Experiment:
Making Hard Soap

 

Purpose:

 

Introduction: 

 

Soap is one of the earliest cleaning agents known to man.  It was first made by the ancient Romans from animal fats and wood ashes about 2,500 years ago.  

 

Animal fats and vegetable oils are substances composed of esters.  Esters are chemical compounds which have the general chemical formula RCOR' where the R and the R' represent long chains of carbon and hydrogen atoms (hydrocarbon groups).  Stearin is the main component of beef fat (lard). 

 

When a fat is boiled with lye (sodium hydroxide), it reacts to form a hard soap and glycerin.  This process is called saponification.  However, if there is too much fat compared to the amount of lye reacted, the process is incomplete - called incomplete saponification.  The end result of this is a goopy mixture which is NOT hard soap (you don't want this!).

 

Let's look at the saponification process a bit more.  The lye (sodium hydroxide) has the job of "cleaving" (cutting off) the long hydrocarbon chains from the glycerin "backbone".  The sodium takes the place of the hydrogen atom at the very end of each hydrocarbon chain.

 

Reactants:

 

Products:

 

Materials:

 

Procedure:

 

1.  Using a permanent marker, label your soap mold with EACH person's name in your lab group and your block period.  

2.  You will need 50 grams of lard.  You needn't be exact - plus or minus a couple of grams won't matter.

If you are using an analytical balance:

If you are using a pan balance:

3.  Melt the lard on the hot plate.

4.  While the lard is melting, under the hood, tare (zero) out the mass of a 250 mL beaker on the analytical balance.  Measure out 10 grams of NaOH.  CAUTION: Sodium hydroxide is very caustic and can cause severe burns if any touches your skin!  Be careful not to allow any "stray" NaOH pellets to get where they shouldn't be! 

5. Using a graduated cylinder, measure out 25 mL of cold tap water and then add this water to the sodium hydroxide.  Mix with glass stirring rod until all the NaOH has dissolved. NOTE: Your beaker will get HOT as you dissolve the sodium hydroxide - this process is quite exothermic.

6. Borax improves the cleaning ability of soap: You will need 1 gram.

If you are using an analytical balance:

If you are using a pan balance:

 

7. Let the lard cool to 40oC.  Then, with continuous stirring of the melted lard, carefully add the sodium hydroxide solution in a very thin steady stream.  You should see the soap begin to saponify.  NOTE: If you add the NaOH too quickly or stir too rapidly, the fat will separate from the NaOH and you will have to START OVER! 

8. As the lard/sodium hydroxide mixture starts to saponify, add the liquid coloring and scenting.

9. When the soap has been thoroughly mixed (complete saponification), carefully pour it into your soap mold.  If you pour too slowly, your soap will look chunky and disgusting. Place mold in the location your instructor states - each class of soaps should be in its own section of the room to dry!  

10. It takes a couple of days for your soap to completely harden.  Remember, if after a couple of days that your soap is still mushy, you have incomplete saponification and you will have to do the lab over!!  Or, if you stirred too long and your soap looks chunky, you will need to redo the lab!

 

Analysis:

 

Visit and learn from the 3 sites below dealing with making hard soap ... If you're gonna go to the trouble of making soap from scratch, you may as well know something about the history of making soap!  You will have a quiz over this information after the lab is completed - see ChemCentral for specific date of the quiz.

Soap Making History

Comparison of Soap Making from Pioneer Times to the Present

Making Soap - The way we used to do it

 

Soap Grading Rubric:

 

Your teacher will be grading your soap product.  Samples of "excellent" or "good" soaps will be available for review prior to grading.  Soaps will be given a single score (range of 0-10).  Please review qualifications below.

 

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