Materials Included In Kit

Additional Materials Required

Safety Precautions

Ethyl alcohol is a flammable liquid and a dangerous fire risk. Addition of denaturant makes the product poisonous—it cannot be made nonpoisonous. Sodium hydroxide is a corrosive solid; skin burns are possible. Considerable heat is evolved when sodium hydroxide pellets are added to water. It is very dangerous to eyes; wear face and eye protection plus gloves when handling and using sodium hydroxide. Do not use the soap prepared in this lab to wash hands because it may contain excess base which is severely corrosive to skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. If the reaction flask contains leftover sodium hydroxide pellets after heating, dissolve the sodium hydroxide in water, then neutralize and dispose of according to Flinn Suggested Disposal Method #10. Flush all soap solutions from the analysis section and the filtrate produced by vacuum filtration down the drain according to Flinn Suggested Disposal Method #26b. Dispose of any remaining soap in the trash according to Flinn Suggested Disposal Method #26a.

Teacher Tips

• If doing this lab activity with middle school students or students with little lab experience, you may want to distribute the sodium hydroxide pellets so that students do not have direct contact with them. Sodium hydroxide is a corrosive solid and skin burns are possible.
• When adding sodium hydroxide pellets to the original reaction flask, there is not sufficient heat generated to require immersing the flask in an ice bath.
• Use the largest Büchner funnel available for vacuum filtration. Enough soap is generated to completely fill a smaller Büchner funnel. For proper filtration, the Büchner funnel should contain only a small layer of soap, which is then rinsed several times with ice-cold water. If smaller Büchner funnels are the only choice, filter in batches so that the funnel is not over-filled.
• If Büchner funnels are not available, standard funnels and filter paper to filter the soap by gravity filtration may be used. Even if this method is used, the soap still needs to be rinsed three times with ice-cold water to remove any excess base.
• If a hot plate is not available, a Bunsen burner may be used to heat the warm water bath.
• Make sure that students do not allow the alcohol to boil over while heating. It should boil, but not boil so rapidly that it boils over.
• The average required reaction time is 20 minutes. It may take a few minutes more or less depending on the exact temperature of the water bath. Once students begin to notice the formation of a white film and the disappearance of most of the liquid in the flask, soap has been generated, even if 20 minutes have not yet passed. If 20 minutes have passed and the mixture in the flask still appears to be a clear, yellow liquid (with no white film), continue heating until the white film appears.
• Student observations and data in the Analysis section may vary because the soap produced each time can vary in composition. It may have a pH of 8 when made today, but a pH of 9 when made tomorrow. Allow for variation in answers.

Further Extensions

Alternate Procedures

• If more than 50 minutes are available for lab, an alternate procedure may be used for preparing the soap. The soaps produced by this procedure will have softer, waxier textures, darker colors and more fragrant smells. Use 25 mL of a 50:50 mixture of 95% ethyl alcohol and water instead of 25 mL of 95% ethyl alcohol in step 4. Heat the mixture for about 45 minutes instead of 20 minutes in Step 6. Follow the procedure as written for all other steps.
• Try using other oils or fats to make a variety of soaps. The same procedure can be followed. Other soaps may have different colors, textures, and/or smells. Possible choices include lard, butter, peanut butter, coconut oil, sesame oil, corn oil or vegetable oil.
• Try using potassium hydroxide in place of the sodium hydroxide. It should generate a softer soap.

Sample Data

• Step 4. When the sodium hydroxide pellets are added to the reaction flask, most will not dissolve. This is okay. Just swirl to get as much dissolved as possible.
• Step 5. Once the oil is added to the reaction flask, the solution will be somewhat cloudy and pale yellow in color. Oil droplets will be visible.
• Step 6. As the reaction flask is heated, the contents of the flask will become darker yellow and have the consistency of paste. A white solid that floats will begin to appear as the flask is heated. This white solid is the soap.
• Step 8. Once the reaction is complete and the contents of the reaction flask have been poured into the sodium chloride solution, a cloudy mixture should form upon mixing.
• Steps 9–10. The final soap will be slightly waxy in texture, with a strong smell of alcohol. It should be pale yellow, off-white or beige in color. As the soap dries overnight, any leftover alcohol in the soap will evaporate and its smell will diminish. As the alcohol evaporates, the texture of the soap will also lose some of its waxy feel and become more brittle and dry.

Observations of Soap Sample Test 1. Determination of pH Test 2. Lathering Ability Test 3. Behavior of Soap in “Hard” Water Test 4. Ability to Cut through Grease

Answers to Questions

1. Based on your data, does the homemade soap behave like the commercial soap sample? How is it similar? How is it different?

   The homemade soaps behave very similarly to the commercial soap sample. They both lather in distilled water, lather only slightly in “hard” water, cut through grease and have a basic pH. They differ in their colors, textures, smells, and perhaps their pH.

2. Compare your homemade soap to a soap that was made from a different oil by a different group. How do these soaps compare?

   The peanut oil soap and the olive oil soap behave similarly. They both lather in distilled water, lather only slightly in “hard” water, cut through grease and have a basic pH. They differ in their colors, textures, smells and perhaps their pH.

3. Why do soaps made from different oils have different properties, even if the properties are only slightly different?

   The soaps have some slightly different properties because they are made from different oils, which are composed of different fatty acids. Therefore, the chemical formula of the two soaps differs according to the fatty acid content of the specific oil. This difference in chemical makeup causes slight changes in color, texture and smell.

4. Generate a list of properties that are important when buying a commercial soap product. (Hint: Think of what brand(s) you buy and why you buy it.)

   Answers will vary. Possibilities include fragrance, texture, color, exfoliant, mildness or moisturizing.

5. Do you think your “homemade” soap would sell as is? Why or why not?

   Answers will vary, but students should mention that their soap may contain excess base, which is severely corrosive to skin.

6. If your goal (as a soap chemist) is to make a soap that is marketable and appealing to the general public, what changes would you make to your “homemade” soap?

   Answers will vary. Possibilities include removing any excess base or adding color or fragrance.

Extension

7. Design a name and a slogan for your new soap. Draw a logo (optional) and write a commercial to market your soap.

   Answers will vary.

References

Dirt Alert—The Chemistry of Cleaning; Sarquis, M., Ed.; Science in Our World; Terrific Science: USA, 1995; pp 39–50.

Phanstiel, O.; Dueno, E.; Wang, Q. X. J. Chem. Ed. 1998, 75, 612–614.

Selinger, B.; Chemistry in the Marketplace; Harcourt Brace: Orlando, FL, 1994; Chapter 2.

Strong, F. M.; Koch, F. H. Biochemistry Laboratory Manual, 3rd ed.; WCB: Dubuque, IA, 1974; pp 119–120.

Introduction

Soap-making is an age-old, relatively simple process. In this laboratory activity, “homemade” soap will be prepared, then tested to determine its pH and to see how well it lathers and cuts through grease. These properties of the “homemade” soap will be compared to those of a commercial soap sample to see how well it performs.

Concepts

• Saponification
• Surfactants

Background

History of Soap-Making

Soaps have been produced for thousands of years. In fact, the soap-making process dates back to the Sumerians in 2500 B.C. The pioneers on the American frontier made their own soap using natural materials they obtained from around the homestead. They collected fat drippings from cooking and combined the fat with potash (potassium carbonate, K₂CO₃), which was extracted from wood ashes with hot water. The fat-potash mixture was boiled to produce a harsh soap that was cut into bars and used in the home. The soap was harsh because it was difficult to determine exactly the right amount of potash to react with the fat. If potash was added in excess, the leftover potash would remain in the soap. Potash is a base and a severe skin irritant; therefore, the soap made by this method often left skin rough and dry. Soap was a precious item on the frontier because of the effort involved and the safety risks associated with making it. Injuries such as blindness and deaths even occurred while making soap because of the corrosiveness of the potash. The pioneers used it primarily for washing clothes and dishes instead of for bathing. The process by which soap is made has been virtually unchanged since the days of the pioneers; although, today’s methods are generally safer and the soap is milder due to some changes and improvements in the soap-making process.

How Do Soaps Work?

Much of the dirt and grease that makes clothes or dishes dirty is composed of nonpolar, hydrophobic (water-fearing) molecules which are not soluble in water. If water alone were used to clean dirty clothes or dishes, the hydrophobic dirt and grease molecules would not dissolve in the water. Instead they would “bead up” in the water and, as a result, would remain on the dirty surface. Soaps are added to water to aid in dissolving the dirt and grease that water alone cannot efficiently remove.

How do soaps help water to remove dirt and grease? Soaps are surfactants, or surface-active agents. A surfactant is a molecule that has two distinct parts. One end is a nonpolar, hydrophobic (water-fearing) carbon chain, resembling a “tail.” The other end is a polar, ionic, hydrophilic (water-loving) “head.” An example of a soap molecule and a representation are shown in Figure l.

The nonpolar tail has an attraction to the nonpolar materials like dirt or grease while the polar head is attracted to the polar water molecules. This “like dissolves like” phenomenon allows the surfactant to form a micelle in a polar solvent like water. A micelle is a spherical cluster of soap molecules that associate with a dirt or grease droplet with all of the nonpolar tails pointing inward to surround the nonpolar dirt or grease droplet. The polar heads form the sphere’s outer suface and thus allow the entire dirt- or grease-containing droplet to be soluble in water (see Figure 2). By enclosing the dirt and grease in a micelle, the nonpolar dirt and grease molecules are effectively transformed into polar particles. Hence, they become water soluble. The soap therefore disperses or breaks up the oil particles and dissolves them in the water. The water soluble micelles can then easily be rinsed away, taking the dirt and grease with them.

Chemistry of Soap-Making

Most fats or oils are composed of one or more triglycerides, which contain three long chain hydrocarbons connected to a 3-carbon backbone (glycerol). Each of these hydrocarbon chains is connected to the glycerol backbone by an ester linkage (see Figure 3). Many times, the long hydrocarbon chains are simply represented by R, as in Figure 4. In the presence of acids and bases, triglycerides are hydrolyzed to yield three fatty acids and a glycerol molecule. This process of hydrolyzing fats is known as saponification. In a saponification reaction, a fat or oil is combined with a base to produce a soap and an alcohol. Sodium chloride is generally added to precipitate the soap through a process called “salting out.” Clearly, the specific chemical formula of the soap depends on the fatty acids in the original triglycerides. Soaps prepared using different oils will contain different fatty acids and will therefore have some slightly different properties, such as texture, color or fragrance. However, the characteristic properties of soap, such as its surfactant action, will be present regardless of the oil used to make the soap.

Materials

Safety Precautions

Ethyl alcohol is a flammable liquid and a dangerous fire risk. Addition of denaturant makes the product poisonous—it cannot be made nonpoisonous. Sodium hydroxide is a corrosive solid; skin burns are possible. Considerable heat is evolved when sodium hydroxide pellets are added to water. It is very dangerous to eyes; wear eye protection plus gloves when handling and using sodium hydroxide. Do not use the soap prepared in this lab to wash hands because it may contain excess base which is severely corrosive to the skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

1. Prepare a water bath by filling a 600-mL beaker about half full with tap water. Place this beaker on a hot plate and bring the water to a temperature of about 80–85 °C. This will be the warm-water bath.
2. Prepare 150 mL of ice-cold water by placing 150 mL of distilled or deionized water in a 250-mL beaker. Partially submerge this beaker in a large container of ice water (the ice-bath) and allow the water to cool until step 8.
3. Prepare a 25% sodium chloride solution by adding 25 g of sodium chloride to 100 mL of cool distilled or deionized water in a 400-mL beaker. Submerge this beaker in the ice bath prepared in step 2 and allow it to cool until step 8.
4. Add 2.5 g of sodium hydroxide pellets to 25 mL of 95% ethyl alcohol in a 125-mL Erlenmeyer flask. Swirl the flask to get as much of the sodium hydroxide to dissolve as possible, but most of it will not dissolve.
5. Add 5 mL of oil to the 125-mL Erlenmeyer flask and swirl.
6. Submerge the flask in the warm-water bath and heat it for 20 minutes. While heating, stir the solution frequently with a stirring rod. Watch the flask carefully to make sure that it does not boil over. Turn down the temperature of the hot plate if necessary to prevent the contents of the flask from boiling over into the water bath.
7. While the flask is heating, set up the vacuum filtration apparatus.
8. After heating the flask for 20 minutes and once a white soapy film has developed, pour the contents of the flask into the cooled 25% sodium chloride solution. Leave any remaining sodium hydroxide pellets behind in the flask. Stir this mixture vigorously. Submerge the beaker in the ice-bath and allow it to cool to room temperature.
9. Collect the precipitate by vacuum filtration and wash it three times with the ice-cold water from step 2. Observe the physical characteristics of your soap such as color, texture, and smell. Record your observations.
10. Allow the filtered soap to air-dry overnight. Observe its physical characteristics again after drying. Record your observations.
11. Observe the physical characteristics of a commercial soap sample. Record your observations.

Test 1. Determination of pH

12. Place about 0.5 g of your soap sample into a test tube labeled “homemade soap.” Use forceps to handle the soap sample until the pH has been determined.
13. Place about 0.5 g of a commercial bar soap into a second test tube labeled “commercial soap.”
14. Add 10 mL of distilled or deionized water to each test tube and stopper the test tubes. Shake both tubes to dissolve the soap in the water.
15. Immerse a stirring rod into each test tube. Remove the stirring rod and touch the wet end of the rod to a strip of pH paper.
16. Compare the resulting color to the chart on the container of pH paper to determine the pH of the solution. Record the pH of both samples. Note: If the pH of your soap sample is above the pH of the commercial soap sample, do not handle the soap with your bare hands for the remaining tests. Instead, use forceps to handle the soap.
17. Save these soap solutions for Test 2.

Test 2. Lathering Ability

18. Use the same samples used in Test 1.
19. Stopper both test tubes and shake each one vigorously 25 times. Record your observations of the lathering abilities. Allow both tubes to sit until the liquid below the soap bubbles appears clear (about one minute).
20. Record the amount of lather formed by measuring the height of the lather in cm above the water with a ruler.

Test 3. Behavior of Soap in “Hard” Water

21. Place about 0.5 g of your soap sample into the test tube labeled “homemade soap.”
22. Place about 0.5 g of a commercial bar soap into the test tube labeled “commercial soap.”
23. Add 10 mL of a 0.1 M calcium chloride solution to each test tube and stopper the test tubes. The calcium chloride solution is similar to “hard” water.
24. Shake each test tube vigorously 25 times. Record your observations.

Test 4. Ability to Cut through Grease

1. Drip about 1 mL of oil onto a watch glass with a pipet. Rub the oil on the watch glass with a cotton swab.
2. Hold the watch glass over the sink or a large beaker. Using a pipet, rinse the watch glass with water. Allow the water to run down the watch glass in an attempt to rinse the oil. Record your observations.
3. Rub a small piece of the “homemade” soap sample on the watch glass. Again run water over the watch glass to rinse the oil/soap mixture. Record your observations. Note: If the pH of your soap sample was above the pH of the commercial soap sample, use forceps to handle the soap.
4. Repeat steps 25–27, this time using the commercial soap sample to clean the watch glass. Record your observations.

Student Worksheet PDF

11810_Student1.pdf