This process of "dropping" out of the solution is called precipitation. Because of this reduced solubility, the soap leaves the solution and forms a solid mass. The sodium ions from the sodium chloride bond with the fatty acid, forming a product that is less soluble in water. The addition of salt (sodium chloride) to the crude form of soap forms fatty-acid salts. This dual nature allows soaps to dissolve fat, grease, and dirt in water. The part that mixes well with fats is the "fatty" part. The part that mixes well with water is the "acid" part. 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"). Fats mixed with strong bases are hydrolyzed into fatty acids. Now to explain, chemically, how soap works to clean things. In the second step, the fatty acids are converted into relatively pure fatty-acid salts by the addition of sodium chloride. The fatty part is the long chain of carbons, shown here as the crooked lines. The "acid" part of the fatty acid is the side with the oxygen (O) atoms. 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). In the second step, the fatty acids are converted into relatively pure fatty-acid salts by the addition of sodium chloride.įigure 1. The figure shows the chemical reaction that is the basis for soap synthesis. This soap was milder and suitable not just for washing clothes or pots, but also for use on skin. The addition of salt caused the soap to form a solid that excluded impurities, such as the sodium hydroxide. It was later discovered that the soap could be purified by adding salt to it. More lye was added and the reaction continued. If there were droplets of fat on the surface of the water, the reaction was incomplete. You could tell when it was "done" by taking a small amount of the mixture and adding it to some clean water. Before soap became available from large companies, people made their own by mixing animal fats with lye in a pot and boiling it. The ashes contain substantial amounts of sodium hydroxide, which dissolves in the water, forming a solution of sodium hydroxide. The traditional way to make lye is to leach ashes with water. Soap is formed by mixing fats or oils with strong bases, such as sodium hydroxide. Chemists now refer to the chemical reaction for making soap as saponification, in honor of the discovery on Mount Sapo. Upon experimentation, they learned that this new substance, later called soap, had useful properties, including the ability to clean surfaces. 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. Fats from the burnt animal flesh mixed with ashes from the fires. Parts of the sacrificed animals were burned as offerings to the gods. An intriguing story about how the Romans learned to make soap involves the tradition of sacrificing animals on Mount Sapo. It might have been invented independently in several regions at different times. Substances believed to be soaps have been found in ancient Egyptian ruins. There are documents suggesting that it was used by ancient Phoenicians over 5,000 years ago.
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