Problem 35 - A Complete Resource Book in Chemistry for JEE Main

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) \(\mathrm{Co}\left(\mathrm{BO}_{2}\right)_{2}\)

Step-by-Step Solution

Answer

The blue color in the borax bead test for co^{2+}\\) is due to co(bo\u00172\u00170)\u00172\u0017\u00171\u00192.

Step 1: Analyze the Borax Bead Test

The borax bead test involves heating a mixture of borax and a metal ion until they melt and form a glassy bead. This bead can be used to identify metal ions based on its color.

Step 2: Identify the Metal Ion

In this exercise, the metal ion involved in the borax bead test is \(co^{2+}\), which is known to impart a distinct color to the bead when involved in the reaction.

Step 3: Formation of the Compound

During the heating process, the \(co^{2+}\) ions react with borate ions to form a complex compound. The distinct color of this compound determines its identity.

Step 4: Recognize the Color and Compound

The \(co^{2+}\) ion forms \(co(bo_{2}\u00170)\u00172\u0017\u0019)\u0019\) when it reacts with the borate ions, which imparts a blue color to the bead.

Step 5: Select the Correct Option

From the given options, \(\post\u001eto(bo\u00172)\u00172\u0017\u0017\u0019\) from option (d) is the compound responsible for the blue color in the borax bead test for \(co^{2+}\).

Key Concepts

Cobalt ions (Co^{2+})

Cobalt ions, denoted as \(\mathrm{Co}^{2+}\), are transition metal ions that can exhibit a variety of colors when dissolved in different environments. These ions possess a +2 oxidation state, which is stable and allows them to form numerous compounds through reactions with other ions or molecules.

• In aqueous solutions, \(\mathrm{Co}^{2+}\) ions usually appear pink, but their color can change depending on their chemical surroundings.
• These ions have a geometric structure that allows them to engage in complex formation, which is an important feature in tests like the borax bead test.

Understanding cobalt ions is essential in identifying their roles in various chemical processes and their behavior in coordination chemistry.

Complex compound formation

Complex compounds are intricate structures formed when metal ions like \(\mathrm{Co}^{2+}\) bind with other molecules or ions, such as borate ions in the borax bead test. These structures typically involve coordinate covalent bonds, where the ligand (such as a borate ion) donates electron pairs to the metal ion.

• This bonding results in the creation of a new compound, which often exhibits distinct physical properties, like a unique color.
• The formation of the complex compound is key to the borax bead test's success in metal identification.
• In this specific test, \(\mathrm{Co}^{2+}\) can form a complex with borate, resulting in a color shift that helps identify cobalt's presence.

Complex compounds are fundamental to understanding how metal ions interact with their environments.

Metal ion identification

The identification of metal ions in chemistry often involves observing reactions that result in color changes or other distinct characteristics. The borax bead test is one classical method to achieve this.

• By heating a metal ion with borax, chemists can create a bead whose color reveals the identity of the metal ion involved.
• In this context, a blue bead indicates the presence of \(\mathrm{Co}^{2+}\) ions, as they form a complex with borate ions.
• This method is simple yet effective, taking advantage of visible changes that indicate specific reactions have occurred.

Such tests are valuable educational tools in chemistry, demonstrating concepts like ion reactions and compound formation.

Color change in chemical reactions

Color changes are crucial indicators in many chemical experiments, linking directly to reactions at the molecular level. In the borax bead test, observing these color shifts can reveal valuable information about the underlying chemical processes.

• The reaction between \(\mathrm{Co}^{2+}\) ions and borate ions results in a blue color due to the formation of the \(\mathrm{Co}(\mathrm{BO}_{2})_{2}\) complex.
• Color changes not only signal the identification of specific metal ions but also provide insight into the nature of the compounds formed.
• The test highlights the atomic interactions that occur when metal ions form bonds with other ions or molecules, leading to visible changes.

Understanding these phenomena enhances comprehension of chemical reactions and the identification of substances based on observable properties.