Question

In the borax bead test of $C{o^{2 + }}$ the blue color of bead is due to.
A) $C{O_3}{B_2}$
B)$Co{\left( {B{O_2}} \right)_2}$
C)$CoO$
D) None of the above.

Answer 1

Hint: We know that borax bead test is used to identify the metal present in the given mineral or salt. $N{a_2}{B_4}{O_7}.10{H_2}O$ is the chemical formula of the borax. We must remember that the borax is heated under the Bunsen flame the water molecule in the borax is lost and forms a transparent glassy bead called borax bead.

Complete step by step answer:We also remember that the meta borates of the many transition metals have characteristic colors and thus, borax bead test is often wont to identify them within the laboratory. We must know about the bead test also called blister test and it is an analytical method to identify the presence of definite metals.
We also remember that heating the glassy bead which leads to the formation of colored metaborate in oxidizing flame. The blue color in the test of cobalt ion is due to the formation of cobalt metaborate \[\left( {Co{{\left( {BO2} \right)}_2}} \right)\].

So the answer to this question is option B .


Note:We know that the mixture of sodium metaborate and boric anhydride is named borax bead and it is used to detect the presence of particular metals. The presence of particular metals is detected by observing the colors given to bead borax. We must remember the following points,
The dark color of the borax bead is due to the presence of cobalt salt.
Copper salt gives sky blue and red color in oxidizing and reducing parts.
Nickel salt gives yellow color and grey color in oxidizing and reducing parts.
Aluminium salt does not answer borax salt.