2\right)_2+ SO _3$

Two reactions may take place in reducing flame (Luminous flame)

$(i)$ The blue-green $Cu \left( BO _2\right)_2$ is reduced to colourless cuprous metaborate as :

$2 Cu ( BO _2 )_2+2 NaBO _2+ C \underset{\text { flame }}{\stackrel{\text { Luminous }}{\longrightarrow}} 2 CuBO _2+$ $ Na _2 B _4 O _7+ CO$

$(ii)$ Cupric metaborate may be reduced to metallic copper and bead appears red opaque.

$2 Cu \left( BO _2\right)_2+4 NaBO _2+2 C \underset{\text { fuminous }}{\stackrel{\text { flame }}{\longrightarrow}} 2 Cu +$ $2 Na _2 B _4 O _7+2 CO$