Introduction:
In order to understand why the particles of a colloidal solution do not settle down when left undisturbed, while in the case of a suspension they do, it is important to first define what colloidal solutions and suspensions are.

Colloidal Solutions:
A colloidal solution is a heterogeneous mixture in which the particle size is intermediate between true solutions and suspensions. The particles in a colloidal solution are larger than individual molecules but smaller than those in a suspension. These particles are called colloids and they do not settle down when left undisturbed.

Suspensions:
Suspensions, on the other hand, are heterogeneous mixtures in which solid particles are dispersed in a liquid or a gas. The particles in a suspension are larger and more easily visible compared to those in a colloidal solution. When a suspension is left undisturbed, the particles tend to settle down at the bottom due to gravity.

Size of Particles:
The key difference between colloidal solutions and suspensions lies in the size of the particles present.

- In a colloidal solution, the particles are typically in the range of 1 to 1000 nanometers in diameter. These particles are much smaller and lighter compared to the particles in a suspension. Due to their small size, the particles in a colloidal solution experience Brownian motion, which is the random movement of particles caused by collisions with surrounding molecules. This Brownian motion prevents the particles from settling down, as they are constantly being jostled and moved around by the surrounding fluid.

- In a suspension, the particles are often larger than 1000 nanometers in diameter. These particles are heavier and more prone to settling down under the influence of gravity. When a suspension is left undisturbed, the particles gradually settle down due to their larger size and weight.

Stability of Colloidal Solutions:
The stability of colloidal solutions is also attributed to other factors such as the charge on the particles and the presence of a dispersing medium.

- Colloidal particles often carry an electric charge, which creates a repulsive force between them. This electrostatic repulsion prevents the particles from coming together and settling down.

- The presence of a dispersing medium, such as water, also helps to keep the colloidal particles suspended. The dispersing medium provides a continuous medium for the particles to move and remain dispersed.

Conclusion:
In summary, the particles of a colloidal solution do not settle down when left undisturbed due to their small size, Brownian motion, electrostatic repulsion, and the presence of a dispersing medium. In contrast, the larger particles in a suspension settle down under the influence of gravity. Understanding these differences is crucial in various fields such as chemistry, biology, and materials science, as it allows us to manipulate and utilize colloidal solutions and suspensions for various applications.