On heating, borax loses its water of crystallisation and decomposes to give sodium metaborate and boric anhydride
Na2B4O7⋅10H2O⟶ΔNa2B4O7+10H2O
Na2B4O7⟶Δ2NaBO2 (sodium meta borate) +B2O3
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
B2O3+CoO→Co(BO2)2
(cobalt (II) metaborate)
Blue Bead