EXPERIMENT 12 Saponification and Soaps
Goals
Prepare soap by the saponification of a fat or oil.
Observe the reactions of soap with oil, CaCl2, MgCl2, and FeCl3. Discussion
A. Saponification: Preparation of Soap For centuries, soaps have been made from animal fats and lye (NaOH), which was obtained by
pouring water through wood ashes. The hydrolysis of a fat or oil by a base such as NaOH is called
saponification and the salts of the fatty acids obtained are called soaps. The other product of hydrolysis
is glycerol, which is soluble in water. The fats that are most commonly used to make soap are lard and tallow from animal fat and coconut,
palm and olive oils from vegetables. Castile soap is made from olive oil. Soaps that float have air
pockets. Soft soaps are made with KOH instead of NaOH to give potassium salts.
B. Properties of Soaps and Detergents
A soap molecule has a dual nature. The nonpolar carbon chain is hydrophobic and attracted to
nonpolar substances such as grease. The polar head of the carboxylate salt is hydrophilic and attracted
to water. When soap is added to a greasy substance, the hydrophobic tails are embedded in the nonpolar
fats and oils. However, the polar heads are attracted to the polar water molecules. Clusters of soap
particles called micelles form with the nonpolar oil droplet in the center surrounded by many polar heads
that extend into the water. Eventually all of the greasy substance forms micelles, which can be washed
away with water. In hard water, the carboxylate ends of soap react with Ca2+, Fe3+, or Mg2+ ions and
form an insoluble substance, which we see as a gray line in the bathtub or sink. Tests will be done with
the soap you prepare to measure its pH, its ability to form suds in soft and hard water, and its reaction
with oils.
Detergents or “syndets” are called synthetic cleaning agents because they are not derived from naturally occurring fats or oils. They are popular because they don't form insoluble salts with ions,
which means they work in hard water as well as in soft water. A typical detergent is sodium lauryl
sulfate. As detergents replaced soaps for cleaning, it was found that they were not degraded in sewage treatment
plants. Large amounts of foam appeared in streams and lakes that became polluted with detergents.
Biodegradable detergents such as an alkylbenzenesulfonate detergent eventually replaced the
nonbiodegradable detergents. In addition to the sulfonate salts, a box of detergent contains phosphate compounds along with
brighteners and perfumes. However, phosphates accelerate the growth of algae in lakes and cause a
decrease in the dissolved oxygen in the water. As a result, the lake decays. Some replacements for
phosphates have been made. Lab Information
Time: 2 hr
Comments: You will be working with hot oil and NaOH. Be sure you wear your goggles. Tear
out the report sheets and place them beside the matching procedures.
Related Topics: Esters, saponification, soaps, hydrophobic, hydrophilic Experimental Procedures Wear your safety goggles! A. Saponification: Preparation of Soap
Materials: 150-mL beaker, hot plate, graduated cylinder, stirring rod or stirring hot plate with stirring
bar, large watch glass, 400-mL beaker, Büchner filter system, filter paper, plastic gloves, fat
(lard, solid shortening, coconut oil, olive or other vegetable oil), ethanol, 20% NaOH,
saturated NaCl solution.
Weigh a 150-mL beaker. Add about 5 g of fat or oil. Reweigh.
Add 15 mL ethanol (solvent) and 15 mL of 20% NaOH. Use care when pouring NaOH. Place
the beaker on a hot plate and heat to a gentle boil, stirring continuously. A magnetic stirring bar may be
used with a magnetic stirrer. Heat for 30 minutes or until saponification is complete and the solution
becomes clear with no separation of layers. Be careful of splattering; the mixture contains a strong base.
Wear disposable gloves, if available. Do not let the mixture overheat or char. Add 5-mL portions of an
ethanol-water (1:1) mixture to maintain volume. If foaming is excessive, reduce the heat. Caution: Oil and ethanol will be hot, and may splatter or catch fire. Keep a watch glass
nearby to smother any flames. NaOH is caustic and can cause permanent eye damage. Wear
goggles at all times.
Obtain 50 mL of a saturated NaCl solution in a 400-mL beaker. (A saturated NaCl solution is
prepared by mixing 30 g of NaCl with 100 mL of water.) Pour the soap solution into this salt solution
and stir. This process, known as “salting out”, causes the soap to separate out and float on the surface.
Collecting the soap Collect the solid soap using a Büchner funnel and filter paper. See Figure
32.1. Wash the soap with two 10-mL portions of cold water. Pull air through the product to dry it
further. Place the soap curds on a watch glass or in a small beaker and dry the soap until the next lab
session. Use disposable, plastic gloves to handle the soap. Handle with care: The soap may still
contain NaOH, which can irritate the skin. Save the soap you prepared for the next part of this
experiment. Describe the soap.
Filter paper Büchner funnel Rubber disk
To vacuum Filter flask Figure 32.1. Apparatus for suction
filtration with Büchner funnel. B. Properties of Soaps and Detergents
Materials: Test tubes, stoppers to fit, droppers, small beakers, 50- or 100-mL graduated cylinder,
stirring rod, laboratory-prepared soap (from Part A), commercial soap product, detergent, pH
paper, oil, 1% CaCl2, 1% MgCl2, and 1% FeCl3.
Prepare solutions of the soap you made in part A, a commercial soap, and a detergent by
dissolving about 1 g of each in about 50 mL of distilled water. If the soap is a liquid, use 20 drops.
B.1
pH test Place 10 mL of each soap solution in separate test tubes. Use 10 mL of water as a
comparison. Label. Dip a stirring rod into each solution, then touch the stirring rod to pH paper.
Determine the pH. Save the tubes for part B.2.
B.2
Foam test Stopper each of the tubes from B.1 and shake for 10 seconds. The soap should form
a layer of suds or foam. Record your observations. Save the tubes for part B.3.
B.3
Reaction with oil Add 5 drops of oil to each test tube from B.2. Stopper and shake each one for
10 seconds. Record your observations. Compare the sudsy layer in each test tube to the sudsy layers in
part B.2. Report Sheet—Lab 32
Date Name Section Team Instructor
Pre-Lab Study Questions
1. What happens when a fatty acid reacts with NaOH? 2. Why is ethanol added to the reaction mixture of fat and base in the making of soap? 3. What is the product of saponification of a salt? A. Saponification: Preparation of Soap
Describe the appearance of your soap. Questions and Problems
Q.1
How would soaps made from vegetable oils differ from soaps made from animal fat? Q.2 How does soap remove an oil spot? B. Properties of Soaps and Detergents
Tests
B.1 pH
B.2 Foam
B.3 Oil
B.4 CaCl2
MgCl2
FeCl3 Water Lab Soap Commercial Soap Detergent

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