What is the main characteristic of a colloidal solution?

A colloidal solution is primarily defined by the presence of particles that are dispersed throughout a medium but are not visible to the naked eye. These particles, typically ranging from 1 nanometer to 1 micrometer in size, remain suspended and do not settle out over time due to gravitational forces. This stability results from Brownian motion, which keeps the particles moving and prevents them from aggregating and settling.

In contrast to other types of mixtures, such as suspensions where larger particles eventually settle, or solutions with dissolved substances that can't be filtered out, a colloidal solution maintains its cloudiness or opacity, giving it distinct optical properties. This characteristic is fundamental because it allows colloidal solutions to scatter light, leading to phenomena such as the Tyndall effect, where light becomes visible when it passes through a colloidal mixture.

Given this understanding, the defining characteristic of colloidal solutions emphasizes their ability to remain dispersed without settling, which is why this option accurately describes the nature of colloidal solutions in water treatment and other applications.