SAPONIFICATION
Pre-Lab Discussion
Fats are esters formed from glycerol (an alcohol) and a long-chain organic acid (a
long-chain fatty acid). The formation of a fat can be represented by the equation:
*Diagram*
Soaps are metallic salts of fatty acids. Soaps are made by boiling solid fats or
liquid fats (oils) with a solution of a strong base. This reaction is called saponification. If
the fat from the equation above reacts with a strong base, such as NaOH, a soap and
glycerol are formed:
*Diagram*
In condensed form, this equation can be written:
(RCOO)3C3H5 + 3NaOH  3RCOONa + C3H5(OH)3
In this experiment, soap will be made by reacting a liquid fat with sodium
hydroxide dissolved in ethanol.
Purpose
Prepare soap from liquid fat and a strong base.
Equipment
beaker, 50-mL
graduated cylinders, 10-mL and 100-mL
evaporating dish
ring stand
iron ring
wire gauze
microspatula
test tube, 13x100-mm
glass stirring rod
funnel
burner
safety glasses
Materials
oil, cottonseed or olive
saturated NaCl solution
ethanol
30% NaOH solution
distilled water
litmus paper
filter paper
Procedure
1.
2.
Measure out 8.0 mL of cottonseed or olive oil and pour it into a clean, dry
evaporating dish. Add 8.0 mL of ethanol and 4.0 mL of 30% NaOH
solution to the oil. CAUTION: Handle this solution carefully.
Set up the ring stand, wire gauze, evaporating dish, and burner as shown in
Figure 50-1. Heat the mixture gently with a very low flame. CAUTION:
Keep the flame away from the ethanol, which is highly flammable.
*Diagram*
Figure 50-1
3.
4.
5.
6.
7.
8.
Continue heating for 10-15 minutes, stirring constantly to prevent
spattering. Stop heating when the odor of fat has dissappeared and the oil
has dissolved.
Allow the mixture to cool. During this time, place 20.0 mL of distilled
water in a 50-mL beaker and heat to just below boiling.
When the evaporating dish is cool enough to touch, add the hot water to
the mixture in the dish. Then add 25.0 mL of saturated NaCl solution and
stir the mixture.
Filter the mixture, collecting the liquid in the beaker. Discard the liquid
and keep the solid on the filter paper.
In a test tube, dissolve 2 microspatulas of your soap in 20.0 mL of distilled
water. Test the solution with litmus paper and record your observations.
Add about 1.0 mL of tap water to the soultion in the test tube. Place your
thumb over the mouth of the tube and shake vigorously. Record your
observations.
Observations
Litmus test:
Shaking test:
Conclusions and Questions
1.
2.
3.
4.
5.
What do the processes of saponification and esterfication have in
common? How do these processes differ?
To what class of compounds do fats belong? Soaps?
Describe the contents of the test tube after shaking it (step 8). Did any
white precipitate (scum) appear? If so, explain its prescence.
The formula for the hydrocarbon radical (R-) in the fat glycerol stearate is
C17H35. Write an equation showing the reaction of glycerol stearate with
sodium hydroxide to produce the soap sodium stearate.
How are soaps treated commercially before they are marketed?