Soap in a Tube Saponification Reaction


Test Tube Soap Bubbles (Jason Hickey)

Test Tube Soap Bubbles (Jason Hickey)

Soap in a tube? Why not! Typically when people make soap, they make a lot of it. However, you can make a small amount of soap in a test tube as a chemistry demonstration. This is a terrific example of the saponification reaction, plus it results in a product you can use.

Soap Materials

  • sodium carbonate (a home canning chemical)
  • calcium oxide (used to control humidity, sold in home stores)
  • sodium chloride (table salt)
  • butter or lard
  • water
  • 2 test tubes (or small heat-safe cups, if you don’t have test tubes)
  • candle or alcohol lamp
  • teaspoon or other measure

Make Soap in a Tube

  1. Add two scoops of sodium carbonate and 3 scoops of calcium oxide into a test tube.

    Once you master soap in a tube, upscale and try making homemade bars of soap. (kahvikis)

    Once you master soap in a tube, upscale and try making homemade bars of soap. (kahvikis)

  2. Fill the test tube about 3/4 full with water.
  3. Heat the test tube in the flame, but don’t bring the solution to the boiling point. As a safety measure, point the open end of the test tube away from your face. This is a good lab practice when heating any chemicals.
  4. Remove the test tube from heat and allow it to cool.
  5. When the liquid has cleared, add a pea-sized amount of butter or lard.
  6. Return the test tube to the flame. First, the butter or lard will melt. When it starts to mix with the water, add 6 scoops of sodium chloride.
  7. Carefully shake the test tube to mix the ingredients. The soap will rise to the top of the test tube.
  8. Allow the tube to cool and then remove the soap. If you rub it between your fingers, you’ll feel the characteristic slippery texture of your creation.

 How Saponification Makes Soap

One of the organic chemical reactions known to ancient man was the preparation of soaps through a reaction called saponification. Natural soaps are sodium or potassium salts of fatty acids, originally made by boiling lard or other animal fat together with lye or potash (potassium hydroxide). Hydrolysis of the fats and oils occurs, yielding glycerol and crude soap.

In the industrial manufacture of soap, tallow (fat from animals such as cattle and sheep) or vegetable fat is heated with sodium hydroxide. Once the saponification reaction is complete, sodium chloride is added to precipitate the soap. The water layer is drawn off the top of the mixture and the glycerol is recovered using vacuum distillation.

The crude soap obtained from the saponification reaction contains sodium chloride, sodium hydroxide, and glycerol. These impurities are removed by boiling the crude soap curds in water and re-precipitating the soap with salt. After the purification process is repeated several times, the soap may be used as an inexpensive industrial cleanser. Sand or pumice may be added to produce a scouring soap. Other treatments may result in laundry, cosmetic, liquid, and other soaps.

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