Question

Which green-colored compound of chromium is formed in the borax bead test?

• A. ${\mathrm{Cr}}_2{\mathrm{O}}_3$
• B. $\mathrm{CrB}$
• C. ${\mathrm{CrBO}}_3$
• D. $\mathrm{Cr}{\left({\mathrm{BO}}_2\right)}_3$

Answer 1

The correct option is D

$\mathrm{Cr}{\left({\mathrm{BO}}_2\right)}_3$

Explanation for the correct options:

Option (D) is the correct answer.

Borax Bead test: “The Borax bead test is used to determine the presence of metals in the sample”.On heating Borax (${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7.10{\mathrm{H}}_2\mathrm{O}$) first loses molecules of water and swells up. When heated further, it changes into a transparent liquid that solidifies into glass-like material known as borax beads On coating the beads with a layer of the mineral sample and again exposed to the flame, produces different colors in the flame. The colors of the flame are used to determine the metal present in the sample.

$$\begin{gathered} {\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7.10{\mathrm{H}}_2\mathrm{O}\stackrel{∆}{\to }{\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7\stackrel{∆}{\to }{\mathrm{NaBO}}_2+{\mathrm{B}}_2{\mathrm{O}}_3(\mathrm{Glassy}\mathrm{beads}) \\ \left(\mathrm{Borax}\right) \\ {\mathrm{Cr}}_2{\mathrm{O}}_3+{\mathrm{B}}_2{\mathrm{O}}_3\stackrel{∆}{\to }\mathrm{Cr}{\left({\mathrm{BO}}_2\right)}_3(\mathrm{Green}\mathrm{colour}\mathrm{due}\mathrm{to}\mathrm{Cr}) \\ (\mathrm{Chromium}(\mathrm{Chromium}\mathrm{meta}\mathrm{borate}) \\ \mathrm{Oxide}) \end{gathered}$$

$\mathrm{Cr}{\left({\mathrm{BO}}_2\right)}_3$ a green-colored compound of Cr which is formed in the borax bead test. Thus, option (D) is the correct answer.