The solute particles settle down when left undisturbed.
Explanation
A colloidal solution is a heterogeneous mixture where the particle size of the dispersed phase is intermediate between that of a true solution and a suspension (typically 1 nm to 1000 nm). While they may appear homogeneous, they possess distinct physical properties that differentiate them from other mixtures.
Analysis of Options:
- It is a heterogeneous mixture. It is a heterogeneous mixture: This is a characteristic property. Although colloids often look like true solutions (homogeneous), they are actually heterogeneous in nature.
- The solute particles settle down when left undisturbed. The solute particles settle down when left undisturbed: This is NOT a characteristic property of colloids. Colloidal particles are constantly in random motion (Brownian motion), which prevents them from settling under the influence of gravity. This stability distinguishes colloids from suspensions, where particles do settle down.
- The particles are too small to be seen with the naked eye. The particles are too small to be seen with the naked eye: This is a characteristic property. Colloidal particles are microscopic and cannot be seen without the aid of powerful microscopes.
- It exhibits the Tyndall effect. It exhibits the Tyndall effect: This is a characteristic property. Colloidal particles are large enough to scatter a beam of light passing through the solution, making the path of the light visible.
Key Takeaway:
Colloids are stable mixtures where particles remain suspended and do not settle down when left undisturbed, unlike suspensions where the solute particles separate over time.