JCE Classroom Activity: #14
Instructor Side
Soapmaking by the Journal’s Editorial Staff
Integrating the Activity into Your Curriculum This activity provides a simple experiment that introduces students to an important reaction of organic chemistry. It also helps students connect chemistry to something that they see and use every day and provides an opportunity for crosscurricular work. It might be connected to history or literature classes that discuss the lives of pioneers and homesteaders who often made their own household products, including soap, and it connects to industrial chemistry (1).
fold here and tear out
About This Activity CAUTION: This activity must be done in a chemistry laboratory with all appropriate safety precautions and careful supervision by an instructor familiar with the properties of strongly basic solutions. It is not a take-home activity. NaOH (sodium hydroxide or lye) is extremely corrosive. Students and instructor must wear splash-proof goggles. Protective aprons and latex gloves are strongly recommended. The instructor should prepare the 6 M NaOH solution, which requires 240 g of NaOH per liter of solution. Add solid NaOH to water, not water to NaOH. The solution will become very hot as the NaOH dissolves. Any spill of NaOH solution should be rinsed with a large volume of cold water. Because the soap students make might be impure and very alkaline, do not allow students to use their soap or to touch it unless its pH is 8.5 or less. The soap should probably not be taken home. It can be disposed of in the trash. The soapmaking reaction is called saponification, from the Latin sapo, soap. When animal fats or vegetable oils, which are esters of long-chain carboxylic acids with glycerol, are heated and stirred with a solution of sodium hydroxide, the ester groups hydrolyze, producing glycerol and sodium salts of the long-chain carboxylic acids. We call this product soap. R1 , R2 , and R3 are long hydrocarbon chains. Ri in the product indicates that the soap produced will be a mixture of salts containing R1, R2, and R3.
O O
H2 C R1 C O
C R2
O
H2 C OH
+ NaOH
CH C O R3 H2 C
HO
CH C OH H2
O
+ Ri
C
O – Na+
O
perforated
Fat
Lye
Glycerol
Soap
This procedure produces a fairly hard, white, opaque soap using vegetable shortening. Other fats could be used, yielding soaps with different colors, textures, and smells (2). Students are likely to be interested in how specialty soaps are made. This can make an interesting class discussion. Soapmakers use additives such as essentials oils, vitamin E, oatmeal, the rind from citrus fruits, dried leaves, ground spices, etc. These items are added to the soap after it thickens and before it is molded. Additives that contain water (such as food coloring) or alcohol (such as vanilla or almond extracts used in cooking) cannot be used. Home soapmakers sometimes color soap by adding melted wax crayons to the fat (3). If students are interested in making soap that they can use, there are many soapmaking kits available. A transparent bar of glycerin soap that can be colored using food coloring can be made with various ingredients (4). A few of the numerous Web sites that give soap recipes for home use are listed on the Student Side of this activity.
Answers to Questions 1. At first, the universal pH indicator paper will probably turn blue. This corresponds to a pH of at least 10. In time, the pH should go down to approximately 8, giving a green color on the indicator paper. The soap should remain basic, even after 4 weeks. 2, 3, and 4. Answers will vary, depending on the soap the students bring from home. The properties of any soap depend on the type of fat used, the ratio of NaOH to fat, and the additives used. 5. Saponification occurs in the drain when the grease combines with the NaOH in the drain cleaner. The products are glycerol and soap, which are then washed down the drain.
References, Additional Information, and Related Activities 1. Lynch, M.; Geary, N.; Hagaman, K.; Munson, A.; Sabo, M. J. Chem. Educ. 1999, 76, 191. 2. Phanstiel, O. P.; Dueno, E.; Wang, Q. X. J. Chem. Educ. 1998, 75, 612. 3. Hill, J. W.; Hall, L. C. J. Chem. Educ. 1978, 55, 516. 4. Hill, J. W.; Hill, C. S. J. Chem. Educ. 1980, 57, 372.
Acknowledgment Kim Kostka, Department of Chemistry, University of Wisconsin College–Rock County, provided valuable advice on soapmaking and suggested Web sites to help students learn more about making soap. This Activity Sheet may be reproduced for use in the subscriber’s classroom. JChemEd.chem.wisc.edu • Vol. 76 No. 2 February 1999 • Journal of Chemical Education
192A
JCE Classroom Activity: #14
Student Side
Soapmaking by the Journal’s Editorial Staff Soap is something that you probably use every day. You can buy it at your local store, and soap manufacturing is an important industrial process. However, in the past, many people have made their own soap at home and some still do. Soap is produced when a fat or oil undergoes a chemical process called saponification. This process involves heating the fat or oil in a basic solution. In this activity, you will make your own soap. The fat you will use is vegetable shortening and the basic solution is sodium hydroxide (NaOH) in water. As the heated mixture is stirred, the fats react with the NaOH to give glycerol and sodium salts, which we call soap.
Try This
CAUTION: Do not attempt this procedure
You will need: vegetable shortening (fat) such as Crisco, 6 M NaOH, outside the laboratory. Splash-proof 10-mL graduated cylinder, two 100-mL glass beakers, balance, thermomgoggles must be worn at all times. A eter, hot-plate, stir stick (glass stirring rod or wooden craft stick), Styrofoam protective apron and gloves are cup, universal pH indicator paper, dropper, two test tubes with stoppers, strongly recommended. During the proand a piece of commercial soap (bring from home). cedure if your hands or equipment feel slippery or soapy, rinse immediately __1. Melt 20 g of vegetable shortening in a 100-mL glass beaker, allow it to with cold water. If soap or NaOH cool to about 45 °C, and pour into a Styrofoam cup. should contact your eyes, rinse with lots __2. CAUTION: NaOH is extremely corrosive. Rinse any spills immediof water and see a physician. ately with large amounts of cold water, and call your instructor. Measure 10 mL of 6 M NaOH with a graduated cylinder and pour it into a 100-mL beaker. Heat the solution gently on a hot-plate until its temperature is nearly the same as that of the liquid fat, about 45 °C. Do not overheat or boil the solution. The temperatures of the liquid fat and the NaOH should be within 5 °C of each other when you mix them. __3. Slowly pour the NaOH solution into the liquid fat while stirring constantly, smoothly, and slowly with a stir stick. Describe what happens as you mix the NaOH solution into the liquid fat. __4. Stir the mixture constantly, smoothly, and slowly for approximately 10–15 minutes. The mixture should thicken to about the consistency of cold honey. If it has not thickened after being stirred for 15 minutes, let it stand for 3–5 minutes and then stir it again for a few minutes. __5. Leave the thickened mixture in the Styrofoam cup, which will act as a mold, for two or three days while the soap hardens. Carefully clean the graduated cylinder and beaker that had NaOH in them by rinsing several times with cold water before you put them away. __6. Test the soap’s pH by wiping a piece of universal pH indicator paper across the top of the thickened mixture without allowing the soap to touch your skin. Is your soap acidic, neutral, or basic? __7. After two or three days, measure the pH again by wetting the soap with a few drops of water from a dropper and wiping a piece of indicator paper across the wetted area without getting your fingers wet. Has the pH changed? __8. Unless your soap is pH 8.5 or less, wear gloves when handling it. Remove it from the cup. You may need to break off pieces of the cup until the soap is free. How does your soap compare to the commercial soap you brought from home? __9. Allow your soap to sit in an open container for 2–4 weeks. Monitor the pH and hardness of the soap every few days. You should not use your soap. Do not allow it to touch your skin unless its pH is less than 8.5. Dispose of it as instructed by your teacher.
Questions __1. Make a table showing the pH of your soap at different times. Explain the pattern of your pH readings. __2. Measure the pH of the soap you brought from home. Compare it with that of the soap you made. __3. Compare the color, texture, smell, and hardness of the soap you brought from home with the soap you made. How and why are they different? __4. Dissolve a small piece of the commercial soap in a test tube half full of water. Do the same with a piece of the soap you made. Stopper the two test tubes and shake them. Compare the “sudsing action” of the two soaps. __5. Commercial drain cleaner contains NaOH. How does drain cleaner work to clear drains of grease?
Information from the World Wide Web
(all sites accessed December 1998)
1. Soapmaking... The Way We Used To Do It: http://www.lis.ab.ca/walton/old/soapold.html 2. Soapmaking Made Easy: http://www.millennium-ark.net/News_Files/INFO_Files/Soapmaking.html 3. Majestic Mountain Sage Lye Calculator: http://www.the-sage.com/services/calculator.html 4. More Than You Ever Wanted to Know about Making Soap: http://www.monsterbit.com/touch/soap1.html
This Activity Sheet may be reproduced for use in the subscriber’s classroom. 192B
Journal of Chemical Education • Vol. 76 No. 2 February 1999 • JChemEd.chem.wisc.edu
