Colloids: Tyndall Effect

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• published: 30 Oct 2014
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Colloid is a heterogeneous substance (with 2 or more phases) consisting of insoluble particles dispersed in continuous medium. It can also be referred as a dispersion phase (e.g. milk powder) in a dispersant phase (e.g. water). The particles are well-distributed in the medium and will only settle down to the bottom of the container after a very long period of time. They can be an aggregate of tiny molecules or one big molecule. The properties of colloid is located in between a suspension and solution. For example, a typical colloidal particle has diameter around 0.002 - 0.2 mm, which is normally bigger than particles found in a solution and smaller than particles in suspension. The particles might be bigger than the solute in solution, but they still too small in size to settle down only by gravitational force. Another significant property of colloid is its ability to scatter or absorb beam of light or better known as Tyndall Effect which is defined as the scattering of light by particles in a colloid. It is due to colloidal particles having diameter which is very close to the wavelength of the light passes through it. Similar result can also be obtained if the distances between colloidal particles are comparable to the wavelength of the light. Visible light has wavelength ranging from 350 to 700 nm, and it is almost the same as the diameter of colloidal particle. The colloids chosen for the experiment is milk particles in water. The aim of this experiment is to prove that light gets scattered in colloids and the path of scattered light can be seen by naked eye. Two beakers, a pack of milk and a laser pointer are prepared for this experiment. In the beginning, two beakers locating side by side are filled with water and light is being shone through both beakers. No light path is observed because the molecules of water are too small to scatter the light. After that, a small amount of milk is poured into the beaker and stirred to obtain a colloid. As a result, a clear bright red light path passing through the beaker is seen. This shows that Tyndall effect only applies to colloid. Tyndall effect is mainly used when dealing with colloidal particles. For example, Tyndall effect is used in waste water treatment to examine the pollution level of water. Besides that, food industry also utilise this effect to detect presence of contaminants in different fluids. Furthermore, it is also very important in pharmaceutical industry to observe microbiological cultures. Last but not least, pulp and paper industry uses Tyndall effect to monitor the clarity of the mild acids used in the production of paper.