Green Colouring Agent in Borax Bead Test for Chromium III

To determine the cause of the green color observed in the borax bead test for a chromium(III) salt, we analyze the chemical reactions involved in the borax bead test:

1. Formation of the glassy bead: When borax ($\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O}$) is heated strongly on a platinum wire loop, it loses its water of crystallization and swells up. On further heating, it melts to form a transparent glassy bead composed of a mixture of sodium metaborate ($\text{NaBO}_2$) and boric anhydride ($\text{B}_2\text{O}_3$): $\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O} \xrightarrow{\Delta} \text{Na}_2\text{B}_4\text{O}_7 + 10\text{H}_2\text{O}$ $\text{Na}_2\text{B}_4\text{O}_7 \xrightarrow{\Delta} 2\text{NaBO}_2 + \text{B}_2\text{O}_3$

2. Reaction with Chromium(III) salt: When a chromium(III) salt is touched to the hot bead and heated again, the chromium oxide ($\text{Cr}_2\text{O}_3$) reacts with the boric anhydride ($\text{B}_2\text{O}_3$) to form chromium(III) metaborate: $\text{Cr}_2\text{O}_3 + 3\text{B}_2\text{O}_3 \xrightarrow{\Delta} 2\text{Cr}(\text{BO}_2)_3$

3. Color Observation: Chromium(III) metaborate, $\text{Cr}(\text{BO}_2)_3$, is responsible for the characteristic green color of the bead in both oxidizing and reducing flames.

Thus, the green coloration is due to $\text{Cr}(\text{BO}_2)_3$.

Correct Option: A