OHSP 3a&b 1. A colloid is a dispersion of particles of one substance, the dispersed phase, throughout a dispersing medium made of another substance. 2. Colloids are classified according to the phases of the dispersed material and dispersing medium. The types of colloids are sol, gel, emulsion, foam and aerosol. 3. A colloid is distinguished from a regular solution by the Tyndall effect, which is the scattering of visible light by colloidal particles. 4. Colloids exhibit special properties like Tyndall effect, Brownian motion, adsorption and electrical charge effect. 5. Brownian motion is the movement of colloidal particles in a random and zigzag fashion. 6. Since colloids have a large surface area, they exhibit high adsorbing capacity. 7. The ions adsorbed on the surface of a colloid produce an electrical charge. This, along with Brownian motion, prevents colloids from coagulating. 8. Dispersion and condensation are two ways of making a given substance disperse to colloidal size. When larger pieces are broken into colloidal size, the process is dispersion. But when tiny particles clump together to form clusters the process is condensation. Colloids in Environment The use of colloids in the environment can be both harmful and beneficial. Some may be natural while others are caused by human activities. Harmful ones can be made beneficial if we just learn how to manipulate colloids. Aerosols are one of the unwanted colloids in the environment. These consist of solid or liquid particles that are dispersed in air smaller than 100m in diameter. These very small particles include carbon black, silver iodide, and sea salt. These suspended particles in air are commonly called particulates or simply particulate matter (PM). Larger particulates include cement dusts and soil dust. Even larger particulates are raindrops, fog and sulfuric acid mist. In some of these particulates, viruses, bacteria and fungal spores may also be present. The presence of these bacteria causing diseases can be a threat to health in the community as what happened in the meningococcemia epidemic in Baguio City on December 2004. Some toxic air pollutants such as unburned hydrocarbons from motor vehicles can cause respiratory irritations when inhaled. Aside from posing a threat to human health, these particulates can also affect the transport industry when road visibility is reduced by during fog or mist. In our waterways, the disposal of detergents and other pollutants can act as protective colloids that stabilize foam formation. When these foams accumulate on the surface of water, they can seriously reduce the amount of sunlight that can penetrate the water surface. In this regard, the photosynthetic activity of aquatic plants is drastically reduced. Lesson 2. What are the Properties of a Colloid? Colloids are a special type of mixture because they exhibit unique properties. That is why many kinds of food products and raw materials are in the form of colloids. These unique properties of colloids include the Tyndall effect, Brownian motion, adsorption and electrical charge effect. Tyndall Effect When a beam of light is made to pass through a colloid, it is scattered by the colloidal particles which appear as tiny specks of light. This light scattering is called the Tyndall effect. No such scattering is observed with ordinary solutions because the solute molecules are too small to interact with visible light. Colloidal particles can scatter light because they are large enough to catch light and reflect it back. This is the reason why oil and water mixture in Activity 1.1 has the ability to scatter light because it is a colloid. The ability of a mixture to scatter light is one way of distinguishing colloids from solutions. Manifestations of Tyndall effect are observed everyday in nature. For instance, when we wake up on a sunny morning, our senses are awakened by sunlight scattered by dust or smoke in the air. Light scattering is often used in determining particle size. The different colors that we see are not due to pigments but rather to the scattering of light by colloidal substances in the iris. Green, brown, and black are due to a combination of light scattering and the presence of yellowish-brown pigment in front of the iris, causing selective absorption of light. Brownian Movement Observe the movement of dust particles floating in air one sunny day. Could you describe their movement? Do they settle on standing? The motion of particles floating in air is random and almost in a zigzag fashion. This interesting property of colloids, called Brownian motion, is due to the constant and continuous collision of colloidal particles against each other. This is also the reason why colloidal particles do not settle on standing. The rate of settling of particles is dependent on the following: a. size of the colloidal particles b. gravitational force acting on the colloidal particles c. viscosity of the medium (ability to resist flow) Water and gasoline mixture has low viscosity as compared to syrup and oil mixture which has high viscosity. The above factors are also useful in identifying viruses, proteins, plastics and other macromolecules. Adsorption How is adsorption different from absorption? If you place a few drops of water onto a cotton ball, the water droplets are immediately soaked up by the cotton. This is absorption. On the other hand, adsorption is when you use the cotton ball against the chalkboard. Observe closely what happens to the cotton. Chalk particles and dust adhered to the surface. This phenomenon is called adsorption. Physical and/or chemical forces may be involved in adsorption. One property of colloidal particles is that they exhibit adsorption. This is due to the large surface areas of colloidal particles. This interesting property makes colloids very useful in everyday life. For example, charcoal is used to remove the bad odor produced by vapors of food in the refrigerator. The network of pores in the charcoal provides extensive surface area that adsorbs the vapors. The adsorptive ability of colloids is used in dyeing fabrics, in the use of aluminum hydroxide in purifying water, in the use of activated carbon in refining sugar and electroplating solutions, and in the use of bone black in gas masks to remove toxic gases in the air. Electrical Charge Effect A colloid may allow ions to be adsorbed on its surface, thereby acquiring an electrical charge. The electrical charge may either be positive or negative. This electrical charge will prevent the particles from clumping together or coagulating.
Posted on: Mon, 23 Sep 2013 02:15:17 +0000