Animated Solution for Chemistry - Salt Analysis: The green colour produced in the borax bead test of a chromium(III) salt is due to –

Imagine you are in a chemistry lab, holding a thin platinum wire with a small loop at the end. You dip this loop into some white borax powder and hold it in the hot flame of a Bunsen burner. The powder sizzles, loses its water, and melts into a beautiful, transparent, glassy bead. This is the famous borax bead test, a classic and elegant method used to identify certain transition metals.

But what exactly is this glassy bead made of? When borax, which is sodium tetraborate decahydrate (${\text{Na}}_2{\text{B}}_4{\text{O}}_7\cdot 10{\text{H}}_2\text{O}$), is heated, it first loses its water of crystallization. Upon further heating, it decomposes into sodium metaborate (${\text{NaBO}}_2$) and boric anhydride (${\text{B}}_2{\text{O}}_3$).

This boric anhydride is the crucial ingredient. It acts as a flux, ready to react with metal oxides to form brightly colored compounds.

Now, let's introduce our mystery guest: a chromium(III) salt. When you touch the hot, transparent bead to a tiny speck of the chromium salt and put it back into the flame, a chemical transformation occurs.

First, the heat of the flame decomposes the chromium(III) salt into chromium(III) oxide (${\text{Cr}}_2{\text{O}}_3$).

Next comes the magic. The newly formed chromium(III) oxide reacts with the boric anhydride present in the glassy bead. They combine to form a new compound called chromium(III) metaborate.

It is this specific compound, chromium(III) metaborate ($\text{Cr}({\text{BO}}_2{)}_3$), that imparts a characteristic, vibrant green color to the bead.

This test is incredibly reliable for transition metals because their metaborates have distinct, signature colors. For instance, cobalt forms a deep blue bead, while copper can form a blue or green bead depending on whether you use an oxidizing or reducing flame. Understanding these reactions not only helps you ace your exams but also gives you a glimpse into the beautiful, colorful world of inorganic chemistry!