Question

In the borax bead test of $C{{o}^{2+}}$, the blue colour of the bead is due to the formation of:
A. ${{B}_{2}}{{O}_{3}}$
B. $C{{o}_{3}}{{B}_{2}}$
C. $Co{{\left( B{{O}_{2}} \right)}_{2}}$
D. $CoO$

Answer 1

Hint: Think about what is the procedure for carrying out the borax bead test and hat all it entails. Consider the metaborate salts that are formed on the bead and impart colour to it after the test has been completed.

Complete answer:
Before moving on to understanding the results of the borax bead test, we should first understand the procedure and principle behind it.
The borax bead test is one of the tests conducted when we want to conduct an inorganic qualitative analysis of a certain salt given to us. It is one of the traditional methods used to detect the presence of certain metals in the salt but is not used often these days due to the discoveries of much more effective methods.
To perform the test, a wire that is made up of an inert metal like platinum is taken. A loop is made at the end of it and dipped in the borax powder. This powder on the wire is heated in the hottest part of the flame of the Bunsen burner and a transparent borax bead is formed. This bead is then dipped into the sample salt so that some of it sticks to the bead but make sure that not too much of the salt sticks as it may cause the bead to become opaque after heating. The bead coated with the salt is then heated in the reducing part of the flame and the colour is observed after cooling. The bead is again heated in the oxidizing part of the flame and then cooled, the colour is observed.
This colour is seen due to the formation of the metaborate salt on the surface of the borax bead. The only metaborate salt that has a deep blue colour is cobalt metaborate. The reaction for this borax bead test when done with respect to cobalt consists of three parts. The borax glass is formed in the first part. The cobalt salt forms cobalt oxide in the second part, and the cobalt oxide reacts with the borax glass in the third part. They are as follows:
\[i)N{{a}_{2}}{{B}_{4}}{{O}_{7}}\cdot 10{{H}_{2}}O\xrightarrow{\Delta }N{{a}_{2}}{{B}_{4}}{{O}_{7}}\xrightarrow{Fused}2NaB{{O}_{2}}+{{B}_{2}}{{O}_{3}}\]
Here, borax is heated to form borax anhydrous, and then fused to form the two components of borax glass.
\[ii)Cobalt\text{ }Salt\xrightarrow{\Delta }CoO+gas\]
Here, the cobalt salt is converted to cobalt oxide on heating and releases the corresponding gas associated with the salt.
\[iii){{B}_{2}}{{O}_{3}}+CoO\xrightarrow{{}}Co{{(B{{O}_{2}})}_{2}}\]
In the final step, boron trihydride which is one of the components of borax glass combines with the formed cobalt oxide to give us the cobalt metaborate salt.

So, the correct answer to this question is ‘C. $Co{{\left( B{{O}_{2}} \right)}_{2}}$’.

Note:
Remember that the cobalt oxide does not combine with the sodium metaborate compound of the borax glass since sodium metaborate is a much more stable compound than cobalt metaborate.