Problem 148 - A Complete Resource Book in Chemistry for JEE Main

Question

Borax \(\left[\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7} .10 \mathrm{H}_{2} \mathrm{O}\right]\) when heated on platinum loop it gives a dark transparent glass like bead. The hot bead is dipped in the salt till it reacts with transition metal oxide. It produces characteristic bead of meta borate. $$ \begin{array}{ll} \text { Colour of the bead } & \text { Ion } \\ \text { Blue green or light blue } & \mathrm{Cu}^{2+} \\ \text { Yellow } & \mathrm{Fe}^{2+} \text { or } \mathrm{Fe}^{3+} \\ \text { Green } & \mathrm{Cr}^{3+} \\ \text { Violet } & \mathrm{Mn}^{2+} \\ \text { Dark blue } & \mathrm{Co}^{2+} \\ \text { Brown } & \mathrm{Ni}^{2+} \end{array} $$ Glassy bead is of (a) \(\mathrm{Na}_{3} \mathrm{BO}_{3}\) (b) \(\mathrm{B}_{2} \mathrm{O}_{3}+\mathrm{NaBO}_{2}\) (c) \(\mathrm{SiO}_{2}\) (d) \(\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7}\)

Step-by-Step Solution

Answer

The glassy bead is composed of B_2O_3 + NaBO_2 (option b).

Step 1: Understand the Reaction

When borax (Na_2 B_4 O_7 .10 H_2O) is heated, it loses water and forms a glassy bead mainly composed of sodium metaborate (NaBO_2) and boric anhydride (B_2O_3). This glassy bead can react with various transition metal oxides to form colored beads characteristic of the metal present.

Step 2: Analyze the Reaction Products

Using the information in the problem, when the borax bead interacts with transition metal oxides, it forms a meta borate bead with a characteristic color. The resulting bead is made of meta borate compounds derived from the reaction of B_2O_3 and NaBO_2, not from unchanged borax or SiO_2.

Step 3: Choose the Correct Chemical Composition

Evaluate the given options: (a) Na_3BO_3, (b) B_2O_3 + NaBO_2, (c) SiO_2, and (d) Na_2 B_4 O_7. Since the process of heating borax leads to the formation of a glassy bead primarily composed of NaBO_2 and B_2O_3, option (b), B_2O_3 + NaBO_2, correctly represents the composition of the glassy bead.

Key Concepts

Transition Metal Colors

In the Borax Bead Test, the colors observed are due to the interaction of transition metals with the heated glassy bead. Transition metals are known for their colorful compounds because of the d-orbitals being split in different energy levels when they interact with ligands, such as the oxygen from the glassy bead.
When the bead, which becomes a transparent glass upon heating, is dipped into various transition metal oxides, it picks up these oxides and reacts with them. This creates different colored beads which are unique colors that help identify specific metal ions involved. Here are some characteristic colors and their corresponding ions:

• Copper (Cu^{2+}): Blue-green or light blue
• Iron (Fe^{2+} or Fe^{3+}): Yellow
• Chromium (Cr^{3+}): Green
• Manganese (Mn^{2+}): Violet
• Cobalt (Co^{2+}): Dark blue
• Nickel (Ni^{2+}): Brown

This characteristic coloration is a useful diagnostic tool in identifying metal ions in a sample by a simple bead test.

Chemical Composition of Bead

The creation of the glassy bead in a Borax Bead Test is rooted in its chemical transformation upon heating. Regular borax, or sodium borate (Na_2B_4O_7 10H_2O), undergoes a physical change when heated, transforming into a distinct chemical composition.
As borax is heated, it loses its water of crystallization and then decomposes into two primary components that make up the glassy bead. These are sodium metaborate (NaBO_2) and boric anhydride (B_2O_3).
This combination, B_2O_3 + NaBO_2, forms a transparent bead. This newly formed matrix is what will react with transition metal oxides, producing the colorful beads.

Borax Heating Process

The first step in the Borax Bead Test involves the transformation of borax through heating. Initially, when borax (Na_2B_4O_7 10H_2O) is exposed to high heat, it undergoes dehydration, losing all its ten water molecules.
Consequently, the anhydrous borax gets converted to sodium metaborate (NaBO_2) which is a rigid component of the bead and boric anhydride (B_2O_3), forming a combination often referred to as glassy or meta borate bead.
This entire process creates a glass-like solid that remains stable even at high temperatures, making it an ideal medium for analyzing transition metal oxides through characteristic color production. Such transformation highlights the thermal stability of these compounds and their importance in chemical assays.

Reactions with Metal Oxides

Once you have the glassy bead formed from heated borax, it serves as a platform to react with various transition metal oxides. The interaction between the bead and the metal oxides leads to the formation of meta borate compounds, which often display unique colors.
This color change is a direct result of the reaction between the metal ions and the constituents of the bead. This chemical reactivity is significant because:

• Each metal oxide produces a colored borate that is distinct, such as Cu^{2+} beads turning blue-green.
• The color helps in identifying metal oxides present in a test sample.
• The reaction can be reproduced consistently, making it reliable for qualitative analysis in laboratory settings.

Through these reactions, the Borax Bead Test turns into an insightful tool for identifying transition metals in a cost-effective manner. Its simplicity and educational value are part of why it has been an enduring experiment in chemistry labs around the world.