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.)
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.