Problem 150 - 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. $$ \begin{array}{ll} \text { Colour of the bead } & \text { Ion } \\ \text { Blue green or light blue } & \mathrm{Cu}^{2+} \\ \text { Yellow } & \mathrm{Fe}^{2+} \text { or } \mathrm{Fe}^{3+} \\ \text { Green } & \mathrm{Cr}^{3+} \\ \text { Violet } & \mathrm{Mn}^{2+} \\ \text { Dark blue } & \mathrm{Co}^{2+} \\ \text { Brown } & \mathrm{Ni}^{2+} \end{array} $$ The flame used in borax bead test is (a) Oxidising (b) Reducing (c) Both (A) and (B) (d) Neither (A) nor (B)

Step-by-Step Solution

Answer

(a) Oxidising

Step 1: Understanding the Borax Bead Test

The Borax Bead Test is a qualitative analysis technique used to identify transition metals. When a borax bead is heated and then dipped in a solution containing metal ions, it forms a colored bead that indicates the presence of certain metal ions based on its color.

Step 2: Analyzing the Flame Requirement

In the Borax Bead Test, heating the bead in a flame is necessary to fuse the salt and to produce a color. The question asks what type of flame is used: oxidizing or reducing. An oxidizing flame is needed to convert the transition metal oxides into meta borates, which results in a characteristic color.

Step 3: Main Components of the Flame

The flame has two main zones: the inner zone (reducing) and the outer zone (oxidizing). An oxidizing flame is hotter and encourages the formation of oxidized states, which are necessary for this test because the transition metal oxides need to react correctly.

Step 4: Conclusion on Flame Type

Since the Borax Bead Test relies on the ability to form a stable, transparent bead that shows the correct reaction in the presence of metal ions, an oxidizing flame is essential. It ensures that metal oxides react to display the accurate coloration for identification based on the list provided.

Key Concepts

Transition Metal Identification

The Borax Bead Test is an important tool in the identification of transition metals by using color. Transition metals have unique properties, including their ability to form compounds with distinct colors when they react with certain chemicals. When a metal ion is present in a solution, it can be detected by the characteristic color it imparts to the borax bead.
In this test, the specific color formed on the bead correlates directly to the type of metal ion present. For instance:

• Blue-green indicates Cu²⁺
• Yellow indicates Fe²⁺ or Fe³⁺
• Green signals the presence of Cr³⁺
• Violet points to Mn²⁺
• Dark blue shows Co²⁺
• Brown is characteristic of Ni²⁺

By analyzing these colors, chemists can easily identify which transition metal ions are present in a sample.

Qualitative Analysis in Chemistry

Qualitative analysis in chemistry is a way to recognize the identity of chemical substances based on their observed properties. Instead of quantifying amounts, it focuses on whether a certain substance is present. The Borax Bead Test excellently showcases qualitative analysis by determining the presence of specific transition metal ions.
This method relies on sensory observations, primarily sight, to make determinations. By observing changes like color transformations in the borax bead when exposed to metal ions, researchers can deduce the specific ions in a sample. Qualitative analysis is crucial in chemical diagnostics for processes like determining unknown samples or confirming the presence of suspected ions.

Oxidizing Flame

An oxidizing flame plays a key role in the Borax Bead Test as it helps convert transition metal salts into their oxide forms. This conversion is necessary for the formation of colored beads, which are used to identify different metal ions.
The flame itself is composed of two zones: the inner reducing zone, which lacks oxygen and is cooler, and the outer oxidizing zone, which is intensely hot and rich in oxygen. The oxidizing zone is essential because it encourages metals to take on higher oxidation states, forming stable oxides that react with the borax to produce color.
Using the oxidizing flame ensures the reaction conditions are ideal for creating the correct metal oxide complexes, crucial for a successful bead test.

Transition Metal Oxides

Transition metal oxides are compounds formed when transition metals react with oxygen. In the Borax Bead Test, these oxides are vital as they are the components that combine with the borax to form colorful beads.
These oxides have unique properties due to the d-electron configurations of transition metals, which allow them to manifest various colors when combined with different ligands like borate ions from the melting borax.
When heated with borax, transition metal oxides convert into colorful metal borates. This conversion process is central to identifying different transition metal ions present in a substance, based on the color displayed on the bead.

Coloration of Beads

The coloration of beads in the Borax Bead Test is a crucial indicator for identifying transition metal ions. Each metal gives a distinct color based on its interaction with borate ions, providing a straightforward visual indication of its presence.
Transition metals exhibit varied oxidation states, which significantly influence the resultant colors. For example, the presence of Fe²⁺ or Fe³⁺ produces yellow beads due to the specific electronic transitions occurring in those ions. Similarly, the dark blue color of Co²⁺ is linked to its d-d electron transitions, influenced by the surrounding borate environment.
Understanding the specific colors associated with each transition metal ion enables chemists to quickly determine and confirm the compositions of unknown samples.

Chemical Reactions of Metal Ions

Metal ions undergo complex chemical reactions when they interact with other elements or compounds, significantly influencing their properties. In the context of the Borax Bead Test, these reactions are evident in the formation of colored beads.
The reaction occurs as the metal ions are heated and oxidized in the flame, encouraging them to react with the boron compound in the borax. This process forms a metal–borate complex, each with a unique color indicative of the metal.

• For instance, the cobalt ion reacts to form a dark blue borate complex.
• Meanwhile, the same process results in a violet bead for manganese ions.

These chemical reactions are pivotal for identifying the presence of specific transition metal ions, given their characteristic color results.