Question

In the borax bead test of \(\mathrm{Co}^{2+}\), the blue colour of bead is due to the formation of (a) \(\mathrm{CoO}\) (b) \(\mathrm{Co}_{3} \mathrm{O}_{4}\) (c) \(\mathrm{B}_{2} \mathrm{O}_{3}\) (d) \(\mathrm{Co}\left(\mathrm{BO}_{2}\right)_{2}\)

Short answer

The blue color is due to the formation of Co(BO_2)_2, so the answer is (d).

Step-by-step solution

Understanding the Borax Bead Test

The borax bead test is used to identify certain metal ions based on the color they impart to a glass bead formed by heating borax and a small amount of the metal. When metal ions are heated with borax, they form a colored bead.

Identifying the Role of Cobalt Ion

In the context of the borax bead test, when cobalt ions (C12+) are present, they impart a characteristic blue color to the bead. This is due to the formation of specific complexes in the bead.

Analyzing the Blue Complex Formation

The blue color in the borax bead test with cobalt is due to the formation of a cobalt borate complex rather than simple oxides. The chemical formulation associated with this blue color is Co ext{(}BO_2 ext{)}_2.

Selecting the Correct Option

Among the given options, the complex responsible for the blue color in the borax bead test with cobalt ions is Co ext{(}BO_2 ext{)}_2. This corresponds to option (d).

Key concepts

Cobalt Ion

The cobalt ion, particularly in the form of its divalent state \( \text{Co}^{2+} \), plays a significant role in the identification of substances through various analytical methods. In chemistry, ions like \( \text{Co}^{2+} \) are known for their ability to form colorful compounds and complexes when combined with other materials. This characteristic is highly useful in identifying and studying metals in various assays and experiments.

Cobalt ions are transition metal ions which are inherently colorful due to their electron configurations. In the visible spectrum, the presence of \( \text{Co}^{2+} \) often results in colors such as pink or blue, depending on the ligands attached to it. This distinctive coloration is not only a visual property but an insightful identifier in both qualitative and quantitative analysis.

Cobalt’s ability to form colored complexes is especially highlighted in classic techniques such as the borax bead test. Here, cobalt's roles become pivotal which makes it easier to study and distinguish from other elements.

Blue Complex Formation

The fascinating blue color observed during the borax bead test with cobalt ions is specifically due to complex formation involving the cobalt ion. In these situations, the cobalt ion interacts with borate, a product of heated borax, leading to complex compounds like \( \text{Co(BO}_2\text{)}_2 \).

The formation of such a complex can be primarily attributed to the coordination chemistry of cobalt. As a transition metal, cobalt readily accepts electron pairs from surrounding ligands (in this case borate) to form stable complexes. These interactions significantly alter the energy levels within the cobalt ion, resulting in the absorption of certain wavelengths of light and the reflection of others, which we perceive as color.

The rich blue hue of \( \text{Co(BO}_2\text{)}_2 \) serves as a clear indication in qualitative tests, confirming the presence of cobalt ions. This is why the color outcome is often utilized as a reliable method for identifying cobalt in materials throughthe borax bead test.

Metal Ion Identification

Identifying metal ions in a mixture or compound is crucial in both academic and industrial chemical analyses. The borax bead test serves as one of the classical methods for identifying metal ions by exploiting the unique color each metal can produce when heated with borax.

In this test, borax (\( \text{Na}_2\text{B}_4\text{O}_7\cdot10\text{H}_2\text{O} \)) is melted to form a clear glassy bead. When a metal sample is then added and heated with the bead, specific metal ions react with the borate support in the bead, creating colorful complexes depending on the metal present.

• Transition metals, like cobalt, reveal their presence by forming clear, vivid colors due to their electronic transitions.
• Analyzing the color produced helps in determining which metal ions are contained in a sample.
• This method is particularly useful for identifying transition elements, thanks to distinct complex formations they exhibit.

Understanding these principles helps chemists and students alike to quickly identify unknown samples in practical laboratory settings.