Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following salts will form colourless beads in borax bead test? (a) \(\mathrm{CoCl}_{2}\) (b) \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\) (c) \(\mathrm{K}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) (d) \(\mathrm{CoC}_{2} \mathrm{O}_{4}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2} \) and (c) \( \mathrm{K}_{2} \mathrm{C}_{2} \mathrm{O}_{4} \) form colourless beads.

Step 1: Understanding the Borax Bead Test

The borax bead test is a qualitative test used to identify certain metal ions in a compound based on the color exhibited when they are heated in a flame. Typically, transition metal salts form colored beads due to the presence of d-electrons that transition between energy levels. Non-transition metal salts often form colorless beads.

Step 2: Identify Transition Metals

Look at each option and determine if it contains a transition metal: - \( \mathrm{CoCl}_{2} \) contains cobalt, a transition metal.- \( \mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2} \) contains barium, not a transition metal.- \( \mathrm{K}_{2} \mathrm{C}_{2} \mathrm{O}_{4} \) contains potassium, not a transition metal.- \( \mathrm{CoC}_{2} \mathrm{O}_{4} \) contains cobalt, a transition metal.

Step 3: Determine Color Formation

Consider which substances will form colorless beads:- \( \mathrm{CoCl}_{2} \) and \( \mathrm{CoC}_{2} \mathrm{O}_{4} \) form colored beads as cobalt ions produce a blue color.- \( \mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2} \) contains barium ions, which do not form colored beads.- \( \mathrm{K}_{2} \mathrm{C}_{2} \mathrm{O}_{4} \) contains only potassium ions, which also do not form colored beads.

Step 4: Select the Correct Options

Based on the above analysis, \( \mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2} \) and \( \mathrm{K}_{2} \mathrm{C}_{2} \mathrm{O}_{4} \) will form colorless beads in the borax bead test, as they do not have transition metals that produce color.

Key Concepts

Transition metals

Transition metals are elements found in the d-block of the periodic table, ranging from groups 3 to 12. These metals are known for their ability to form various colored compounds. This is because of their partially filled d-orbitals. As electrons move between these orbitals, they absorb and release energy in the form of light. This light often appears as color to our eyes.
For example, a well-known transition metal is cobalt, which produces a blue color during certain reactions, like the borax bead test.

• These metals participate in a wide range of oxidation states.
• They also form complex compounds with interesting colors.
• Due to their versatile nature, they are utilized in various industrial and scientific applications.

Identifying transition metals in a salt can help predict the color outcome in qualitative tests.

Colorless beads

In the borax bead test, the formation of colorless beads occurs when non-transition metals are present. Non-transition metals do not have partially filled d-orbitals, so there is no electron movement to emit light energy. As a result, when these metals are heated in the test, they won't display colors.
For instance, barium or potassium salts tend to form colorless beads.

• Such metals can't transition between energy levels to produce visible light colors.
• The lack of visible color helps in distinguishing them from transition metals during qualitative analysis.

Recognizing these characteristics aids in the quick identification of non-transition metals.

Qualitative analysis

Qualitative analysis refers to the process of determining the presence of certain species or elements in a sample based on their properties, rather than their quantity. It's a powerful tool in chemistry for identifying unknown substances.
In qualitative analysis, the borax bead test is a useful method to identify metal ions based on the color they emit when heated.

• This method is often used to detect transition metals, which produce distinctive colors.
• By observing the bead's color, chemists can make educated guesses about the metal ion present.

Thus, qualitative analysis provides essential clues in chemical identification using simple observational techniques.

Metal ions

Metal ions in their charged form are pivotal in many chemical processes. When a metal loses one or more electrons, it forms a positive ion. Metal ions play a critical role in forming compounds and conducting chemical reactions.
In the context of the borax bead test, understanding metal ions' behavior helps in predicting the bead's color.

• Transition metal ions, due to their electronic structure, are known for their vibrant colors.
• Non-transition metal ions tend to be colorless, aiding in rapid analysis.

As you delve deeper into chemistry, recognizing different metal ions and their characteristics will become an essential skill.