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In borax bead test which compound is formed
A
Ortho borate
B
Meta borate
C
Double oxide
D
Tetra borate
Detailed Solution
On heating, borax loses water and swells, then melts into a transparent glassy bead of sodium metaborate and boric anhydride:
$Na_2B_4O_7 \cdot 10H_2O \xrightarrow{\Delta} Na_2B_4O_7 \rightarrow 2NaBO_2 + B_2O_3$
When this bead is heated with a coloured metal salt, the metal oxide combines with $B_2O_3$ to form a coloured metaborate, e.g. $CuO + B_2O_3 \rightarrow Cu(BO_2)_2$ (blue), $CoO + B_2O_3 \rightarrow Co(BO_2)_2$ (blue).
So in the borax bead test, a metaborate is formed.
$Na_2B_4O_7 \cdot 10H_2O \xrightarrow{\Delta} Na_2B_4O_7 \rightarrow 2NaBO_2 + B_2O_3$
When this bead is heated with a coloured metal salt, the metal oxide combines with $B_2O_3$ to form a coloured metaborate, e.g. $CuO + B_2O_3 \rightarrow Cu(BO_2)_2$ (blue), $CoO + B_2O_3 \rightarrow Co(BO_2)_2$ (blue).
So in the borax bead test, a metaborate is formed.