Question

In Borax bead test characteristic colour appears due to formation of:

• A. metaborate of transition metals
• B. sodium metaborate
• C. boron oxide
• D. boric anhydride

Answer 1

The correct option is D boric anhydride

Borax on strong heating loses its water of crystallization and then shrinks forming a transparent 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:

$N{a}_2{B}_4{O}_7.10H_{2}O\to N{a}_2{B}_4{O}_7+10H_{2}O$

$N{a}_2{B}_4{O}_7\to 2NaB{O}_2+B_2{O}_3$

Boric anhydride, being non-volatile displaces more volatile acidic oxides and combine with basic oxides present to form metaborates which are identified through their characteristic colours.