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

Answer: (C)

metal metaborates

Borax on strong heating loses its water of crystallization and then shrinks forming a transparent glassy bead of sodium metaborate ($NaB{O}_2$) and boric anhydride ($B_2{O}_3$) 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.