Problem 65 - A Complete Resource Book in Chemistry for JEE Main

Question

The aqueous solution of the following salts will be coloured in the case of (a) \(\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{LiNO}_{3}\) (c) \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\) (d) potash alum

Step-by-Step Solution

Answer

The solution of \(\mathrm{Co(NO_{3})_{2}}\) will be colored.

Step 1: Identify the Metal Ions

Determine the metal ions present in each of the given salts: - \(\mathrm{Zn(NO_{3})_{2}}\) contains \(\mathrm{Zn^{2+}}\) ions.- \(\mathrm{LiNO_{3}}\) contains \(\mathrm{Li^{+}}\) ions.- \(\mathrm{Co(NO_{3})_{2}}\) contains \(\mathrm{Co^{2+}}\) ions.- Potash alum is a double salt consisting of \(\mathrm{KAl(SO_{4})_{2}}\), which includes \(\mathrm{Al^{3+}}\) and \(\mathrm{K^{+}}\) ions.

Step 2: Determine Ion Color Properties

Evaluate the typical coloration properties of the metal ions in an aqueous solution:- \(\mathrm{Zn^{2+}}\): Generally colorless.- \(\mathrm{Li^{+}}\): Generally colorless.- \(\mathrm{Co^{2+}}\): Typically produces a pink or red color in solution.- \(\mathrm{Al^{3+}}\) and \(\mathrm{K^{+}}\): Both are generally colorless.

Step 3: Identify the Salt with Colored Solution

From the observations above, only the aqueous solution of the salt containing \(\mathrm{Co^{2+}}\) ions (i.e., \(\mathrm{Co(NO_{3})_{2}}\)) will be colored. Therefore, \(\mathrm{Co(NO_{3})_{2}}\) will produce a colored solution, specifically pink or red.

Key Concepts

Aqueous Solutions

When we talk about aqueous solutions, we are referring to a solution where water is the solvent. This is quite common because water is a universal solvent and can dissolve a wide range of substances. The word "aqueous" comes from "aqua," which means water. These solutions can conduct electricity if the dissolved substances are ionic. For instance, salts like those mentioned in the exercise, when dissolved in water, create an aqueous solution. A few things happen when ionic salts dissolve in water:

• The salt breaks down into its individual ions.
• The water molecules surround and separate the ions.
• This process is known as dissociation.

This dissociation is crucial because it allows the ions to interact with light, leading to visible color changes in the solution. When the metal ions from these salts interact with water, they take on distinctive characteristics that we can observe, such as color, if the ions are of transition metals.

Color Properties of Metal Ions

Metal ions in aqueous solutions can exhibit various colors based on their specific electronic configurations. The color a metal imparts to a solution is often due to its ability to absorb certain wavelengths of light. Transition metals, such as cobalt (\( \mathrm{Co^{2+}} \)), typically form colored ions if they are dissolved in water. This color formation can be explained by the absorption of light:

• When light passes through the solution, certain wavelengths are absorbed by the metal ions.
• The wavelengths that are not absorbed are what we see as color.
• For cobalt ions, the result is a pink or red hue when they are in solution because of how they absorb and emit light.

Other metal ions like \( \mathrm{Li^{+}} \), \( \mathrm{Zn^{2+}} \), \( \mathrm{Al^{3+}} \), and \( \mathrm{K^{+}} \) do not significantly absorb visible light and thus appear colorless to our eyes. The presence of transition metals often results in vibrant colors, making them interesting subjects in coordination chemistry.

Coordination Chemistry

Coordination chemistry is the study of complexes formed between metal ions and ligands. Ligands are ions or molecules that can donate lone pairs of electrons to the metal, forming coordinate bonds. This type of chemistry is particularly significant when dealing with transition metals, which include elements in the group that contains metals like cobalt and zinc.In coordination complexes, several factors affect color:

• The type of ligand bound to the metal.
• The oxidation state of the metal ion.
• The number of d-electrons in the metal ion.
• The geometry of the complex.

The pink hue seen in \( \mathrm{Co(NO_{3})_{2}} \) aqueous solutions results from its cobalt ions interacting with the nitrate ions, forming a specific coordination complex in water. This interaction alters the metal's electronic environment, which in turn changes the wavelengths of light absorbed, causing the distinct coloration. Understanding these interactions and formations helps us predict and explain the behaviors of such metal ions in aqueous solutions.