Heat on a hot plate in a
boiling water bath for 15 minutes. Periodically add some 95% ethanol to replace the
ethanol evaporated during boiling. This will keep the level of the liquid in the test tube
roughly constant.Purpose:
To synthesize two soaps and compare their properties with the properties of a detergent.Wastes: All of the chemicals from this experiment can be disposed of down the drain or in the trash. The water should be running before disposal of excess KOH solution.
Procedure:
Preparation of the Potassium Soap. Weigh 1.5 g of vegetable oil or
shortening into a large test tube. Add 10 ml of the 10% potassium hydroxide solution in
ethanol (EtOH). Heat on a hot plate in a
boiling water bath for 15 minutes. Periodically add some 95% ethanol to replace the
ethanol evaporated during boiling. This will keep the level of the liquid in the test tube
roughly constant.
After 15 minutes, test the reaction for completion by adding a few drops of the reaction mixture to water. If fat is visible on the water surface, boil for an additional 15 minutes. When the reaction is complete, pour the mixture into a 100 ml beaker and put in the boiling water bath until all of the ethanol is gone. The material which is left will be thick and gooey. This is a potassium soap. Add 30 ml of deionized water and stir to dissolve it. Do not touch it with your hands! Record the solubility of your potassium soap in water in your data table. Divide this solution into two parts. Set one aside for later testing purposes. This is your potassium soap solution. The other portion will be used for the synthesis of the sodium soap.
Preparation of the Sodium Soap. To the second portion of potassium soap solution from the previous part add 15 ml of saturated sodium chloride (NaCl) solution. Stir vigorously until large curds appear (approximately 10 minutes). Filter out the sodium soap using a Buchner funnel and filtering flask. Press the soap dry on a paper towel. Wash the funnel and filtering flask before returning to the Stockroom. Dissolve the sodium soap you have just prepared in 15 mL of deionized water. Record the solubility of this sodium soap in deionized water in your data table. Save this solution for later tests you will perform.
Testing the Soap:
1. Comparison to Detergent. Weigh out approximately 1 gram of the detergent provided in a 100 mL beaker. Dissolve this detergent in 30 mL of deionized water. Record your observations of its solubility in deionized water. Save this detergent solution for use in the following tests.
2. Test the effects of different water hardness with the following procedure: Add 1 ml of CaCl2 solution to each of three test tubes. This is artificial hard water. Add 1 ml of tap water to three other test tubes. Now, add 1 ml of sodium soap solution to one of the CaCl2-containing test tubes and 1 ml of sodium soap solution to a tap water containing test tube. What happens? See what happens when you repeat these tests with the potassium soap solution. Finally, repeat these tests with the detergent solution. (*Note: as your soap sits for long periods of time it may thicken considerably. It may be necessary to add some water to thin it out to perform this test.)
3. Testing the Properties. a) Add about 2 mL of deionized water and 20 drops of vegetable oil to each of two test tubes. To one test tube add 1 mL of your soap solution, and to the other add 1 mL of the detergent solution. To a third test tube add 3 mL of deionized water and 20 drops of vegetable oil. Shake each test tubes for about 10 s and note the formation of an emulsion. In which test tube is the emulsion more stable?
b) Determine the solubility of the original fat in these soaps, the detergent and in deionized water. Spread a thin coating of the fat on a watch glass and test the ability of pure water and the solutions of these soaps/detergent to wash it off. Which would you choose to wash your glassware in lab or your dishes at home? Why? Compare and contrast your results and enter them in your data table.
Record all your observations in tabular form (perhaps a table like the one below will be useful) in the Results section of your notebook.
| Test | Sodium Soap Solution | Potassium Soap Solution | Detergent Solution | Water |
|---|---|---|---|---|
| Behavior in Deionized Water |
not applicable |
|||
| Behavior in Tap Water ( with 1 ml of tap water) | not applicable |
|||
| Behavior in Hard Water (with 1 ml CaCl2) | not applicable |
|||
| Effectiveness in dissolving fat (Solid) | ||||
|
Effectiveness in stabilizing emulsion |
Conclusion
In your Conclusion answer the following questions in complete sentences.
1. Compare the solubility of the original fat with that of your soaps. Explain the difference in terms of intermolecular forces and the rule of thumb that "like dissolves".
2. Explain, at the molecular level, why soap is an effective cleaning agent.
3. What is an emulsion? Which is the best emulsifying agent, water, your soap or the detergent? Justify your answer, and give an explanation at the molecular level.
4. Explain chemically, what happens when soap is added to hard water, and the problems that can occur when washing.
5. Which is the more effective cleaning agent in hard water, soap or detergent? Explain.
Revised on 9/10/98.