EXPERIMENT 16: COLLOIDS PURPOSE Some colloids will be prepared and their unique properties investigated. BACKGROUND Most people have seen a muddy river or muddy stream. As the water flows, the clay and silt remain evenly distributed. Other things, like fog, smog, mayonnaise, milk, gelatin, glue, blood, paint and ink, are members of a special type of mixture called colloids. A colloid is neither a true solution nor a suspension, but in some ways possesses the characteristics of both. The material being dispersed, liquid or solid, has a particle size of 10 7 to 10 4 cm. The gas, liquid, or solid, into which these particles are dispersed is called the dispersion medium. The small particle size allows the dispersed material to remain uniformly distributed due to the effects of Brownian motion. Every type of colloid has a name and various dispersions can be classified by these titles: aerosol solid aerosol liquid foam emulsion sol or gel solid foam solid emulsion solid sol liquid in gas solid in a gas gas in a liquid liquid in a liquid solid in a liquid gas in a solid liquid in a solid solid in a solid Material dispersed in a liquid will often form a true solution instead of a colloid. A mixture of two gases always forms a true solution. INTERACTION OF LIGHT WITH COLLOIDS The scattering of light by colloidal particles was first explained by John Tyndall in 1869. When a beam of light is passed through a colloidal dispersion, the transmitted light appears different from the light scattered to the side. This scattering of light is an example of the Tyndall Effect.
The color of the scattered light depends upon the size of the particles in the dispersion. Scattering is caused by particles that are about the same size as the wavelength of the light being scattered. Blue light has the shortest wavelength and appears to be scattered first by the formation of the small particles. The transmitted light contains all other wavelengths and appears yellow. As the particles grow in size longer wavelengths are scattered and the color of the light that is transmitted gradually changes. Those wavelengths of light which are not scattered are transmitted. A light beam passed through a bowl of Jell-o would exhibit the Tyndall Effect. This is because gelatin belongs to a type of colloid called a gel. Photographic film, silica gel (used for the adsorption of gases), and some living tissues are gels. STABILIZATION OF COLLOIDS Under ordinary conditions, some substances do not form solutions because of insolubility or immiscibility. These substances can be colloidally dispersed with the aid of a stabilizer. These stabilizers prevent precipitation or separation by deterring particle growth. Salad dressings contain stabilizers which prevent the oil from separating from the vinegar. COMMERCIAL COLLOIDS All brands of toothpaste are colloids, dispersions of many ingredients in methyl cellulose. The ingredients usually consist of calcium carbonate (chalk or limestone), magnesium carbonate (used in dental abrasives, and also used in medicines, cosmetics and silver polish) and mineral oil (a high-boiling petroleum fraction). Another form of methyl cellulose is used as a thickening or binding agent in products such as ice cream and cosmetics. Sterno or canned heat is a colloid of ethanol dispersed in saturated calcium acetate solution. This results in a gel of solid calcium acetate dispersed in ethanol. MATERIALS 2 test tubes Mineral oil 2 100 ml beakers Dishwashing detergent 1 50 ml beaker Calcium acetate 1 10.0 ml graduated cylinder Ethanol, 95% 1 100.0 ml graduated cylinder Methyl cellulose Bunsen burner Magnesium carbonate Ring stand Wire gauze Calcium carbonate Methyl salicylate Food coloring
PROCEDURE A. THE SUNSET EXPERIMENT Note: This experiment may be performed as a demonstration. If so, you should record observations from the demonstration into your notebook. 1. Add 5mL of 0.1M Na 2 S 2 O 3 to a 100mL beaker. Dilute to 50mL by adding deionized H 2 O. Add 5mL of 6M HCl to the beaker and briefly stir. Shine a bright light through the beaker. Within 1 to 5 minutes the solution will start to become hazy. At right angles to the beam, the path of the scattered light will become visible. 2. At the same time, observe the color of the transmitted beam by projecting it onto a screen. Note the change in color of the transmitted light as time passes (5 to 10 minutes). B. FORMATION OF PROTECTED COLLOIDS 3. Place 5mL H 2 O in a test tube. Add 1mL mineral oil. Observe the layers. Stopper and shake. Allow the test tube to stand for a few minutes. Carefully note whether the oil and water separated back into layers. 4. Place 5mL H 2 O in another test tube. Add 1mL mineral oil and 1mL of a liquid soap or detergent solution. Stopper and shake. Allow the test tube to stand for a few minutes. Compare with the previous mixture of just oil and water. 5. This is an illustration of stabilization of an oil and water emulsion. Note the difference with and without the liquid soap. C. FORMATION OF GELS 6. Sterno. Prepare a saturated solution of calcium acetate by dissolving 2g of Ca(C 2 H 3 O 2 ) 2 in 7mL of deionized water in a 100mL beaker. Pour 25mL of 95% ethanol, C 2 H 5 OH, into another 100mL beaker. Pour the saturated Ca(C 2 H 3 O 2 ) 2 solution into the ethanol and then transfer the mixture back to the first beaker. Repeat the back-and-forth transfer until no further gel forms. Remove a small piece of the gel, place it on a heat-resistant pad, and ignite it. Record observations. Discard the rest of the gel into the labeled waste jar. 7. Toothpaste. Heat 12.0mL of water to boiling in a 50mL beaker. Remove from the heat, add 0.5g of powdered methyl cellulose and stir until the powder is fully wet and dispersed. Cool the mixture in an ice-water bath, stirring vigorously until a thick gel has formed. Stir in 5 drops of mineral oil. When completely mixed, stir in 0.35g of powered calcium carbonate, and then 0.35g of powdered magnesium carbonate. Stir in one drop of a flavoring such as methyl salicylate (oil of wintergreen) and one drop of food coloring. Record the appearance of the toothpaste at each step. Do not use the toothpaste. Discard the toothpaste into the labeled waste jar..do not wash the toothpaste down the drain.
DATA
Name: Date: COLLOIDS RESULTS 1. What color of light was most readily transmitted by the colloidal dispersion in the sunset experiment? What color was scattered? Why do we call this the sunset experiment? 2. Shaking the oil and water mixture had what effect? Did the oil and water separate again? Shaking the oil-water-soap (detergent) mixture had what effect? Did the oil and water separate again in this step? 3. What happened when the saturated Ca(C 2 H 3 O 2 ) 2 and ethanol were mixed? Why is Sterno called "canned heat"? What is the safety advantage of the gel over liquid ethanol as fuel? 4. Describe your observations and the results of the preparation of toothpaste.
POST-LAB QUESTIONS 1. Why is the sky blue? Answer this question based on your observations during the sunset experiment. Think about all of the components of the atmosphere. 2. Classify each of the following materials into one of the colloid types from the background: a. milk b. smoke c. Jell-o d. fog 3. How does a soap (or detergent) act as a stabilizer for an oil in water emulsion? 4. What is happening when Sterno is burned? Write and balance an equation for the burning of ethanol (C 2 H 5 OH).