Experiment: Making Hard Soap

Purpose:

• To experience making soap from scratch.

• To experience an "everyday application" of a chemical reaction - from a historical perspective.

• To understand the chemical reaction involved in the making of soap.

• To create a bar of soap that you would be proud to give to your Mom.

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:

• centigram balance

• electronic balance

• hot plate

• 250 mL beakers

• 25 mL graduated cylinder

• glass stirring rod

• thermometer

• lard

• sodium hydroxide pellets

• borax (sodium borate decahydrate)

• oil based scent - Each group will need to bring this to class on the day of the experiment.

• oil based coloring

• SOAP MOLD - THESE ARE CLEAR PLASTIC AND CAN BE PURCHASED AT CRAFT STORES LIKE MICHAELS, JO-ANN FABRICS, CRAFT WORLD, ETC.  Each group must bring ONE soap mold to class on the day of the experiment.  I recommend buying a "candy mold" type - these will be smaller and so you can get several soaps made.  If you buy the "soap mold" which will be a bath bar type, if it cracks when you try to get the soap out, you will be in trouble.  When you use the multiple mold type, you will be popping out a soap for me to grade - if one cracks or looks yucky, you have several more as "backups."

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:

• Use a 250 mL beaker and place it onto the analytical balance pan.

• Tare it out.

• Measure out 50 grams.

• Don't get any lard on the OUTSIDE of your beaker.

• Go to step #3.

If you are using a pan balance:

• Measure the mass of a 250 mL beaker on the pan balance at your lab table.

• Record that value here: _____________ grams

• Add lard until you have a mass of 50 g of lard.

• Record your beaker + lard value here: ________________ grams

• Make the appropriate subtraction above to be sure you have measured out about 50 grams of  lard: ___________________ g lard.

• Don't get any lard on the OUTSIDE of your beaker.

• Continue to step #3.

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:

• Use a small beakers and place it onto the analytical balance.

• Tare it out.

• Measure out 1 gram of borax.

• Add the borax to the NaOH solution.

• Stir well.

If you are using a pan balance:

• Measure the mass of one small beaker: ___________ grams

• Add borax so your value from above is 1 gram more.

• Add the borax to the NaOH solution.

• Stir well.

7. Let the lard cool to 40^oC.  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!  

• NOTE #1: You want to make sure that your soap product is completely saponified - BUT, don't stir for too long or your soap will begin to harden.  If this happens, when you try to pour it into the mold, it will be "chunky" and disgusting.

• NOTE #2: On the other hand, if your soap is not completely saponified (not enough stirring), you will get a goopy mess and you will be unable to get the soap out of the mold.  You don't want this situation either!

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 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.

• Does the soap look like a decorative hard soap?*

  • Consistent appearance

  • Dye color the same throughout

  • "Soft" goopy soap will NOT get any points (due to incomplete saponification) (or if the soap won't come out of the mold!)

• Does the soap smell like a soap?

• Did you dissolve most (all) of the borax?

• Does the soap have feel like a hard soap?

  • No grittiness

  • Smooth

  • Slippery

• Does the soap have an attractive shape relative to its color and smell?

  • a flower shape should not be brown, spotted and smell like ammonia...