What is Soap?
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Barbara Daiker Brookwood Junior High
201 East Glenwood-Lansing Road
Glenwood, IL 60425
The student will:
1. Observe soap-making in order to learn this process
2. Discover how soap behaves through experimentation
3. Develop a model of a soap molecule from observations
4. Apply information learned to soap's uses
Soap making Soap/Food Coloring Soap/Grease
15 grams lard petri dishes transparent jars
50 mL NaOH (6M) milk with lids
5 grams NaCl food coloring water (300 mL)
500 mL beaker toothpicks oil (5 mL)
watch glass liquid soap liquid soap (5 mL)
hot plate stirrer (homemade soap solution) liquid detergent
homemade soap chips water (homemade soap)
1. SOAP-MAKING is both dangerous and time-consuming. Therefore soap should be
prepared as a DEMONSTRATION as follows: Place the beaker on a hot plate and
add the lard. When the lard is almost melted, add the NaOH. You will
notice the layers are immiscible at the start. Place a watch glass on top
of the beaker, removing occasionally to stir, thereby avoiding heat buildup
and sputtering. The reaction is complete when there is no longer any
evidence that the layers are separated, which takes about 30 minutes.
Remove from heat and add 5 g of NaCl. This makes the solution more dense
and causes the soap to precipitate. Cool. Scoop out the soap onto paper
towels and blot excess grease. Rinse several times in cold water (to remove
excess hydroxide). Let dry for 24 hours.
TRY THIS ON YOUR OWN BEFORE THE CLASS DEMO! WEAR PROTECTIVE CLOTHING!
While setting up this DEMO, give some history of soap-making and challenge
the students to research the topic for extra credit.
2. Suggest that in order to understand how soap works, it is important to
discover its chemistry. This "puzzle" can be solved with some simple
experimentation. Pass out lab sheets which provide a list of materials,
methods of procedure, data table and "leading" questions. Call attention to
the technique to be used in dipping the toothpick into the various liquids.
Mention that the soap chips will be used for cleanup, not experimentation.
3. LAB: Place milk in the lid of the petri dish, to cover bottom. Carefully
add a drop of food coloring. Dip a toothpick into the H2O, then center of
food coloring. Note what happens. Using another tooth-pick, dip the tip
into liquid soap, then the center of dye. Note the reaction. Repeat using
a fresh source of milk, perhaps more than one color, and homemade soap
solution. Allow about ten minutes for the activity. At the finish, ask the
students to wash their hands with homemade soap, noticing in particular how
it feels and performs.
4. Regroup students for discussion. First ask for observations regarding the
hand soap. (It usually retains some physical properties of each ingredient,
i.e. feeling slippery and greasy.)
5. Go over LAB observations. (Milk, which is a suspension, will support the
drop of food coloring better than water and will allow for a more visible
reaction. When touched with H2O, there should be no reaction. It is used
as a control. When touched with a liquid soap, the dye spreads rapidly over
the surface. The movement should radiate from a center point, although once
it reaches the edge of the petri dish, the dye may be made to travel inward.
The negatively polar end of the soap molecule disrupts the surface tension
of the milk/water.) Ask for a visual description of the movement, leading
to the term "repel". Introduce the term "hydrophobic" - having a strong
dislike for H2O. Conclude that this is a part of the molecule.
6. Suggest that, to arrive at the structure of a soap molecule, it is necessary
to observe the way soap performs in a "real-life" situation. Solicit from
the students uses for soap. Choose removing grease. DEMONSTRATE as
follows: Fill two jars with equal amounts of H2O. Add 10 mL of motor oil to
each. Shake. Observe that oil is immiscible in water. Add 10 mL of liquid
soap to one jar. Shake both again. Observe. (The jar with the soap has an
emulsion and no evidence of oil remaining.) The soap molecule must then
have a part that will "dissolve" the oil, yet also like the water.
Introduce the term "hydrophilic" - having a strong liking for H2O. Review
the nature of a water molecule as being polar. In order for a soap molecule
to get along in water, it must also be polar and that part of the molecule
is hydrophilic. Then the part that was hydrophobic must be the part that
likes the oil [lipophilic]! Repeat procedure using homemade soap (and
detergent). The jar with no soap is used as a control.
7. Using the overhead projector, unveil the chemical formula for a soap
molecule. Highlight the COO- end of the molecule as POLAR:
H H H H H H H H H H H H H H H H H O
| | | | | | | | | | | | | | | | | i
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-O - Na+
| | | | | | | | | | | | | | | | |
H H H H H H H H H H H H H H H H H
Often a SYMBOLIC representation is used to show this molecule:
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\( - )
hydrocarbon chain polar end
8. Introduce a candy sucker to visualize this symbol. Tape a paper oil
molecule on the board and ask students to place the suckers around the oil
molecule. REMIND them which part is hydrophobic/lipohilic (stick) and which
is hydrophilic (candy). (The stick should face inside and the candy the
outside of the oil molecule.) Use paper H2O to interface with the polar
end of the soap molecule (candy).
9. To understand the difference between soap and detergent, perform the
following DEMONSTRATION. Using two glass jars, fill one with tap H2O and
another with "hard"/well H2O. Shake. Observe. (Hard water has ions which
react with the polar end of a soap molecule, causing a precipitate and tying
up some of the soap. It will have less bubbles. Detergents have a more
complicated "head" which does not react this way, allowing the "soap" to do
its job.) Ask the students when it would be advisable to use a detergent
over a soap and when it would not matter. (Soap: hands; detergent: hair,
clothing, etc. where the precipitate cannot be washed away.)
10. For further investigation, challenge the students to think of any substance
that neither a soap nor a detergent will work on. (Natural stains: coffee,
"Soap" by Clair Wood; Chem Matters, February 1985.
"Detergents" by Clair Wood; Chem Matters, April 1985.
Soap and Film Aims Fresno, California.