The concept of colloid is used to name that substance that, being in a liquid, disperses little by little. A colloid is made up of two phases: a dispersing or dispersing phase and a disperse phase.
The dispersing phase is a fluid: a continuous substance. The dispersed phase, on the other hand, is made up of colloid particles. In a colloidal system, the colloid particles – which are usually very small solid elements – are dispersed in the dispersing phase. It should be noted that, in certain cases, the dispersing phase of the colloid is not a liquid, but a matter in a different state of aggregation.
Colloid particles are microscopic and therefore cannot be detected with the naked eye. That is why colloids differ from suspensions, whose particles are visible without the need to resort to a microscope. Furthermore, suspensions can be filtered (colloids cannot) and their particles tend to separate at rest (something that the particles of a colloid do not do).
It can be said that a colloid is a non-homogeneous system. According to the force of attraction between the dispersing phase and the dispersed phase, colloids have different characteristics and receive different names, some of which are listed below, with their respective definitions:
* emulsion: is a system in which the continuous and dispersed phases are liquid. It occurs when a liquid is colloidally suspended in another with which it cannot be mixed. For its preparation, you can start by placing the two liquids in a suitable container and shaking the mixture. Another possibility is to run it through a homogenizer, a name given to a colloidal mill;
* sun: despite what this name may suggest at first, a colloid of relative instability is called lyophobic sun (they are described as meta stable). In general, raising the temperature or adding a moderate amount of electrolyte is sufficient to cause the dispersed particles to coagulate and precipitate;
* aerosol: according to its definition in the field of environmental engineering, it is a colloidal or colloid system whose particles can be solid or liquid and are dispersed in a gas with a marked subdivision. In fact, when saying aerosol it is possible to speak both of the particles and of the gas itself. Today, the word aerosol is used in everyday speech to refer to a metal container whose contents are pressurized;
* gel: it is a colloid in which the dispersed phase is liquid, while the continuous phase is solid. Its density can be compared to that of liquids, although its structure is more similar to that of a solid. Edible gelatin is probably one of the most common gels on a general level. Some gels are capable of changing their colloidal state, depending on whether they are left standing (to maintain their solid state) or shaken (to make them become liquid);
* foam: defined as a globular liquid layer that encloses gas or vapor. Although it resembles an emulsion, one of its differences is that its dispersed phase is a gas, and not a liquid. Furthermore, its gas bubbles are much larger than the globules of the emulsion. An example of this type of colloid can be seen on the coastline.
The gelatin, the cheese, the shaving cream (shaving) and fog are examples of colloids. If we focus on the case of gelatin, we will notice that it is a mixture that, at room temperature, is semi-solid, which is translucent and colorless. This colloid is produced by boiling collagen.