The Synthesis and Properties of Soaps

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.


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.

Comparison to Detergent.  Weigh out approximately 1 gram of the detergent provided from the stockroom 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.

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 tapwater 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 tapwater containing test tube. What happens?  See what happens when you repeat these tests with the potasium 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.)

Testing the Properties.  Also determine the solubility of the original fat in these soaps and the detergent. Spread a thin coating of the fat on a watch glass and test the ability of 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 Data section of your notebook. In your Conclusion section use your observations of solubilities and washing ability to compare the washing action of soaps and detergents. Do your observations of the effect of hardwater (CaCl2) and tapwater on soaps and detergents suggest an advantage to using a particular one?   What advantages might each particular type have?

Test Sodium Soap Solution Potassium Soap Solution Detergent Solution
Behavior in Deionized Water (with 1 ml of water)
Behavior in Tap Water ( with 1 ml of tap water)
Behavior in Hard Water (with 1 ml CaCl2)
Effectiveness Dissolving Fat (Solid)

Revised on 9/10/98.

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