EXPERIMENT 2
Lipids
Objectives
1.
2.
To prepare a simple soap.
To investigate some properties of a soap,
Preparation and properties of a soap
Background
A soap is the sodium or potassium salt of a long-chain fatty acid. The fatty acid usually contains 12 to 18
carbon atoms. Solid soaps usually consist of sodium salts of fatty acids, whereas liquid soaps consist of the
potassium salts of fatty acids. A soap, such as sodium stearate, consists of a nonpolar end (the hydrocarbon
chain of the fatty acid) and a polar end (the ionic carboxylate).
O

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2C-O- Na+
nonpolar (dissolves in oils)
polar (dissolves in water)
Because "like dissolves like," the nonpolar end (hydrophobic or water hating part) of the soap molecule can
dissolve the greasy dirt, and the polar or ionic end (hydrophilic or water loving part) of the molecule is
attracted to water molecules. Therefore, the dirt from the surface being cleaned will be pulled away and
suspended in water. Thus, soap acts as an emulsifying agent, a substance used to disperse one liquid (oil
molecules) in the form of finely suspended particles or droplets in another liquid (water molecules).
Treatment of fats or oils with strong bases such as lye (NaOH) or potash (KOH) causes them to undergo
hydrolysis (saponification) to form glycerol and the salt of a long-chain fatty acid (soap).
O
||
H2C-O-C-(CH2)16CH3

O

||
HC-O-C-(CH2)16CH3 + 3 NaOH
 O
 ||
H2C-O-C-(CH2)16CH3
tristearin
→
CH2OH
|
CHOH +
|
CH2OH
O
||
3 CH3(CH2)16C-O- Na+
glycerol sodium stearate (a soap)
Because soaps are salts of strong bases and weak acids, they should be weakly alkaline in aqueous solution.
However, a soap with free alkali can cause damage to skin, silk, or wool. Therefore, a test for basicity of the
soap is often important. Soap has been largely replaced by synthetic detergents during the last two decades
because soap has two serious drawbacks. One is that soap becomes ineffective in hard water, which contains
appreciable amounts of Ca2+, Fe3+, or Mg2+.
O
O
||
||
2 CH3(CH2)16C-O- Na+ +
M2+ →
(CH3(CH2)16C-O-)2M2+ ↓ + 2Na+
soap
(Ca2+, Fe3+, or Mg 2+)
soap scum
The other is that, in an acidic solution, soap is converted to free fatty acid that is insoluble in water and
therefore loses its cleansing action.
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O
O
||
||
+
+
CH3(CH2)16C-O Na + H → CH3(CH2)16C-OH ↓ + Na+
soap
free fatty acid
Procedure
1. Preparation of a soap
Measure 23 mL of a vegetable oil into a 250 mL Erlenmeyer flask. Add 20 mL of ethyl alcohol (to act as
a solvent) and 20 mL of 25% sodium hydroxide solution. While stirring the mixture constantly with a
glass rod, the flask with its contents is heated gently in a boiling water bath. Decrease stirring rate if
mixture gets close to foaming out of the flask. A 600 mL beaker containing about 200 mL of distilled
water can serve as a water bath (Fig. 2.1).
2.1 Experimental set-up for soap preparation.
CAUTION! Alcohol is flammable
After being heated for about 20 min. the odor of alcohol will disappear, indicating the completion of the
reaction. A pasty mass containing a mixture of the soap, glycerol, and excess sodium hydroxide is obtained.
Use an ice-water bath to cool the flask with its contents. To precipitate or “salt out” the soap, add 150 mL of
a saturated sodium chloride solution to the soap mixture while stirring vigorously. This process increases the
density of the aqueous solution therefore, soap will float to the surface of the aqueous solution. Clamp a 500
mL filter flask to a ring stand. Filter the precipitated soap with the aid of suction and wash it with 10 mL of
ice cold water. Observe the appearance of your soap and record your observation.
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2. Properties of a soap
a. Emulsifying Properties. Shake ten drops of mineral oil in a test tube containing 5 mL of distilled water.
A temporary emulsion of tiny oil droplets in water will be formed. Repeat the same test, but this time add
a small piece of the soap you have prepared before shaking. Allow both solutions to stand a while.
Compare the appearance and the relative stabilities of the two emulsions. Record your observations on
the Data Sheet.
b. Hard Water Reactions. Place about one-third spatulaful of the soap you have prepared in a 100-mL
beaker containing 35 mL of distilled water. Warm the beaker with its contents to dissolve the soap. Pour
5 mL of a soap solution into each of five test tubes (nos. 1, 2, 3, 4, and 5). Save the remainder of your
soap solution for parts c and d. Add 2 drops of a 5% solution of calcium chloride to tube 1; add 2
drops of a 5% solution of magnesium chloride to tube 2; add 2 drops of a 5% solution of iron chloride to
tube 3; and add 2 drops of tap water to tube 4. Compare each of these with the solution in tube 5. Make
a sixth test tube using 5 mL of distilled water and 1 drop of synthetic liquid detergent. Add 2 drops of
CaCl2 to tube 6. Record all of your observations on the Data Sheet.
c. Alkalinity (Basicity). Test the soap solution you made in part b with a wide-range pH paper. What is
the approximate pH of your soap solution? Record your answer on the Data Sheet.
d. Acidity. Add 5 mL of soap solution from part b into a test tube and add 5 drops of 3M sulfuric acid.
Record your observations on the data sheet.
Chemicals and Equipment
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Hot plate
Ice cubes
Buchner funnel in No.7 one-hole rubber stopper (check out from stockroom)
500 mL filter flask (check out from stockroom)
Filter paper, 7 cm diameter
pH paper
95% ethanol
Saturated sodium chloride solution (~2L per lab section)
25% NaOH
Vegetable oil
5% FeCl3
5% CaCl2,
5% MgCl2
Mineral oil
3M H2SO4
Liquid detergent
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Fatty Acid Components of Some Common Fats and Oils
Lauric
Component Fatty Acids (%)
Myristic Palmitic Stearic Oleic Linoleic
Δ9
(C12:0 )
(C14:0 )
1-4
8-13
Tallow
Lard
(C16:0 )
Δ9, 12
Linolenic
Δ9, 12,15
(C18:0 )
(C18:1 )
(C18:2 )
(C18:3
25-32
8-13
22-29
2-4
2-3
24-32
20-25
37-43
2-3
1-2
25-30
12-16
40-50
3-8
13-18
7-10
1-4
5-8
1-3
1-6
32-47
1-6
40-52
2-11
0-2
7-20
2-3
53-86
4-22
Peanut oil
0-1
6-11
3-6
40-65
17-38
Cottonseed oil
0-3
17-23
1-3
23-44
34-55
Corn oil
1-2
8-12
2-5
29-49
34-56
Soybean oil
0-1
6-10
2-5
20-30
50-60
2-10
6-7
2-3
12-14
75-80
0-2
5-9
4-7
9-29
8-29
45-67
Fats
Butter
Edible Oils
Coconut oil
40-50
Palm oil
Olive oil
0-1
Safflower oil
Nonedible Oil
Linseed oil
0-1
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)
NAME
DATE_________________
PARTNER
SECTION______________
DATA SHEET
Experiment 2: Lipids
Preparation and Properties of Soap
1. Preparation of Soap: Appearance of your soap ____________________________
2. Properties of Soap:
a. Emulsifying Properties
Which mixture, oil-water or oil-water-soap, forms a more stable emulsion? _____________
b. Hard Water Reaction
Observations
Tube 1 + CaCl2 _____________________________________________________
Tube 2 + MgCl2 ____________________________________________________
Tube 3 + FeCl3 _____________________________________________________
Tube 4 + tap water _________________________________________________
Tube 6 (liquid detergent) + CaCl2______________________________________
c. Alkalinity
pH of your soap solution: ____________
Is your soap solution good enough for washing hands? _____________________
d. Acidity
Observations
soap solution + H2SO4 ______________________________________
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POST-LAB QUESTIONS
1. If you react vegetable oil with KOH what chemical species will you find at the end of the reaction?
2. In soap making, first you dissolved vegetable oil in ethanol. What happened to the ethanol during the
reaction?
3. If corn oil is used for making soap, draw either the condensed structural formula or the line formula of the
most abundant soap you formed (see the table on page 14 of the manual).
4. Indicate which end of the soap molecule in question 3 is nonpolar and which end is polar.
5. Why is the use of a soap in hard water impractical? Explain with a chemical equation.
6. What is the major chemical structure difference between soaps made from lard (animal fat) and soaps
made from vegetable oil?
7. Explain how soap acts as a cleansing agent?
8. Why doesn’t soap work well under acidic conditions?
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