Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

In borax bead test which compound is formed (a) ortho borate (b) meta borate (c) double oxide (d) tetra borate

Step-by-Step Solution

Answer

The compound formed in the borax bead test is meta borate.

Step 1: Understanding Borax Bead Test

The borax bead test is a qualitative analysis method to identify transition metals. A metal salt is heated with borax which forms a glass-like bead. The color of the bead indicates the presence of certain metal ions.

Step 2: Chemical Reaction

When borax \[ \text{Na}_2\text{B}_4\text{O}_7\cdot 10\text{H}_2\text{O} \] is heated, it loses water molecules and first converts to metaborate. \[ \text{Na}_2\text{B}_4\text{O}_7 \rightarrow 2\text{NaBO}_2 + \text{B}_2\text{O}_3 \] Thus forming sodium metaborate ( \( ext{NaBO}_2\)).

Step 3: Compound Formed

The compound that is formed when borax is melted and used in the bead test is sodium metaborate. Therefore, the correct answer is (b) meta borate.

Key Concepts

Transition Metal Identification

The Borax Bead Test is a valuable technique used in qualitative analysis, particularly for identifying transition metals. This test involves exposing a small amount of borax, typically in the form of a bead, to a sample of metal salt. Upon heating in a flame, the borax fuses to form a glassy bead. The color relationship between the bead and the transition metals reveals their presence.

Transition metals are known for their striking colors and varied oxidation states, which is why the Borax Bead Test is so effective. Different metals will impart different colors to the bead, such as:

• Cobalt gives a blue bead.
• Chromium results in a green bead.
• Iron produces a yellow to brown bead.

Recognizing these color changes helps chemists not only identify but also confirm the existence of specific transition metals in a sample. Thus, this method provides quick insights into the composition of unknown samples.

Sodium Metaborate Formation

The transition of borax to sodium metaborate during the bead test is central to the chemical process. Borax itself, known chemically as \\( \text{Na}_2\text{B}_4\text{O}_7\cdot 10\text{H}_2\text{O} \), is a hydrated compound. Upon heating, it undergoes dehydration and subsequently breaks down.

This reaction can be represented as:

• Water molecules are driven off as it is heated.
• The remaining \\( \text{Na}_2\text{B}_4\text{O}_7 \) converts into \\( 2\text{NaBO}_2 + \text{B}_2\text{O}_3 \).

The transformation results in sodium metaborate (\\( \text{NaBO}_2 \)), which is essential for the bead test.
The sodium metaborate forms a viscous, glassy bead that will react with metal ions to display specific colors, assisting in the identification process.

Qualitative Analysis Methods

Qualitative analysis methods focus on identifying the components of a substance without quantifying them. The Borax Bead Test is a fine example of such a method, providing insights into the composition of metal ions based on visual cues.

These methods are invaluable in analytical chemistry for:

• Identifying the presence of transition metals due to their unique color properties.
• Allowing quick testing, usually requiring minimal equipment beyond a simple flame and a support wire.
• Providing a relatively straightforward technique that doesn’t rely heavily on complex instrumentation.

The simplicity of qualitative methods like the Borax Bead Test lies in their straightforwardness to understand and apply, making them accessible to both beginners and experts in chemistry. Despite being simple, these methods effectively lay the groundwork for more detailed quantitative analyses.