Colloids

Solutions are homogeneous, and a suspension (like sand stirred into water) settles out with time. Between these two extremes lies a large family of systems called colloidal dispersions, or simply colloids.

• A colloid is a heterogeneous system in which one substance — the dispersed phase — is spread as very fine particles through another substance, the dispersion medium.

Particle size is the key difference

The essential difference from a true solution is particle size. In a solution the particles are ions or small molecules; in a colloid the dispersed particle may be a single macromolecule (a protein or synthetic polymer) or an aggregate of many atoms, ions or molecules. Colloidal particles are larger than simple molecules but still small enough to stay suspended, with diameters between 1 and 1000 nm ($10^{-9}$ to $10^{-6}$ m).

Enormous surface area …