A colloidal suspension is a mixture of usually two materials where one is dispersed in the other at a microscopic level, but not chemically bonded to it. Liquids, solids, and gasses can all be part of a colloidal suspension with the exception that one gas cannot be colloidally suspended in another gas. The particles that act as the colloid in a suspension are typically larger in size than those found in solutions and range from 1 to 1,000 nanometers or billionths of a meter in diameter. They tend to be evenly distributed throughout the suspension if it has been recently mixed or stirred, but will settle to the bottom of the solution due to gravity if it is allowed to sit undisturbed for an extended period of time.
Good examples of colloidal suspensions include milk, paint, and smoke. Milk from cows, goats, and other animals is a suspension of fat globules in liquid water, as an example of a liquid colloid suspended in a liquid. Paint is an example of a solid colloid suspended in a liquid where paint pigments, often composed of heavy metal powders such as chromium oxide or zinc, are suspended in oil, water, or hydrocarbon solvent bases. Smoke is an example of a solid colloid suspended in a gas, where the smoke is composed of fine particles of ash as combustible residue dispersed in air. Even human blood is considered to be a colloidal suspension, with biological materials such as solid proteins suspended in liquid blood serum or plasma.
A unique property of many colloids is that they can have an opaque or translucent appearance to them. This is due to the fact that the suspended particles are often large enough individually or as collections of molecules that they act to partially scatter light as it passes through the colloidal suspension. Blue light tends to be much more effectively scattered than longer wavelengths of light, such as red light, so colloidal suspensions often have a blue-tinged appearance to them, such as fog in air. This feature of a colloidal suspension has been used to create special types of paint and varnish that have a reflective quality that lends a glow to the coated surface. The suspension of particles and glow can be maintained to some degree, even after drying has taken place.
The properties of a colloidal suspension exist in a rather delicate balance. The colloid particles will only remain in suspension and avoid clumping together if the repulsive charges they carry balance out against natural attractive Van der Waals forces in the suspension. Artificial means of maintaining this balance to create a colloidal suspensions have been developed. An example of this is a hydrophobic molecule of detergent that can act as a colloid in water with a natural ability to avoid bonding to water molecules, yet possessing an attractive charge towards dirt and oil particles also suspended in the water.