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Soaps and Detergents

Directions: Please read the following passage and answer all of the accompanying questions.

1 Soaps and detergents are used frequently in our daily life. We use them to wash our hands and clean our clothes without ever really paying attention to how they work. Beneath the plain white surface of a bar of Ivory soap lies an intriguing history and a powerful chemistry.
2 It is hard to say when soap was first invented. Some hypothesize that even prehistoric man had a primitive form of it at his disposal. Whether the hunters and gathers had soap is debatable, but it is certain that soap was within the grasp of the ancient Babylonians. Soaps have been excavated in clay cylinders that date back to 2800 B.C. By 1500 B.C. Egyptians medical scrolls recommend a soap made from alkaline salts and animal and vegetable oils for skin conditions.
3 Later, the ancient Romans discovered the cleaning power of soap accidentally. At Mount Sapo, where animals were sacrificed, rain mixed animal fats, wood ashes and clay in to the soil. Incidentally, women washing their clothes by the stream found it was much easier to wash their clothes with some of this clay mixture. Legend links Mount Sapo with the process of soap making (saponification). Interestingly, although Romans are famous for their baths, they actually did not use soap to wash. They coated themselves in oils and then used a scraping tool called a strigil to clean their bodies. However, bars of soap were found in the ruins of Pompeii and archeologists believe soap was used for laundry and occasionally on the body.

4 By the 7th century soap making was an established craft in Europe, and soap making centers flourished in France, Spain and Italy. In the New World, soap makers found work in Jamestown as early as 1608. However, most of the early settlers made their own soap by boiling ashes and animal fats. As time passed, the soap industry continued to grow fairly steadily until the 20th century. In 1916, there was a chemical breakthrough that promised to change the role of soap for years to come; the Germans, suffering from a shortage of materials, resorted to synthetic detergent. It was amazingly successful and by 1946 laundry detergent became available to the entire American population. Shortly thereafter, in 1953, sales of detergent had surpassed those of soap.
5 Soaps and detergents are very similar in their chemical properties. However, there is a significant difference between them; soaps are produced from natural products, and detergents are synthetic, or man-made.
6 To make soap, the first step is to start with fats and oils (obtained from plants or animals) that are reduced to fatty acids and glycerine with a high pressure steam. The fatty acids then combine with either sodium or potasium salts (an alkali or base) to produce soap and water. This is exactly what happened when our early American settlers combined ashes, containing lye, a base, with animal fats.

7 After this process, the soap possesses a hydrophilic end that is attracted to water and a hydrophobic end that is repelled by water, allowing the soap to break down materials that dissolve in both oil and water. Sodium soaps are harder and appear as bar soaps, while the potassium soaps are softer and are used in liquid hand soaps and shaving creams.

8 Detergents are created through a similar process and produce an almost identical product, a sodium salt. The first ingredient used in creating detergents is the compound propylene, CH3-CH=CH2, which used to just be burnt off as waste by the petroleum industry. Propylene molecules are manipulated to form a compound that will react with sulfuric acid. Next, sodium hydroxide is added to neutralize the sulfuric acid resulting in a sodium salt similar to the one present in soap. In general, since soaps are a more natural product, they are used on the body, and detergents are used on clothes. But detergents are the more prevalent of the two and are often used in combination with natural soaps.
9 Both soaps and detergents share a critical chemical property - they are surface-active agents, or surfactants. In other words, they reduce the surface tension of water. Normally, water molecules have a strong attraction to each other, which causes water to bead on counters and on clothes. By reducing this effect, water soaks more easily in clothes and removes stains faster.
10 There are some differences between them, however. Soaps possess a number of qualities that make them preferable to detergents. First, as mentioned earlier, they are natural products and less harmful to the human skin and the environment. Soaps are biodegradable and do not create pollution in our rivers and streams. On the other hand, soap will combine with the magnesium and calcium ions in hard water to create an insoluble residue that can clog drains and stick to clothing. The hardness of a water sample can be gauged by the amount of calcium carbonate that is present. Soft water is relatively free of dissolved calcium carbonate.

Calcium carbonate in milligrams per liter (mg/L)
soft0-75 mg/L (or 0 - 4.4 grains per gallon)
moderately hard75-150 mg/L (or 4.4 - 8.8 grains per gallon)
hard150-300 mg/L (or 8.8 - 17.5 grains per gallon)
very hard> 300 mg/L (or >17.5 grains per gallon)

Hard Water Facts courtesy of mid kent water

Since the soap does not rinse out as well as detergent, it tends to build up on clothes, and over a long period of time, causes the fabric to deteriorate while leaving an odor. Another shortcoming of soap is that it is less powerful than synthetic detergent and tends to lose its cleaning power over time. So don’t wash your laundry with soap!

An added benefit of detergents is that they can be specially engineered for each cleaning task and for use in different types of machines. Front loading washers produce better cleaning results when the clothes strike the wall of the washer's tub as it rotates. This type of machine will clean your clothes better if there are fewer bubbles to cushion their impact. Top loading washers, on the other hand, function better with more bubbles since the bubbles trap excess dirt and keep it from re-attaching to your clothes. Many people also have a fascination with the spectacular colors produced by bubbles. Little do they realize that bubbles are an important aspect of the detergents cleaning action.
image courtesy of
Conan The Bubbleman

  General Questions
Under which set of conditions are soaps preferable to detergents?


Clothing stained with oil cannot be effectively cleaned with water alone. It is well known that oil does not mix well with water. Why is the addition of soap or detergent helpful in removing a greasy stain?


As stated in paragraph 6 in the passage, soap is made form a combination of chemicals. One of these is fatty acids. In order to get fatty acid, one hydrogen atom combines with two oxygen atoms and one carbon atom to get a carboxylic acid molecule that is combined with straight chain carbon and hydrogen molecules. What is the maximum amount of carboxylic acid molecules that can be made with 1200 Hydrogen atoms, 2000 Oxygen atoms, and 1500 Carbon atoms?


What is the maximum amount of difference in calcium carbonate between the softest soft water and the softest hard water? ( in mg/L).


Which of the following statements is true of both soap and detergent?


What is the meaning of the word hydrophobic?


Which of the following statements does NOT support the main idea of this passage?


All of the following statements are true of soap EXCEPT:


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