Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

The simplest way, to check whether a system is colloidal or not is by (a) Tyndall effect (b) electrodialysis (c) finding out particle size (d) Brownian movement

Step-by-Step Solution

Answer

(a) Tyndall effect

Step 1: Understand Colloids

A colloid is a mixture with particle sizes between those in true solutions and suspensions. The particles are big enough to scatter light but too small to settle under gravity.

Step 2: Identify Tyndall Effect

The Tyndall effect is the scattering of light as it passes through a colloidal dispersion, making the beam of light visible. This occurs due to the size of the colloidal particles being comparable to the wavelength of visible light.

Step 3: Consider Electrodialysis

Electrodialysis is a process that separates ions from solutions using an electric field. It is not specifically a method for checking if a system is colloidal but rather for purifying or desalting.

Step 4: Evaluate Particle Size Measurement

Finding out particle size directly can determine if a dispersion is colloidal, as colloids have particle sizes typically between 1 nm and 1000 nm. However, specialized equipment is needed for this purpose.

Step 5: Understand Brownian Movement

Brownian movement is the random motion of particles in a fluid. While it is observed in colloidal systems, it can also occur in other types of mixtures and is not exclusively indicative of a colloidal system.

Step 6: Conclusion

Among the options, observing the Tyndall effect is the simplest and most direct way to check if a system is colloidal. It can be done with basic optical equipment and readily shows the presence of colloidal particles.

Key Concepts

Tyndall effect

One of the simplest methods to determine if a mixture is colloidal is to observe the Tyndall effect. The Tyndall effect is a phenomenon where light is scattered by particles within a mixture, making the path of the light beam visible. This happens because the colloidal particles have a size similar to the wavelength of visible light.

• When a beam of light passes through a colloid, the light scattering occurs, producing a visible beam.
• In true solutions, the particles are too small to scatter light, so the Tyndall effect is not observed.

Looking for the Tyndall effect is practical and easy. You can shine a flashlight or any coherent light source through the mixture. If the light path appears as a visible beam, a colloid is likely present. This test is especially useful because it doesn't require complex equipment.

Particle size measurement

Colloids are defined by their particle size, which generally ranges from 1 nm to 1000 nm. Determining particle size is crucial for identifying colloidal systems, but it requires specialized methods and equipment. Here's why understanding particle sizes matters:

• An accurate measurement allows you to differentiate between solutions, colloids, and suspensions.
• Typical methods for measuring particle size include dynamic light scattering (DLS) and electron microscopy.

Dynamic light scattering is based on analyzing the pattern of scatter caused by particles. This non-invasive technique provides the size distribution of particles in suspension. Although practical and accurate, measuring particle size might not be as quick or cost-effective as observing the Tyndall effect. However, it remains a critical approach for detailed analyses and research settings.

Brownian movement

Another interesting phenomenon related to colloids is Brownian movement, which describes the random motion of particles suspended in a fluid. This movement results from collisions with fast-moving molecules in the liquid or gas, creating a seemingly erratic path.

• Brownian movement is more vigorous in smaller particles due to their lower mass and higher velocity collisions.
• This effect helps stabilize colloids as it prevents the settling of particles under gravity.

Despite being a characteristic of colloids, Brownian motion is observed in other mixtures as well. Therefore, while it's indicative of a colloidal system, it's not exclusive to it. This means that while Brownian movement provides insights into the behavior of colloids, it is not the sole criterion for their identification.