Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

Colloidal solution of gold prepared by different methods are of different colours because of (a) variable valency of gold (b) different concentrations of gold particles (c) impurities produced by different methods (d) different diameters of colloidal gold particle

Step-by-Step Solution

Answer

Gold colloids have different colors due to varying particle diameters (option d).

Step 1: Understanding the Concept of Colloidal Solutions

Colloids are mixtures where one substance is dispersed evenly throughout another. In the case of gold colloids, gold particles are dispersed in a medium such as water.

Step 2: Analyzing the Influence of Particle Size

The color of a colloidal solution, particularly gold, often changes with the size of the particles. Smaller particles might appear red, while larger particles take on a purple or blue hue. This is due to surface plasmon resonance, where light absorption differs based on particle size.

Step 3: Evaluating Other Factors

The valency of gold, concentration of particles, and impurities can affect physical properties but do not primarily influence the color of gold colloids. Variable valency or impurities would affect chemical behavior, while concentration changes opacity rather than color.

Step 4: Narrowing Down to the Correct Option

Based on the factor influencing the color change in colloids, it is the difference in particle size that affects the optical properties and thus the color. Therefore, the correct reason for different colors in gold colloids is the variation in particle diameter.

Key Concepts

Particle Size

One of the primary reasons the color of gold colloidal solutions can vary lies in the size of the particles present. Colloidal particles are incredibly tiny, often in the nanometer range, but even slight changes in size can dramatically affect their appearance.
This phenomenon is because light interacts differently with particles of different sizes.

• Smaller particles typically lead to red or pink hues.
• Larger particles, on the other hand, shift toward blue or purple hues.

The color change is related to the way particles scatter and absorb light. When light hits the particles, the electrons on the surface oscillate at a certain frequency. This frequency changes based on the particle size, altering the light's wavelength that is absorbed or scattered, and hence, changing the perceived color.

Surface Plasmon Resonance

Surface plasmon resonance (SPR) is a key concept in explaining why the color of gold colloids changes with particle size. This resonance occurs when light causes the electrons on the surface of the metallic nanoparticles to oscillate. These oscillations reach a peak when the frequency of the incoming light matches the natural frequency of the surface electrons' movement.
This is what causes the unique absorption of certain wavelengths of light by the particles. Since this phenomena is highly dependent on the particle size, the color of the colloidal solution changes as:

• Smaller particles have resonances at longer wavelengths, making them appear redder.
• Larger particles absorb shorter wavelengths, appearing more toward blues and purples.

SPR is a distinctive characteristic of metal nanoparticles and provides insight into how nanoscale materials interact with light.

Gold Colloids

Gold colloids are fascinating due to their varying colors and unique properties. These are a type of colloidal solution where tiny gold particles are dispersed within a fluid, usually water. The optical properties of these colloids are greatly influenced by their size, which connects to the concepts discussed earlier.
Some key features of gold colloids:

• They are often used in scientific research and medical diagnostics due to their distinctive optical properties and ability to bind to specific molecules.
• The versatility in their color based on particle size offers potential for visual indicators in experiments.
• The preparation method can afford control over particle size, and therefore, the resultant color of the solution. This is critical for applications requiring precision.

Furthermore, manipulating the factors that dictate particle size and SPR allows the customization of gold colloids' optical characteristics, making them valuable in both practical applications and experimental settings.