Question

What happens when a mixture of cobalt oxide and borax is heated in a flame on a loop of platinum wire?

• A. A transparent white bead is formed
• B. A bright pink coloured $NaB{O}_2$ bead is formed
• C. A blue coloured $Co(B{O}_2{)}_2$ bead is formed
• D. A red coloured $Co(B{O}_2{)}_2$ bead is formed

Answer 1

Answer: (C)

A blue coloured $Co(B{O}_2{)}_2$ bead is formed

Borax $\left({Na}_2{B}_4{O}_7.10H_{2}O\right)$, on heating gets fused and loses water of crystallisation. It swells up into a fluffy white porous mass which then melts into a colourless liquid and forms a clear transparent glass bead consisting of boric anhydride and sodium metaborate.

${Na}_2{B}_4{O}_7.10H_{2}O\stackrel{△}{\to }{Na}_2{B}_4{O}_7\stackrel{△}{\to }2NaB{O}_2+\underset{\text{Boric anhydride}}{B_2{O}_3}$

Boric anhydride is non-volatile. When it is reacted with a coloured metallic salt, a characteristic coloured bead of metal metaborate is formed.

Thus, when reacted to cobalt oxide, forms blue coloured $Co{\left(B{O}_2\right)}_2$ bead.

$B_2{O}_3+CoO⟶\underset{\text{Cobalt metaborate (blue bead)}}{Co(B{O}_2{)}_2}$

Hence, the correct answer is option $\text{C}$.