Question

A minute quantity of cupric salt is heated on the borax bead in reducing flame of Bunsen burner, the color of the bead after cooling will be:
(a)- Blue
(b)- Red
(c)- Colourless
(d)- Green

Answer 1

Hint: In the quantitative inorganic analysis, a borax bead test is used to test for the presence of few metals. Only a few metals can be detected by this test but not all metals can be detected. Different metals give different colors.

Complete answer:
When the borax is heated, the borax loses its water of crystallization and swells up to form a puffy mass. When it is further heated, it melts into a liquid which solidifies to a transparent glass-like bead which contains sodium metaborate ($NaB{{O}_{2}}$) and boric anhydride (${{B}_{2}}{{O}_{3}}$). The reaction is given below:
$N{{a}_{2}}{{B}_{4}}{{O}_{7}}\to 2NaB{{O}_{2}}+{{B}_{2}}{{O}_{3}}$
This glassy bead is the borax bead. Some radicals like $N{{i}^{2+}},C{{o}^{2+}},C{{r}^{3+}},C{{u}^{2+}},M{{n}^{2+}}$, etc. When the cupric salt like copper oxide is treated with borax bead, there is a formation of cupric metaborate which is dark blue. The reaction is given below:
$CuO+{{B}_{2}}{{O}_{3}}\to Cu{{(B{{O}_{2}})}_{2}}$
This cupric metaborate is reduced to the free metal by the carbon present in the reducing flame of the burner. There are two reactions involved in this reduction:
(i)- First, colorless cuprous metaborate is formed.
$2Cu{{(B{{O}_{2}})}_{2}}+C\to 2CuB{{O}_{2}}+{{B}_{2}}{{O}_{3}}+CO$
(ii)- Second, red metal is formed.
$2CuB{{O}_{2}}+C\to 2Cu+{{B}_{2}}{{O}_{3}}+CO$
So on cooling the bead changes into red color.

Therefore, the correct answer is option (b)- Red.

Note:
Metaborates are formed when the colored salt containing the metal cations is heated with a borax bead on a platinum wire. On heating, they give corresponding colored metaborates. This test is known as the borax bead test.