Suspension Properties and Tyndall Effect

The question describes a mixture that is heterogeneous, meaning it has visibly distinct parts or phases. It initially exhibits the Tyndall effect, which is the scattering of light as a light beam passes through the mixture. This scattering occurs because the particles within the mixture are large enough to deflect light. However, the mixture stops showing the Tyndall effect after some time, indicating a change in its composition or particle distribution.

Understanding the Tyndall Effect

The Tyndall effect is a characteristic property observed in colloids and fine suspensions. It happens when light encounters dispersed particles that are large enough (typically between 1 nm and 1000 nm) to scatter the light rays, making the path of the light visible. Homogeneous mixtures, like true solutions, do not show the Tyndall effect because their solute particles are too small (less than 1 nm) to scatter light effectively.

Analyzing Mixture Behavior: Suspension vs. Colloid

Let's analyze the behavior described in the question:

• The mixture is heterogeneous. Both suspensions and colloids are heterogeneous.
• It shows the Tyndall effect initially. Both suspensions and colloids can show the Tyndall effect.
• It stops showing the Tyndall effect after some time. This suggests that the particles responsible for scattering light are no longer dispersed in the medium.

Evaluating the Options

We need to determine which type of mixture fits this behavior:

• Option 1: It is a colloid which changes into a solution on keeping for some time. While colloids can sometimes aggregate, changing into a true solution is not a typical transformation. If it became a solution, it would stop scattering light, but the initial state description points more strongly towards a suspension.
• Option 2: It is a suspension which does not scatter light when its particles settle down. Suspensions contain larger particles (typically > 1000 nm) that are dispersed but eventually settle out due to gravity. When the particles settle, they are no longer suspended in the medium to scatter light, causing the mixture to become clearer and stop showing the Tyndall effect. This perfectly matches the description: heterogeneous, shows Tyndall effect initially, and stops showing it as particles settle.
• Option 3: It is a colloid which stops scattering light as it changes into a suspension. This is contradictory. If it changes into a suspension, it might still scatter light, or if the particles aggregate and settle, it would be similar to a suspension settling. The core issue is stopping the Tyndall effect, which settling achieves.
• Option 4: It is a suspension which changes into a solution on keeping for some time. Similar to option 1, suspensions don't typically transform into true solutions. The settling of particles is the more common phenomenon that leads to the cessation of the Tyndall effect.

Conclusion on Mixture Type

The most accurate description for a heterogeneous mixture that initially shows the Tyndall effect but stops doing so over time is a suspension whose particles have settled. Option 2 correctly identifies this behavior, linking the cessation of light scattering to the settling of suspension particles.