Question
State with equations what happens when borax is heated on a platinum wire loop and the resulting transparent mass is heated with CoO in Bunsen burner.
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Answer
On heating borax on a platinum loop, borax first loses its water of crystallisation and swells upto form a puffy mass. On heating further, it melts to a transparent liquid which solidifies to a transparent mass.
This transparent solid is called borax bead. Whenever a coloured salt of $Co^{2+}, Cu^{2+}, Ni^{2+}$ etc. is heated with borax bead, the salt decomposes to form the corresponding metal oxide which combines with $B_2O_3$ present in the bead to form coloured metaborates e.g.,
$\text{CoO}\ \ +\ \ \text{B}_2\text{O}_3\ \xrightarrow{\ \ \ \text{heat}\ \ \ }\ \text{Co(BO}_2)_2\\ ^\text{Cobalt oxide} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Cobalt metaborate (blue)}$
This transparent solid is called borax bead. Whenever a coloured salt of $Co^{2+}, Cu^{2+}, Ni^{2+}$ etc. is heated with borax bead, the salt decomposes to form the corresponding metal oxide which combines with $B_2O_3$ present in the bead to form coloured metaborates e.g.,
$\text{CoO}\ \ +\ \ \text{B}_2\text{O}_3\ \xrightarrow{\ \ \ \text{heat}\ \ \ }\ \text{Co(BO}_2)_2\\ ^\text{Cobalt oxide} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Cobalt metaborate (blue)}$
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