What Is a Colloid

A colloid is a mixture, wherein the minute particles of one substance are evenly distributed throughout the second substance. The particles within a colloid are of the dimensions between 1 and 1000 nanometers, thereby, making it very difficult for the naked eye to recognize the individual particles. However, these tiny particles that make up the colloid can be observed through an optical microscope. A colloidal system can exist in any of the three forms, solid, liquid or gaseous. The credit of the discovery of colloids can be given to the scientist, Thomas Graham who in the year 1860 clearly distinguished between colloids and mixtures. According to him, the particles of a colloid do not diffuse through a semi permeable membrane and the only method that can be used to separate the particles is dialysis.

There are various ways in which colloids can be classified based on the properties of the dispersed substance and the medium of dispersion. When a gas is dispersed in a liquid is termed ‘foam’ and when it is dispersed in a solid it is referred to as ‘solid foam’. When a solid is dispersed in a gas the resultant mixture is termed a ‘solid aerosol’ and when immersed in liquid it is termed a ‘sol’. A colloidal solution of one liquid in the other is called an emulsion and that of a liquid in a gas is termed ‘aerosol’. Many examples of colloids can be found in the daily life.

Another method of classification of colloids happens on the basis of the size of the particles in them. When the medium of dispersion is a liquid, classification is done based on the affinity of the particles of the colloid to get dissolved in the solvent. Systems, wherein the particles have a great affinity towards the dispersing medium and tend to get dissolved easily, are referred to as lyophilic systems and the systems where the process of dissolving does not happen easily owing to the low affinity of the particles to the solvent are termed as lyophobic systems.

Two important properties of a colloid that are worth mentioning are the Brownian movement and Tyndall Effect. The property of Tyndall effect was discovered by John Tyndall. As per the Tyndall effect, when a beam of light is passed through a liquid, there is a scattering of light and this phenomenon distinguishes the colloids from true solutions. The tiny particles within the colloid when subjected to light appear to be points of light moving freely within the solution.

It is to be noted that the colligative properties of solution like melting point and boiling point are not affected by the presence of particles of the colloid within the solution.