OPTIONAL ASSIGNMENT
SOAP (50 points)
Due: Monday, June 9, 2014
This assignment has three parts. Please follow all directions for each
part.
Part 1 (20 points)
Read the soap article and answer questions completely. The answers
must be typed.
Part 2 (10 points)
A. Complete the wash this way lab at home. You will need to have a
picture showing you performing the lab at home as documentation.
B. Create a data table with your results.
C. Answer the question pertaining to the lab.
Part 3 (10 points)
Bring in the assigned soap for your class. The bottle must be new and
unopened.
SOAP
Soaps are cleaning agents that are usually made by reacting alkali (e.g., sodium
hydroxide) with naturally occurring fat or fatty acids. The reaction produces sodium
salts of these fatty acids, which improve the cleaning process by making water better
able to lift away greasy stains from skin, hair, clothes, and just about anything else. As a
substance that has helped clean bodies as well as possessions, soap has been remarkably
useful.
History of Soap
The discovery of soap predates recorded history, going back perhaps as far as six
thousand years. Excavations of ancient Babylon uncovered cylinders with inscriptions
for making soap around 2800 B.C.E. Later records from ancient Egypt (c. 1500 B.C.E. )
describe how animal and vegetable oils were combined with alkaline salts to make soap.
According to Roman legend, soap got its name from Mount Sapo, where animals were
sacrificed. Rain would wash the fat from the sacrificed animals along with alkaline
wooden ashes from the sacrificial fires into the Tiber River, where people found the
mixture helped clean clothes. This recipe for making soap was relatively unchanged for
centuries, with American colonists collecting and cooking down animal tallow (rendered
fat) and then mixing it with an alkali potash solution obtained from the accumulated
hardwood ashes of their winter fires. Similarly, Europeans made something known as
castile soap using olive oil. Only since the mid-nineteenth century has the process
become commercialized and soap become widely available at the local market.
Chemistry of Soap
The basic structure of all soaps is essentially the same, consisting of a long hydrophobic
(water-fearing) hydrocarbon "tail" and a hydrophilic (waterloving) anionic "head":
CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 COO − or
CH 3 (CH 2 ) n COO −
The length of the hydrocarbon chain ("n") varies with the type of fat or oil but is usually
quite long. The anionic charge on the carboxylate head is usually balanced by either a
positively charged potassium (K + ) or sodium (Na + ) cation. In making soap,
triglycerides in fat or oils are heated in the presence of a strong alkali base such as
sodium hydroxide, producing three molecules of soap for every molecule of glycerol.
This process is called saponification and is illustrated in Figure 1.
Like synthetic detergents, soaps are "surface active" substances ( surfactants ) and as
such make water better at cleaning surfaces. Water, although a good general solvent, is
unfortunately also a substance with a very high surface tension. Because of this, water
molecules generally prefer to stay together rather than to wet other surfaces. Surfactants
work by reducing the surface tension of water, allowing the water molecules to better
wet the surface and thus increase water's ability to dissolve dirty, oily stains.
Figure 1.
In studying how soap works, it is useful to consider a general rule of nature: "like
dissolves like." The nonpolar hydrophobic tails of soap are lipophilic ("oil-loving")
and so will embed into the grease and oils that help dirt and stains adhere to surfaces.
The hydrophilic heads, however, remain surrounded by the water molecules to which
they are attracted. As more and more soap molecules embed into a greasy stain, they
eventually surround and isolate little particles of the grease and form structures called
micelles that are lifted into solution. In a micelle, the tails of the soap molecules are
oriented toward and into the grease, while the heads face outward into the water,
resulting in an emulsion of soapy grease particles suspended in the water.
With agitation, the micelles are dispersed into the water and removed from the
previously dirty surface. In essence, soap molecules partially dissolve the greasy stain to
form the emulsion that is kept suspended in water until it can be rinsed away (see
Figure 2).
As good as soaps are, they are not perfect. For example, they do not work well in hard
water containing calcium and magnesium ions, because the calcium and magnesium
salts of soap are insoluble; they tend to bind to the calcium and magnesium ions,
eventually precipitating and falling out of solution. In doing so, soaps actually dirty the
surfaces they were designed to clean. Thus soaps have been largely replaced in modern
cleaning solutions by synthetic detergents that have a sulfonate (R-SO 3 − ) group instead
of the carboxylate head (R-COO − ). Sulfonate detergents tend not to precipitate with
calcium or magnesium ions and are generally more soluble in water.
Uses of Soap
Although the popularity of soap has declined due to superior detergents, one of the
major uses of animal tallow is still for making soap, just as it was in years past. Beyond
its cleaning ability, soap has been used in other applications. For example, certain soaps
can be mixed with gasoline to produce gelatinous napalm, a substance that combusts
more slowly than pure gasoline when ignited or exploded in warfare. Soaps are also used
in "canned heat," a commercialized mixture of soap and alcohol that can be ignited and
used to cook foods or provide warmth. Overall, soap is a remarkably useful substance,
just as it has been for thousands of years.
David A. Dobberpuhl
Figure 2. How soap works: The hydrophobic tails of soap molecules embed in grease
and oil, breaking it up into particles called micelles that lift off the surface and disperse
into water.
Bibliography
Brady, James E.; Russell, Joel W.; and Holum, John R. (2000). Chemistry: Matter and
Its Changes, 3rd edition. New York: Wiley.
Internet Resources
"The History and Chemistry of Soaps and Detergents." The Soap and Detergent
Association. Available from the SDA Kids Corner at http://www.sdahq.org/ .
Soap Article Questions
1. What is reacted with alkalis to make soap?
2. What ingredients were used during ancient Egypt to make soap?
3. Explain the terms hydrophobic and hydrophilic.
4. Give an example of an alkali base used in soaps.
5. What is another name for “surface active” substances?
6. Explain the importance of surfactants.
7. Soap works by the explanation “like dissolves like.” The nonpolat hydrophobic
tails are described as being lipophilic. What does this mean?
8. Using context clues, explain the term emulsion used in this article.
9. Some hard water contains calcium and magnesium ions. Write the formula for
these ions.
10. List one other application for soap other than cleaning.
Wash This Way! LAB
When you wash your hands, how do you do it? With soap and water? With water
alone? Do you scrub your hands or simply rinse them under the faucet? Does it even
matter?Yes, it does!
What You’ll Need:
• nonstick cooking spray
• ultrafi ne glitter
• timer or watch with a second hand
• access to a sink with warm and cold water,
soap, and paper towels
• data sheet (Draw 8 small outlines of hands
on a sheet of paper.)
What You’ll Do
»1. Choose four volunteers.
»2. Have each volunteer “dirty” their hands as follows:
a. Coat hands lightly with nonstick cooking spray. Spray over a
sink or newspaper because the floor will get very slippery if
the spray gets on the floor. Rub hands together to thoroughly
coat the front and back of the hands and between all fingers.
b. Sprinkle one hand with glitter and rub the hands together to
cover the front, back, and fingers of both hands. The glitter
represents germs typically found on our hands.
»3. Have the volunteers wash their hands as follows:
a. Volunteer 1: Wash with warm water, rinsing only (no scrubbing or soap) for 5
seconds.
b. Volunteer 2: Wash with warm water, scrubbing for 20 seconds under the
faucet without using soap.
c. Volunteer 3: Wash with warm water and soap, scrubbing hands (while not
under the faucet) for 5 seconds, and rinsing just until no soap is left on the
hands.
d. Volunteer 4: Wash with warm water and soap, scrubbing hands (while not
under the faucet) for 20 seconds, and rinsing just until no soap is left on the
hands.
»4. Observe the cleanliness of each volunteer’s hands. Record observations on
the data sheet by shading in the outline of a hand to indicate where you still see glitter.
»5. Tell each volunteer to dry their hands thoroughly with a paper towel. Again,
observe and record the cleanliness of each volunteer’s hands.
»6. Once you’ve recorded all observations, have all the volunteers completely wash
up with soap and water to remove all glitter and cooking spray.
Some Variations:
Small amounts of baby oil or petroleum jelly (such as Vaseline®) can be used instead of
the nonstick cooking spray. Finely ground spices such as cinnamon, nutmeg, or paprika
can be used instead of ultra fine glitter.
Questions to Consider:
• Which volunteer used the hand washing technique that you normally use or that you
think most people normally use?
• Which hand washing method was the most effective? Which was the least effective?
• What factor do you think is most important in hand washing?
Department of Health and Human Services, National Institutes of Health
Supported by a Science Education Partnership Award (SEPA) from the
National Center for Research Resources
©2004