Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

Which metal salt gives a violet coloured bead in the borax bead test? (a) \(\mathrm{Co}^{2+}\) (b) \(\mathrm{Mn}^{2+}\) (c) \(\mathrm{Ni}^{2+}\) (d) \(\mathrm{Fe}^{2+}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Co}^{2+}\) gives a violet colored bead in the borax bead test.

Step 1: Understanding the Borax Bead Test

The borax bead test is used to identify certain metallic ions based on the color they impart to a bead formed by melting borax with a metal salt. Different metal ions produce distinct colors when heated in a flame.

Step 2: Identifying Characteristic Colors

Each metal ion corresponds to a specific color in the borax bead test. - \(\mathrm{Co}^{2+}\) (Cobalt) usually gives a blue to violet color.- \(\mathrm{Mn}^{2+}\) (Manganese) gives an amethyst or violet red color.- \(\mathrm{Ni}^{2+}\) (Nickel) gives a brown bead.- \(\mathrm{Fe}^{2+}\) (Iron) gives a yellow to brown bead.

Step 3: Evaluation of Provided Options

Considering the characteristic colors of the metal ions in the borax bead test, check which option matches a violet color. While \(\mathrm{Mn}^{2+}\) is associated with violet-red or amethyst shade, \(\mathrm{Co}^{2+}\) closely relates to a blue to violet color.

Step 4: Conclusion and Result

Given these correlations, \(\mathrm{Co}^{2+}\) is the ion that most commonly matches a violet color bead in the borax bead test. Therefore, the answer is option (a).

Key Concepts

Metal Ion Identification

When it comes to identifying metal ions, the Borax Bead Test is a handy method used in chemistry laboratories. This involves melting borax with a small amount of metal salt to form a "bead." Depending on the metal ion present, the bead displays a distinctive color when heated in a flame. Think of it as a mini color-coded system, where each metal ion presents itself in a different hue.

Identifying metal ions through this method can be useful because:

• It provides a quick visual cue for analysis.
• Minimal equipment is required, making it accessible.
• It helps distinguish between different metals in a mixture.

Understanding these color cues is crucial since they provide vital clues about the metal ions present. This tool helps chemists quickly identify unknown metal ions just by observing the color change.

Cobalt (Co) Bead Color

Cobalt, or \( \text{Co}^{2+} \), stands out due to the distinct blue to violet color it produces in the borax bead test. This transition metal uniquely imparts this characteristic color, which can vary slightly depending on the specific experimental conditions, such as the intensity of the flame and purity of the sample.

Other noteworthy aspects of cobalt's bead color include:

• The vibrant color can help differentiate it from other metal ions that produce similar shades, like manganese.
• This color change occurs due to the formation of \( \text{CoBO}_3 \), resulting from cobalt reacting with borax.
• A consistent observation in controlled conditions ensures that this method remains a reliable identification tool.

The presence of cobalt is easy to spot due to this unique hue, making it a reliable method in laboratory settings for identifying cobalt ions.

Flame Test Analysis

The flame test is another useful technique for identifying metal ions based on the colors they produce when exposed to a flame. Though similar to the borax bead test, the flame test is distinct in its application. Instead of forming a bead, this test involves directly observing the color a metal salt imparts to a flame.

Important points about the flame test include:

• Different metals emit various colors, which are easily remembered as their "signatures."
• A flame test is straightforward—it simply needs a clean wire loop dipped in metal salt and held in the flame.
• Although it is less specific than the borax bead test, it serves as a quick first step in metal ion identification.

By using the flame test in combination with other tests like the borax bead test, chemists can narrow down possible metal ions more precisely. This synergy of methods increases the accuracy and reliability of the identification process.