Question

Highly pure dilute solution of sodium in liquid ammonia (a) exhibits electrical neutrality (b) produces sodium amide (c) produces hydrogen gas (d) show blue colour

Short answer

A highly pure dilute solution of sodium in liquid ammonia shows a blue color.

Step-by-step solution

Understanding the Context

A dilute solution of sodium in liquid ammonia is a classic example in chemistry, where sodium metal is dissolved in liquid ammonia.

Identifying Characteristics

Sodium in liquid ammonia doesn't stay as neutral atoms but dissociates into sodium ions and free electrons, causing certain properties.

Color Observation

Due to the presence of solvated electrons, which are electrons released from sodium atoms, the solution exhibits a distinct blue color.

Reason Behind Blue Color

The blue color occurs because the solvated electrons absorb in the visible region of light, thus imparting a blue hue to the solution.

Key concepts

solvated electrons

When sodium metal dissolves in liquid ammonia, it releases free electrons from the sodium atoms into the solution. These electrons don't just float around randomly. Instead, they interact with the surrounding solvent molecules, creating what are called "solvated electrons."
Solvated electrons are essentially free electrons that are stabilized by the solvent, in this case, liquid ammonia. The interaction is primarily due to attraction forces between the negatively charged electrons and the partial positive charges on the hydrogen atoms of ammonia.

• These electrons become surrounded by ammonia molecules, forming a kind of "cage" that stabilizes them.
• This stabilization keeps them from immediately recombining with the positive sodium ions.

In essence, solvated electrons are a unique form of electron behavior seen in certain chemical environments such as those provided by liquid ammonia.

electrical conductivity

The presence of solvated electrons in the solution not only gives the solution a blue color but also contributes to its ability to conduct electricity. In this context, electrical conductivity refers to the ability of the solution to carry an electrical current.
Here's how it works:

• Sodium metal initially dissociates into sodium ions and electrons when dissolved in liquid ammonia.
• While the sodium ions provide positive charges, the solvated electrons contribute freely moving negative charges within the solution.

Because both positive and negative charges are present and can move, the solution facilitates the flow of current. This makes the sodium-ammonia solution a good conductor of electricity, comparable to many solutions of ionic compounds in water.
Overall, the movement of both solvated electrons and sodium ions contributes to this unique property.

blue color of solution

One of the most striking properties of a solution of sodium in liquid ammonia is its vivid blue color. This blue color arises due to the presence of solvated electrons, which are highly influential in the way the solution interacts with light.
Solvated electrons absorb certain wavelengths of light, specifically in the visible spectrum, which leads to:

• Absorption of lower-energy light, particularly in the red and yellow regions.
• What is not absorbed is reflected, which is primarily the blue part of the spectrum.

Thus, the solution appears blue to our eyes.
This optical feature of sodium in liquid ammonia makes it an interesting subject of study in physical chemistry. Not only does it visually signify the presence of solvated electrons, but it also underscores the interactions between light and matter at the molecular level.