Colloidal particles are generally aggregates of numerous atoms or molecules. They pass through most filter papers, but can be detected by light-scattering, sedimentation and osmosis. A characteristic of colloids is absorption, as the finely divided colloidal particles have a large exposed surface area.

The chemical and physical properties of inorganic colloids can be changed dramaticaly when their size is reduced to a number of nanometers. This effect is due to the increasing importance of the colloid surface.

The presence of colloidal particles in a solution has only minor effects on its colligative properties (boiling, freezing point, etc.)

Toothpaste shows thixotropic properties. This image was copied from without permission Certain paints show thixotropic properties. This image was copied from without permission

Thixotropy is a property exhibited by certain gels. This is where a gel appears solid and maintains its own shape until it is subjected to some force or disturbance, such as shaking. It then tends to act as a sol, flowing freely. This behavior is reversible, and the sol will return to a gel if left undisturbed. Examples of thixotropic gels include certain paints, printing inks and clays.

The particles of a colloid selectively absorb ions and acquire an electric charge. The existence of an electric charge on the surfaces of the colloidal particles is a source of kinetic stability for colloids. All of the particles of a given colloid are repelled by one another as they all take on the same charge. The movement of collo! idal particles through a fluid under the influence of an electric field is known as electrophoresis.

The Tyndall effect