How to make your own soap – A practical for science lessons

Following my last article about the chemistry of soap, I have become interested in making my own soap. From my research I have put together a freely available practical/experiment that can be used in science lessons for students to make their own soap. The available scents with this recipe are coconut and cocoa.

Here is the link for the full practical with images: https://www.tes.com/teaching-resource/saponification-practical-make-your-own-soap-11683222.

The experiment works via the saponification reaction between fats from vegetable oils and sodium hydroxide from lye. The students will learn about saponification, how soaps work and how they can be made industrially.

1. Introduction

Humans have been been making and using soap for at least 2300 years. In all that time the manufacturing process has not changed much. Fats from animals or plants are reacted with sodium hydroxide in a reaction called saponification where soap is produced and glycerol is a biproduct.

Soaps are the sodium salts of fatty acids. It is the combination of the non-polar, long carbon chain of the fatty acid and the polar, ionic end consisting of carbonic acid anion and the sodium cation that let soaps dissolve dirt particles in water. The soap molecules are arranged around the dirt particles in spheres called micelles as the non-polar carbon changes are drawn to the non-polar dirt. The polar, ionic ends face the polar water and enable the removal of the dirt.

Today almost all liquid body washes and some bar soaps do not contain actual soap any more. Instead, they are made of soap-like molecules that are derived from petroleum and work the same way soap does. However, bar soaps can be produced in a more environmentally friendly process using only renewable sources and less energy.

You are going to make our own soap via a saponification reaction between vegetable fats (from olive oils, coffee and coconut oil or cocoa butter) and sodium hydroxide in lye. This is how many bar soaps are still made today.

2. Materials

  • Coconut oil or cocoa butter Depending on which smell you want for your soap. It will give your soap its smell, and also provide fats that the sodium hydroxide can react with.

  • Olive oil The main fat that will react with the sodium hydroxide.

  • Lye Sodium hydroxide solution in water of 25 mass-%, that can be bought in the cleaning section of supermarkets.

  • Coffee Not powder, the actual drink. This will provide the reaction medium, but also some fats that the sodium hydroxide can react with.

  • Coffee grounds They are a filler that will give the soap bar the right consistency. Otherwise it would be too soft.

  • Oat bran It is a filler that will give the soap bar the right consistency. Otherwise it would be too soft.

  • Scale

  • Thermometer

  • Protective gloves Latex gloves will suffice, but you can also use nitrile gloves.

  • Lab coat

  • Lab goggles

  • Glass stick, magnetic stirrer or stick or hand blender for stirring

  • 1 large beaker for cooking or stainless steel pot

  • Beakers or stainless steel bowls for mixing

  • Measuring cylinders

  • Spoon or spatula

  • Soap mold A form in which you can pour the soap to let it set. For school classes baking trays for muffins are suitable. One or two students can share one spot on the tray.

  • Heating plate

  • Baking paper

  • Towels

  • Sharp knife Only if students share mold forms.

3. Procedure

The quantities used here will make soap for two students. They will have to be adjusted if larger groups works together.

  1. Using a scale, weigh out 48 g coconut oil or cocoa butter in a beaker. With a measuring cylinder, measure 77 ml (71 g) olive oil. In a second one, measure 45 ml (45 g) coffee. Weigh out 18 g lye in a beaker or bowl on the scale. Mix 1/2 tee spoon coffee grounds and 1 table spoon oat brain in another beaker or bowl.

  2. Work under a fume hood or in another well-ventilated area. Make sure to put on lab coat, lab goggles and protective gloves and keep them on until the soap is poured into the mold.

  3. Pour the coffee in a beaker or bowl and carefully add the lye to the coffee, and stir to dissolve. (Always add lye to liquid, not the other way around.) This will start a chemical reaction and the mixture will get quite hot.

  4. Melt coconut oil or cocoa butter in a beaker or stainless steel pot over low heat on the heating plate until no solids remain. Add the olive oil. Check the temperature of the two fats with the thermometer. Compare against the temperature of the lye solution, and adjust until they are within a degree or two of each other, in a range between 40 and 45 degrees Celsius.

  5. Pour the lye solution into the beaker/bowl with the oils, and use a glass stick, magnetic stirrer or if you have a stick or hand blender to stir until the mixture begins to thicken and the texture resembles pudding. Once the soap has thickened, stir in the coffee grounds and oat bran with a spoon or spatula.

  6. Slowly pour the mixture into a soap mold. Carefully, lift and tap the mold against the table a few times to release air bubbles. Cover with baking paper, and then wrap the entire mold with a towel to insulate it. As a soap mold for a school class baking trays for muffins are suitable. One or two students can share one spot on the tray.

  7. After 24 to 48 hours, remove the towel, and take the soap out of the mold. Let the soap loaf sit for another day to harden further before cutting into bars with a sharp knife. Cutting only needs to be done if students shared one form for their soap.

  8. After the soap is cut into bars, you will need to let them cure in a cool and well-ventilated area for four weeks. This allows the soap to finish saponification and for all the excess water to evaporate.

    References

  1. M. Caudill. Homemade Coconut Oil Soap, 15 September 2015. Retrieved from: www.rodalesorganiclife.com/wellbeing/homemade-coconut-oil-soap (28 July 2017).

  2. A.S. Davidsohn, Soap and detergent, from Encyclopaedia Britannica Online, 2017. Retrieved from: https://www.britannica.com/science/soap (28 July 2017).

  3. American Cleaning Institute, Soaps & Detergents: Chemistry (Surfactants), 2017. Retrieved from: http://www.cleaninginstitute.org/clean_living/soaps__detergents_chemistry_2.aspx (28 July 2017).

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