Question

Colour of the bead in borax bead test is mainly due to the formation of:

• A. Metal oxides
• B. Boron oxide
• C. Metal metaborates
• D. Elemental boron

Answer 1

The correct option is C Metal metaborates

Borax on strong heating loses its water of crystallization and then shrinks forming a glassy bead of sodium metaborate $\left(NaB{O}_2\right)$ and boric anhydride $\left(B_2{O}_3\right)$ as shown in reaction given below:
$N{a}_2{B}_4{O}_7.10H_{2}O\to N{a}_2{B}_4{O}_7+10H_{2}O$
$N{a}_2{B}_4{O}_7\stackrel{{740}^{o}C}{⟶}2NaB{O}_2+B_2{O}_3$
$CuO+B_2{O}_3\to Cu(B{O}_2{)}_2\text{blue bead}$
Boric anhydride, being non-voltile displaces more volatile acidic oxides and combine with basic oxides present to form metaborates which are identified through their characteristic colours.