Question

The compound formed in the borax-bead test for $C{u^{2 + }}$ ion in oxidizing flame is:
A) Cu
B) $CuB{O_2}$
C) $Cu{\left( {B{O_2}} \right)_2}$
D) None of these.

Answer 1

Hint: Try to recall that the borax bead test is used in qualitative analysis for the detection of certain coloured basic radicals such as $C{u^{2 + }}$ and the cuprous metaborate is formed in a borax-bead test of $C{u^{2 + }}$. Now, by using this you can easily find the correct option from the given ones.

Complete step by step solution:
It is known to you that on heating, borax loses its water of crystallization and swells up to form a puffy mass. On further heating, it melts into a clear liquid that solidifies to a transparent glass-like bead which consists of sodium metaborate and boric anhydride.
The glassy bead is commonly known as borax bead and is used in qualitative analysis for the detection of certain basic radicals such as $C{u^{2 + }}$ , etc.
The transparent glassy bead consists of sodium metaborate and boric anhydride (${B_2}{O_3}$) .
Whenever a coloured salt containing $C{u^{2 + }}$ cation is heated with borax bead on a platinum wire, the salt decompose to form the corresponding copper oxide which then combines with ${B_2}{O_3}$ present in glassy bead to form $Cu{\left( {B{O_2}} \right)_2}$ which is a dark blue coloured metaborate. This test is called a borax test. Thus, $CuO + {B_2}{O_3} \to Cu{\left( {B{O_2}} \right)_2}$ .
Certain metaborates are reduced to the free metal by the carbon in the reducing flame of the burner.

Therefore, from above we can conclude that option B is the correct option for the given question.

Note: It should be remembered that the cation of nickel gives brown coloured metaborate compounds on heating in an oxidising flame.
Also, you should remember that naturally occurring borax is called tincal or suhaga and is used in the softening of hard water.