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Question

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

A
Metal oxides
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B
Boron oxide
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C
Metal metaborates
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D
Elemental boron
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Solution

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 (NaBO2) and boric anhydride (B2O3) as shown in reaction given below:
Na2B4O7.10H2ONa2B4O7+10H2O
Na2B4O7740oC2NaBO2+B2O3
CuO+B2O3Cu(BO2)2 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.

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