(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. CuSO4+B2O3‌
‌ Non -luminous flame ‌
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(‌ Oxidising ‌)
‌Cu(BO2)2+SO3 Two reactions may take place in reducing flame (Luminous flame) (i) The blue-green Cu(BO2)2 is reduced to colourless cuprous metaborate as: ‌2‌Cu(BO2)2+2NaBO2+C‌
‌ Luminous ‌
→
‌ flame ‌
‌2CuBO2+Na2B4O7+CO (ii) Cupric metaborate may be reduced to metallic copper and bead appears red opaque. ‌2‌Cu(BO2)2+4NaBO2+2C‌