Problem 149 - 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 colour of bead \(\mathrm{Ni}\left(\mathrm{BO}_{2}\right)_{2}\) is of (a) Brown (b) Blue (c) Green (d) Violet

Step-by-Step Solution

Answer

The color is brown (a).

Step 1: Understand the question

The task is to determine the color of the borax bead when it reacts with nickel, \( ext{Ni}^{2+} \). We are given a list of colors associated with various transition metal ions.

Step 2: Identify the relevant information

Among the listed ions, we need to find the color corresponding to \( ext{Ni}^{2+} \). According to the provided table, the color of a borax bead with \( ext{Ni}^{2+} \) is brown.

Step 3: Verify the conclusion

Confirm that the information aligns with typical chemistry knowledge of transition metal colors. The color associated with nickel borate in a bead is indeed brown, consistent with the table provided.

Key Concepts

Transition Metal Ions

Transition metal ions are a fascinating subject in chemistry because of their unique properties. These ions come from elements in the d-block of the periodic table, which includes metals like iron, copper, and nickel. A distinct feature of these ions is that they have partially filled d-orbitals. This results in their ability to exhibit a variety of oxidation states and form colored compounds. When transition metal ions interact with ligands or other compounds, their d-orbitals split into different energy levels. This process is known as crystal field splitting. The energy difference between these levels is what can lead to the absorption of visible light, thus giving rise to the rich colors observed with these ions. These colors are often used in the identification and testing of different metals, such as in the Borax Bead Test. When heated, borax forms a glass-like bead that can react with metal oxides to produce distinctively colored compounds. This makes it incredibly useful to identify transition metal ions based on their color signatures.

Color Identification in Chemistry

Colors play a significant role in chemistry, especially when it comes to identifying substances. The color seen in compounds is due to the specific wavelengths of light absorbed by an element’s electrons. Electrons move up and down between energy levels, and the specific energy difference corresponds to a specific color. In practical applications, color change is a key indicator in various analytical methods. For example:

• Acid-base indicators: These change color at different pH levels, signifying the acidity or basicity of a solution.
• Transition metal complexes: As mentioned, transition metals can produce characteristic colors that can help in identifying them.
• Titrations: Various substances show color changes signaling the endpoint of a reaction.

The Borax Bead Test is a classical method leveraging color change, providing a visual way to understand and identify various metal ions. Each ion leaves its characteristic color on the bead, simplifying the identification process.

Nickel Ion Characteristics

Nickel ions, especially in the +2 oxidation state, have certain distinguishing features in chemistry. Known for its silvery appearance in its pure metallic form, nickel is widely used in alloys such as stainless steel. When it comes to ions, the \(\mathrm{Ni}^{2+}\) ion shows specific characteristics:

• Color of Compounds: Nickel ions in different complexes can appear green, but specifically, in the Borax Bead Test, the nickel ion produces a brown bead.
• Chemical Reactions: Nickel can form various complexes with organic and inorganic ligands, which can affect the color and properties of the resulting compounds.
• Magnetism: While \(\mathrm{Ni}^{2+}\) is not magnetic, metallic nickel is ferromagnetic, meaning it is attracted to magnets and can be magnetized.

Understanding these characteristics helps in differentiating nickel from other transition metals, especially those with similar properties or appearances.