Soapmaking In Chemistry Class

Saponification Is The Word



Cups of soap "curing"



As a part of our "Organic Chemistry" unit, I decided to have students make soap. I wanted to do this activity because it connects chemistry to real life, but the last time I tried it I had not felt successful with it. Not being one to give up, I decided to try again. I reviewed numerous classroom saponification recipes and discovered that there is a wide variety of approaches. I tried five different methods, some cold process, some using heat, some using ethanol, some not. I decided on the simplest cold process "recipe", with a modification of my own: adding a little borax. I would have liked to use coconut oil, but only had olive oil available at the time, so olive oil it was. I also had corn oil available in case the olive oil ran out.

I had all students handstir the oil and lye for at least ten minutes, to get them to see the emulsion occur. Then I allowed them to use the magnetic stirring plates.  I did have one handheld emulsifier that I used to help anyone "finish up" the emulsion. Some students wanted to add color, so I allowed them to melt some crayon and mix it in with the handheld emulsifier. The melted crayon immediately solidified in the soap emulsion, so it came out more flecked than colored, but the students liked it.

The cups of soap are now sitting in my prep room. I will take them out of the plastic cups when they set up a bit, and form them into cakes wrapped in wax paper. Then they will sit until they cure more, and the pH is about 8.

So for kicks, here is the lab approach I used for my classes. I kept a squirt bottle of vinegar handy to neutralize spills.

Background

Evidence of soap making dates back to the Egyptians and Babylonians. In Europe during the Middle Ages, soap was very expensive, and in some locations, it was heavily taxed. Only the rich could have much of it, and few people knew how to make it, because guilds kept tight control of their recipes and the market for soap.

Things changed when people migrated to North America. There were no guilds or taxes to prevent the making of soap by the common person, enough people came to the US that knew how to make it, and the information was shared. Consistency of a soap product, however, was a problem. People used rough measurements or lacked enough fats or lye, and produced soap that was either overly fatty or harsh. Homemade soaps were usually made from kitchen fats or lard. The fats were broken down using ash soaked in water. The soaking ash produced the needed lye, or hydroxide.

The process is called saponification. Saponification is a process in which a fat molecule is broken down by sodium hydroxide (lye) into four smaller molecules; three of the new molecules are soap and one is glycerol. The glycerol molecule keeps the soap moist. Soap molecules have one polar end and one nonpolar end, giving it the ability to attach to oily substances in water. Emulsion is a temporary mixing of two insoluble liquids such as oil and water.

The soap you are making today is fairly crude, and still contains sodium hydroxide at the end. It could therefore be dangerous if you got it in your eyes. I would NOT recommend it for washing your face! It needs to cure for one to three weeks before it is useable.

Materials and Apparatus

•  Safety goggles, apron, gloves
• 62 mL of olive oil, 100 mL graduated cylinder to measure oil
• 400 mL beaker
• 50 mL of water, 50 mL graduated cylinder to measure water
• 250 mL erlenmeyer flask
• 10 g of NaOH, Weigh boat for massing NaOH and borax
• 1 g of Borax Glass rod for stirring
• Salt (if required)
• Stirring magnet and magnetic stirring plate (if available)
• Handheld emulsifier (if available)
• Essential oil or perfume (optional)
• Plastic cup for setting soap

 Procedure

 Measure out the oil and water. Pour oil into a 400 mL beaker. Pour water into a 250 mL Erlenmeyer flask. Mass the NaOH in weigh boat or on a piece of paper towel. DO NOT TOUCH IT! Squeeze the weigh boat so that the solid NaOH drops into the water in the Erlenmeyer flask and swirl until completely dissolved. Mass 1 g borax and dissolve in the NaOH solution. The borax will add to the foaming ability of the soap.

 SLOWLY trickle the lye (NaOH) into the oil, stirring continuously to emulsify. DON’T TOUCH IT! Keep stirring to “trace” or a thick paste forms. This could take half an hour. If you still don't have a paste after half an hour, stir in 2 spatulas of salt. Other methods of stirring include using a magnetic stirring plate and magnet, or a handheld emulsifier. Hand stirring can take anywhere from ½ hour to 3 hours; the handheld emulsifier takes just a few minutes to reach trace. Once the proper consistency is reached, you may stir in essential oil or perfume, if you want. Test the pH with a pH strip.

 Pour into plastic cup and leave to cure. After a day or two, we will remove it from the plastic cup and wrap in waxed paper to aid the curing process. This type of soap needs to mature to lose its alkalinity (pH10-12 when new). Use only after at least 3 weeks of “curing”, or when pH level is around 8.

 Note: The Secret is in the Stirring ! Don't Give Up!

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Miscellaneous links
Fight Club Soap
A more complex soap recipe