A colloidal solution, sometimes known as a colloidal suspension, is a solution in which a material is evenly suspended in a liquid. In other words, a colloid is a microscopically small substance that is equally dispersed throughout another material. This graphic provides a good introduction to colloidal materials:
While colloidal systems can exist in any one of the three main forms of matter, solid, liquid, or gas, a colloidal solution specifically refers to a liquid mixture. The differentiating factor between a true solution and a colloidal solution is essentially the size of the particles. In a true solution, such as salt water, sodium chloride molecules are fully dissolved into water, and the solution can pass through a membrane without becoming separated. In a colloidal solution, on the other hand, the particles are larger and don’t dissolve, but rather become equally dispersed throughout a liquid. As such these particles will not pass through a membrane as the liquid does.
There are three sub classifications of colloidal solutions: foams, emulsions, and sols. A foam in this context is formed by trapping a gas in a liquid. The material being distributed would be the gas, causing the liquid to become foamy. An example of this would be shaving cream. An emulsion is a mixture of liquids; it’s essentially when one liquid is evenly distributed throughout another liquid. An example of this would be milk or mayonnaise. The third form is called a sol, which is when a solid is evenly distributed throughout a liquid. Examples of sols include blood, paint, and silver aquasols.
A hydrocolloid is a more specific type of colloid solution in which the liquid a specific material is being dispersed in is water. These colloidal solutions can be further described as being reversible, in which the two substances can be easily separated again, or irreversible, in which they cannot. Hydrocolloids are commonly used in food products to manipulate texture, like in gelatins or sauces.
In the natural environment, colloidal solutions are important in many processes, and can often serve as transport vectors. Various particles being transported through water can essentially attach themselves to the particles suspended in the water. By the same token, colloidal solutions can serve to transport more damaging substances through the water table, such as radioactive material.
Colloidal solutions are very important in the medical field because they can be used to manipulate blood conditions. More specifically, they are often used to regulate colloidal osmotic pressure, a pressure applied by proteins in the blood to pull water in the vascular system.