Introduction
It’s not clear who first invented soap. There are documents suggesting that it was used by ancient Phoenicians over 5,000 years ago. Substances believed to be soaps have been found in ancient Egyptian ruins. It might have been invented independently in several regions at different times. An intriguing story about how the Romans learned to make soap involves the tradition of sacrificing animals on Mount Sapo. Parts of the sacrificed animals were burned as offerings to the gods. Fats from the burnt animal flesh mixed with ashes from the fires. When it rained, the Roman’s noticed that a substance formed in the pools of water that ran from the ashes that had been mixed with the animal fats. Upon experimentation, they learned that this new substance, later called soap, had useful properties, including the ability to clean surfaces. Chemists now refer to the chemical reaction for making soap as saponification, in honor of the discovery on Mount Sapo.
Soap is formed by mixing fats or oils with strong bases, such as sodium hydroxide. Sodium hydroxide is also called lye. The traditional way to make lye is to leach ashes with water. The ashes contain substantial amounts of sodium hydroxide, which dissolves in the water, forming a solution of sodium hydroxide. Before soap became available from large companies, people made their own by mixing animal fats with lye in a pot and boiling it. You could tell when it was “done” by taking a small amount of the mixture and adding it to some clean water. If there were droplets of fat on the surface of the water, the reaction was incomplete. More lye was added and the reaction continued. It was later discovered that the soap could be purified by adding salt to it. The addition of salt caused the soap to form a solid that excluded impurities, such as the sodium hydroxide. This soap was milder and suitable not just for washing clothes or pots, but also for use on skin. The figure shows the chemical reaction that is the basis for soap synthesis.

Figure 1. Saponification of a fat molecule. The bonds that connect the long chains of the fat molecule to the “backbone” are broken by the reaction of sodium hydroxide (and heat), yielding glycerol and three fatty-acid molecules (soap). The “acid” part of the fatty acid is the side with the oxygen (O) atoms. This end mixes well with water. The fatty part is the long chain of carbons, shown here as the crooked lines. This end mixes well with fats and oils. In the second step, the fatty acids are converted into relatively pure fatty-acid salts by the addition of sodium chloride.
Now to explain, chemically, how soap works to clean things. Fats mixed with strong bases are hydrolyzed into fatty acids. Fatty acids have the very useful property of having one end that mixes well with water (it is hydrophilic, or “water-loving”) and another end that mixes well with oils and fats (it is hydrophobic, or “water-hating”). The part that mixes well with water is the “acid” part. The part that mixes well with fats is the “fatty” part. This dual nature allows soaps to dissolve fat, grease, and dirt in water. Without soap, oil and water don’t mix. With soap, they do.
Making Soap: A Basic Chemical Reaction
- Melt and pour: This easy process involves melting pre-made blocks of soap and adding your own fragrance.
- Cold process: The cold process is the most common method of making soap from scratch using oils and lye.
- A microwave or double boiler
- A heat-resistant bowl for the microwave
- Measuring spoons and whisks
- Fragrance, color, or additives, as desired
- A mold
- Animal fats or vegetable oils
- A pitcher of lye-water
- A soap pot
- Fragrance or essential oil, as desired
- Natural or synthetic colorant, as desired
- A mold to pour the raw soap into
- Safety gear
Safely Working With Lye
Questions
- What are other oils, besides coconut, that are used to make bath soaps?
- Based on your research, why is potassium hydroxide used instead of sodium hydroxide to make certain kinds of soaps?
- Based on your research, what is a micelle?
- What is the chemical name of the most common fatty-acid molecule found in coconut oil?
- How does the “split-personality” of a soap molecule make it a good cleaning agent?
- What is a triglyceride?
- The procedure for this science project uses 3-molar (3M) sodium hydroxide. What does the term molar mean?
Credits
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