Making a Soap

 

 

Discussion:

The hydrolysis of a fat or oil by a base such as sodium hydroxide (NaOH) is called saponification.  The

products are glycerol  and the salts of the fatty acids, which are called soaps.  The soap is forced out of

solution (precipitated) by the addition of a NaCl solution.  Several tests will be made with the soap that

you will be making in this experiment.

 

Materials:

400-mL beaker, hot plate, filter system, filter paper, solid fat(lard), ethanol, 6 M NaOH,

boiling chips, saturated NaCl solution, commercial soap or detergent, pH paper, safflower oil, 5% CaCl2,

5% MgCl2, 5% FeCl3, ice for ice bath.

 

Procedure:

 

To prepare a hot water bath:  Fill the 400-mL beaker with approx. 150 mL of water.  Place on a hot plate

and begin heating the water to a gentle boil.  This will be the way that you heat your reaction mixture.

 

To prepare your reaction mixture:  Place about 3 g of a solid fat in a 125 mL Erlenmeyer flask. 

Add 15 mL ethanol and 15 mL of 20% NaOH.  Use care when pouring NaOH.  Add a few boiling chips

to the Erlenmeyer flask to prevent foaming.

 

Place the flask containing your reaction mixture into the gently boiling water bath.  Heat the mixture for

30 minutes stirring often with a glass rod.  Be carefully of splattering, since the mixture contains a very

strong base.  The solution should eventually become clear with no visible layers.  If the foaming is

excessive, lower the heat.

 

Remove the flask from the hot water bath.  To test for completeness of your saponification reaction,

carefully transfer a few drops of the mixture to a few mL of water in a test tube.  If you see droplets of fat

or oil, you will need to add more NaOH; add 5 mL of 20% NaOH and heat for another 10 minutes, if this

is the case.  Otherwise, your reaction is complete and you can proceed directly to the next step.

 

Cool your flask by placing it in an ice bath to cool.

 

Obtain 50 mL of a saturated NaCl solution.  Add this solution to the soap mixture and stir vigorously. 

This process of Asalting out@ or precipitating out the salt increases the density of the solution.  This

causes the solid soap curds which form to float on the surface.

 

Collect the solid soap by pouring the mixture into through a filter; you may wish to use a Buchner funnel. 

Wash the soap curds with two 10 mL portions of cold water.  Use plastic gloves to carefully pack the

solid pieces of soap together to form a small cake.  Describe the appearance of the soap.  Handle with

care.  The soap may still contain NaOH, which can irritate your skin.

 

Perform the following tests with your soap:

 

1.         pH test

Prepare a solution from the soap you made by dissolving a small piece of soap in 30 mL of warm

water.  Also prepare a solution of commercial soap or detergent in the same amounts.  Using pH

paper, determine the pH of each soap solution.

 

2.         Foam test

Pour 5 mL of your soap solution in a test tube, stopper and shake for 10 seconds.  A foamy layer

should form.  Record your observations.  Repeat with 5 mL of the commercial soap.

 

 

 

 

 

3.         Emulsification test

Add 10 mL of safflower oil to two test tubes.  Place 5 mL of water in the first test tube.  Stopper

and shake for 10 seconds.  What happens to the oil layer?  Record your observations.  Place 5

mL of your soap solution in the second test tube.  Stopper and shake for 10 seconds.  Compare

the foamy layers in the two text tubes.  Is the oil dispersed?  Record your observations.

 

 

4.         Hard water test

            Place 5 mL of your soap solution in four separate test tubes.  To the first test tube, add 20 drops

            (1 mL) of tap water.  To the second test tube, add 20 drops 5% calcium carbonate, CaCl2,

solution.  To the third test tube, add 20 drops of 5% iron(III) chloride, FeCl3, solution.  To the

fourth test tube, add 20 drops of 5% magnesium chloride, MgCl2, solution.  Stopper each test

tube and shake 10 seconds.  Compare the foamy layer in each of the test tubes.  Record your

observations.

 

 

 

Soap prepared in lab

Commericial soap

1.  pH

 

 

 

2.  Foam

 

 

 

 

 

Safflower oil + water

Safflower + soap

3.  Emulsification

 

 

 

 

4.  Hard water

Amount of foam

     1) tap water

 

 

     2) CaCl2

 

 

     3) FeCl3

 

 

     4) MgCl2

 

 

 

Questions:

1.         A solid fat used in the saponification contain high quantities of tristearin (glyceryl tristearate).

            Write an equation for the saponification of this triglyceride with NaOH.

 

 

 

 

 

 

 

 

 

 

 

 

 

2.         What happened to the glycerol produced during the saponification?

 

 

 

 

 

 

3.         Why is the other product of saponification called a salt?

 

 

 

 

 

 

4.         Were the soap samples acidic or basic?

 

 

 

 

5.         What is the effect of soap on oil?

 

 

 

 

6.         How does the soap react with CaCl2, MgCl2, and FeCl3?  Explain.

 

 

 

 

7.         Why are phospholipids more soluble in water than other lipids?

 

 

 

 

8.         Steroids do not contain any fatty acids.  Why are they classified as lipids?

 

 

 

 

9.         Why are soaps more soluble in water than other fatty acids?

 

 

 

 

10.        How does soap remove an oil spot?

 

 

 

 

11.        How could you determine if a water sample is soft or hard water?

 

 

 

 

12.        How would soaps made from vegetable oils differ from soaps made from animal fat?