Question

The green colour produced in the borax bead test of a chromium (III) salt is due to:
a.) $\text{CrB}$
b.) $\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$
c.) $\text{Cr(B}{{\text{O}}_{\text{2}}}{{\text{)}}_{\text{3}}}$
d.) $\text{C}{{\text{r}}_{\text{2}}}{{\text{(}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}\text{)}}_{\text{3}}}$

Answer 1

Hint: To solve the given question we first need to know what a borax bead test is. Then we need to determine how the borax bead test of chromium III salt is achieved. With this information in hand, we can conclude why the borax bead test of chromium III salt gives us a green colour resultant. Accordingly, the solution below, provides certain chemical equations involving the Borax bead test of chromium III salt and the compound responsible for the green coloration.


Complete step by step solution:
 The Borax bead test is used to determine the presence of metals, particularly in minerals. When exposed to flame the borax forms a bead which is then coated with a film of the sample mineral and exposed to the flame again. The colour of the flame is then used to determine the metal present.
In the borax bead test, metal oxide reacts with a glassy bead to form metaborates. Here are the equations:

$\text{N}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}\cdot \text{10}{{\text{H}}_{\text{2}}}\text{O}\xrightarrow{\text{ }\!\!\Delta\!\!\text{ }}\text{N}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}\xrightarrow{\text{ }\!\!\Delta\!\!\text{ }}\text{NaB}{{\text{O}}_{\text{2}}}\text{+}{{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}$
$\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{+}{{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}\xrightarrow{\text{ }\!\!\Delta\!\!\text{ }}\text{Cr(B}{{\text{O}}_{\text{2}}}{{\text{)}}_{\text{3}}}$
$\text{Cr(B}{{\text{O}}_{\text{2}}}{{\text{)}}_{\text{3}}}$which is produced in the reaction, is green in colour.

Hence, the correct answer is Option C.

Note: The borax bead test is also known as blister test. This is because, in this test a small loop is created at the end of a Platinum or Nichrome wire, and then it is heated in a Bunsen burner flame until it becomes red hot. A stick made up of an inert material such as magnesium stick (MgO), might also be used. Then it is dipped into borax powder and is held at the top of a flame where the temperature is the most. Here, it swells up while it loses the water of crystallization and then it shrinks forming a transparent, colourless, glass-like bead resulting in a mixture of sodium metaborate and boric anhydride.