Sliding Down the Slippery Soap

Our fourth experiment of the year in introduction to organic chemistry was making soap. This is a fun and practical experiment that involves some good chemical principles. The students could vary the composition of their soap by mixing lard or vegetable shortening with olive, corn, or coconut oil. For each combination the appropriate amount of water and sodium hydroxide has to be calculated. Frankly, it is still a mystery to me how to get soap making to turn out perfect every time. It seems that getting the right amount of sodium hydroxide is critical. You want to use enough to saponify all the esters without adding an excess that would make the end product too caustic to be useable. However, the saponification reaction seems to take a long time to go to completion – or maybe it never goes to completion. It seems to me that one would want to keep the amount of water used to a minimum, since that would drive the hydrolysis equation towards the reactants. Stirring the soap for thirty minutes or more seems beneficial but not sufficient to have soap that is homogeneous and firmly set within a week. This year, I bought color and scent from a soap-making website to enhance the attractiveness of the product. The color and scent certainly made the soap more satisfactory even if the bottom of the soap cake was not entirely solid after a week of “curing.” The home-made soap was tested in reference to its pH and sudsing qualities and compared with liquid soap and powdered detergent.


One Comment

  1. Posted March 13, 2010 at 5:49 pm | Permalink

    That does not look like my fruit smoothie I saw that week! And I love the one soap that someone molded into a heart shape.

    The idea of finding the right *ratio* of water, sodium hydroxide, shortening, and a type of oil reminds me of a lab I saw at a public school on *mole rockets*:

    Students had to take cut out pipette bulbs and draw out different portions of water, H2, O2 (the H2 and O2 were in squeeze bottles with reactions that released H2 and O2 gases)and test their ability to be launched from a projectile launcher… thing. They had to calculate the distance each pipette bulb traveled and record down the different ratios of H2, H2O and O2 were in the bulb for each trial. After that, all the class data was compiled together and people had to decide based on their individual results and class results what would be the ‘ideal’ ratio to make a mole rocket travel the longest distance.

    But wouldn’t you compare the homemade soap pH to bars of soap from the store? (I would imagine powdered detergent and liquid soap might have different pHs than a solid bar of soap)

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