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

(d) Different diameters of colloidal gold particle cause different colors.

Step 1: Understanding Colloidal Solution of Gold

Colloidal solutions involve particles dispersed throughout a medium. In the case of a gold colloid, the gold particles are dispersed in a liquid.

Step 2: Exploring Color Variation in Colloids

The color of colloids often depends on the size and distribution of the particles. For gold colloids, variations in particle size can lead to different interactions with light, resulting in various colors.

Step 3: Examining Each Option

Consider each given option: (a) variable valency of gold is unlikely to affect color without a change in particle interaction. (b) Different concentrations typically affect opaqueness, not color. (c) Impurities can alter color but are generally not the primary cause. (d) Different diameters of particles directly affect how light is scattered, leading to varying colors.

Step 4: Identifying the Primary Cause

While impurities and concentration can have effects, the primary reason colloidal solutions of gold are of different colors is due to the different particle sizes. These different sizes cause different scattering of light, hence the variations in color.

Key Concepts

Particle Size

In the fascinating world of colloids, particle size plays a crucial role.
When we talk about particle size in colloidal solutions, we're referring to the very tiny particles dispersed in a medium.
These particles are usually in the range between 1 and 1000 nanometers.
This small size is what gives colloids their unique properties.
- **Importance of Size and Distribution:**
In a colloidal solution like a gold colloid, particles can vary in size, and even small changes in these dimensions can lead to remarkable differences in properties.
- **Effects on Properties:**
These particles are large enough to scatter light but not large enough to settle out by gravity, which is why they stay dispersed.
The size directly influences color, stability, and the ability to scatter light, which is key to understanding their behavior in solutions.
When studying colloidal solutions, always keep in mind that particle size is a primary factor that determines how the colloid will behave and appear.

Light Scattering

One of the most intriguing aspects of colloids is their interaction with light.
This is where light scattering becomes a central topic.
Colloidal particles are capable of scattering light due to their size, which is on the scale of the wavelength of visible light.
- **Tyndall Effect:**
When light passes through a colloid, it scatters in all directions.
This is known as the Tyndall effect, named after the scientist John Tyndall who first observed this phenomenon.
- **Significance of Scattering:**
The scattering of light not only causes the colloid to be visible but also contributes to its apparent color.
Different particle sizes will scatter different wavelengths of light, resulting in color variations.
In gold colloids, for instance, smaller particles might appear red as they absorb and scatter shorter wavelengths, while larger ones can appear blue or purple due to the scattering of longer wavelengths.
This tells us a lot about the particle size and composition just by observing the color, thanks to light scattering.

Color Variation in Colloids

The beautiful range of colors seen in colloidal solutions, especially those involving metals like gold, is a result of multiple factors, with particle size being the most significant.
- **Size Dependent Colors:**
In gold colloids, different particle sizes absorb and scatter different wavelengths of light.
For example, particles around 5-10 nanometers often exhibit a red color, while those that are larger can shift to a blue or purple hue.
- **Other Influencing Factors:**
While impurities and particle concentration might slightly affect the color, they are not the primary influencers.
Instead, impurities may dull the color and concentration can affect the opacity and intensity of the color. Thus, when we see a colorful colloidal gold solution, it is primarily the particle sizes that dictate the color.
This variation is a striking example of how slight changes in nanoparticle dimensions can manifest as completely different phenomena in the observable world.