The solute particles settle down when left undisturbed.
Explanation
Mixtures are classified based on particle size and stability into true solutions, colloids, and suspensions. A true solution is a homogeneous mixture with very small solute particles (<1 nm). A suspension is a heterogeneous mixture containing solid particles large enough for sedimentation (>100 nm).
Option Analysis
- The mixture is homogeneous. is incorrect: A true solution is a homogeneous mixture where the composition is uniform throughout. A suspension is a heterogeneous mixture where the particles remain suspended in the bulk of the medium and are not uniformly distributed.
- The particles are invisible to the naked eye. is incorrect: In a true solution, particles are extremely small (less than 1 nm in diameter) and cannot be seen with the naked eye. In a suspension, the particles are relatively large (greater than 100 nm) and can often be seen with the naked eye.
- The solute particles settle down when left undisturbed. is correct: Suspensions are unstable. The solute particles are heavy enough that gravity causes them to settle down at the bottom of the container when the mixture is left undisturbed. In contrast, true solutions are stable, and particles do not settle.
- The particles can pass through filter paper. is incorrect: The particles in a true solution are small enough to pass through filter paper. In a suspension, the particles are large enough to be retained by filter paper, allowing separation by filtration.
Key Takeaway
The defining characteristic of a suspension is its instability; the solute particles are large enough to settle down under gravity when undisturbed, unlike the stable particles in a true solution.