Materials Included In Kit

Additional Materials Required

Safety Precautions

Sodium hydroxide solution causes severe skin burns and eye damage. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Avoid contact of all chemicals with eyes and skin. All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any material in the lab and do not remove any remaining food items after they have been used in the lab. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Check the pH and appearance of the soaps to decide if students may be permitted to take their soaps home with them. Excess sodium hydroxide or other basic solutions may be neutralized with acid according to Flinn Suggested Disposal Method #10.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 student groups. Both parts of the Introductory Activity can reasonably be completed in two 50-minute class periods. The most convenient stopping point is after step 10 in Part A. The prelaboratory assignment may be completed before coming to lab. Allow more time for the Guided-Inquiry Design and Procedure.
• Remind students to be very careful when adding the solid fat into the beaker. Fat left on the outside of the beaker makes the beaker very slippery when heated. Beef tallow melts at about 35 °C—heat gently and do not overheat. Handle the beaker very carefully when removing it from the heat to avoid spilling any hot oil.
• The amount of sodium hydroxide used in step 5 should be precisely measured in order to avoid having excess base in the soap product.
• The soap is ready to pour in step 8 when it has the consistency of pea soup or thin pudding. Waiting too long, however, before pouring the soap will result in a grainy-textured product. In addition, cooling the soap mixture too quickly may create a “false trace,” resulting in a grainy-texture and unreacted base. For this reason, a cold tap water bath is recommended for step 7, not an ice water bath.

Teacher Tips

• Fats and oils are all triglycerides. The distinction between a fat and an oil rests on the source of the triglyceride—fats are obtained from animal sources, while oils are obtained from plants. Most, but not all, oils are liquids at room temperature because they contain a greater proportion of unsaturated fatty acids. (Introducing double bonds into a triglyceride puts “bends” or “kinks” in the shape of the molecule, which lowers the melting point.) The exceptions to this general rule are coconut oil and palm oil, which contain relatively low molecular weight saturated fatty acids.
• This experiment offers many opportunities for a cooperative class project to investigate how different variables will affect the characteristics and quality of soap, including the nature of various fats and oils, adding coloring or fragrance to the soap, the temperature of the oil and the NaOH solution prior to mixing, the rate of cooling during the curing phase, etc.
• If students create a personalized soap, use your judgment in deciding whether the students should be allowed to take their soaps home. Their soaps may contain excess sodium hydroxide solution and thus be irritating to the skin. If allowed, be sure to allow plenty of time for the soaps to cure and check the pH. Most commercial bar soaps have a pH of 9–10.

Answers to Prelab Questions

1. Define the following terms.
   1. Saponification

      Reaction of a fat or oil with sodium or potassium hydroxide. The strong base hydrolyzes or splits apart the ester linkages in the fat or oil to produce sodium or potassium salts of fatty acids (called soaps). Glycerol is a byproduct of the saponification reaction.

   2. Surfactant

      Also called a surface-active agent. A substance that lowers the surface tension of water and acts as an emulsifying agent. Surfactants are used as soaps and detergents.

   3. Micelle

      A spherical aggregate of soap molecules in aqueous solution. A micelle has two opposing features. The hydrophobic “tails” of soap molecules are arranged inward, facing each other. The exterior surface of the micelle is made up of hydrophilic ionic groups.

   4. What is the principal safety hazard in this experiment?

      Sodium hydroxide solution, a strong base, is caustic and corrosive and can cause severe skin burns.

   5. Explain why soap made by primitive methods was likely to be very harsh.

      When soap was made using primitive methods, the starting materials were not pure and the chemistry was not well understood. As a result, it was difficult to know exactly how much strong base (potash) was needed to make the soap. The soap was therefore likely to contain excess base and to be caustic.

   6. If the saponification value of olive oil is 190 mg KOH per gram, what is the SAP value of olive oil when using NaOH to make soap? Hint: What is the ratio of the molar mass of NaOH to the molar mass of KOH?
      {12616_Answers_Equation_2}

Sample Data

Answers to Questions

Guided-Inquiry

1. Research frequently used fats and oils in soap recipes. How does the nature of various fats and oils affect the characteristics and quality of soap?

   Soaps prepared using different oils will contain different fatty acids, and will therefore have some slightly different properties, such as texture, color, hardness, lathering ability or fragrance.

2. Explain why substituting the same quantity of one oil for another in a soap recipe may not have the desired results.

   Most fats and oils contain a mixture of fatty acid residues of different chain lengths and therefore have different saponification values. The amount or concentration of sodium hydroxide solution may also need to change.

3. Most soap recipes call for superfatting—either adding more fats or oils than needed to react completely with the sodium hydroxide (usually 5% excess fat) or using less sodium hydroxide than required for the amount of oil in the recipe (sometimes called the lye discount). Explain the benefits of superfatting.

   Using excess fat has two main benefits in soap-making. (1) Excess fat helps control the pH of soap. Excess fat ensures all the sodium hydroxide that was added is used up in the reaction. By eliminating the excess base, the soap will be mild and not irritating to skin. (2) Excess fat gives soap a smooth feel. Too much fat, however, will leave the soap greasy.

Post-Lab Questions

1. Explain why soap is used for washing instead of just using water alone.

   Dirt and grease are nonpolar, hydrophobic substances that are not soluble in water. If water alone were used for washing or cleaning, the hydrophobic dirt and grease molecules would not dissolve in the water. In soapy water, however, dirt and grease molecules become trapped or suspended within the hydrophobic core of a micelle. The soap thus disperses or breaks up the dirt particles and dissolves them in the water. The dirt-containing micelles are water-soluble and are rinsed away in the wash.

2. Compare the color, texture and appearance of the homemade soap versus your favorite brand of hand soap.

   The homemade soap is similar in hardness and appearance to commercial soap. Commercial soaps come in all colors and many have special fragrances added.

3. Is the homemade soap solution acidic or basic? Explain.

   The soap solution is basic—pH 10.5.

4. Consider the results of the emulsification test.
   1. Compare the results between the olive oil with water and the olive oil with the soap solution.

      The soap solution formed a stable emulsion with olive oil. The soap and oil emulsion appeared to be a single liquid layer and was cloudy. No oil droplets were visible. A mixture of water and oil separated into two layers immediately after being shaken.

   2. Explain the difference in the results in terms of the ability of soap to form micelles.

      Soap molecules form micelles, which are able to dissolve or entrap oil molecules within their hydrophobic cores.

5. Why do soaps made from different oils have different properties, even if the properties are only slightly different?

   The specific chemical formula of a soap depends on the fatty acids in the original triglycerides. Soaps prepared using different oils will contain different fatty acids, and will therefore have some slightly different properties, such as texture, color, hardness, lathering ability or fragrance.

6. Describe the properties of the soap made with your own recipe.

   Student answers will vary. One soap tested in the lab was made with 10 g of coconut oil and 6.4 mL of 6 M NaOH. The color was white and the texture very smooth. Good lather with soft conditioning. After one week, pH was 8.5.

References

This activity was adapted from Flinn ChemTopic™ Labs, Vol. 19, Chemistry of Organic Compounds; Cesa, I., Editor; Flinn Scientific, Inc; Batavia, IL (2006).

Introduction

Soap-making is an ancient craft and one of the oldest known chemical reactions involving organic compounds. Soaps are sodium and potassium salts of fatty acids. They are prepared by reacting fats and oils with a strong base, such as sodium hydroxide or potassium hydroxide.

Concepts

• Soaps and soap-making
• Saponification
• Surfactants

Background

The earliest written reference to soap comes from the Roman historian Pliny the Elder in the first century C.E. Pliny described the preparation of sapo from goat fat and wood ashes and attributed the invention to the Gauls, who used it to make hair shiny rather than for bathing or cleaning. Historical references to soap may be found in ancient Babylonian and Egyptian artifacts dating as far back as 2500 B.C.E.

Soap-making is also associated with colonial America and pioneers on the American frontier. The soap was made by boiling fat with a concentrated solution of potash (potassium carbonate) extracted from wood ashes with hot water. Potassium carbonate solutions are caustic—strongly basic and irritating to the skin and eyes. Soap made in this way was likely to contain excess (unreacted) potassium carbonate and was therefore quite harsh, leaving the skin rough and dry. This frontier method of soap-making may appear primitive, but it is still used in almost the same form today to make both commercial and handmade soaps. The methods are safer, however, and soaps are milder because the starting materials are pure, the chemistry is well-understood, and the reactants can be mixed in the right ratio.

The process of making soap is called saponification and is one of the earliest examples of using organic chemistry to produce a man-made product. Saponification involves the reaction of natural fats and oils, called triglycerides, with sodium or potassium hydroxide. Triglycerides are esters, containing three fatty acid groups attached via ester linkages to a glycerol backbone (see Figure 1).

The products of a saponification reaction are sodium or potassium salts of fatty acids and glycerol (Equation 1). Each type of oil or fat has a saponification value (SAP value), which is the amount of sodium or potassium hydroxide in mg that it takes to react completely with 1 gram of the oil or fat. Most fats and oils contain a mixture of fatty acid residues of different chain lengths. The most common fatty acids have 12–18 carbon atoms and may be saturated or unsaturated. Unsaturated and polyunsaturated fatty acids contain one or more C=C double bonds, respectively, in their structures while saturated fatty acids contain no C=C double bonds.

Soaps belong to a class of compounds called surface-active agents or surfactants, which also include detergents and emulsifying agents. A surfactant is defined as a compound that reduces surface tension when dissolved in water or in aqueous solutions. All surfactants have two basic features in common. One end of a surfactant molecule is usually a long, nonpolar hydrocarbon chain, resembling a “tail.” The hydrocarbon tail is said to be hydrophobic (water-fearing) because it tends to repel or exclude water and will not dissolve in water. The other end of a surfactant molecule is a small ionic or polar group that is hydrophilic (water-loving). The hydrophilic group will tend to be surrounded by water molecules and will dissolve in water. These two competing structural features give soaps and other surfactants their unique properties.

When dissolved in water, soaps and other surfactant molecules spontaneously self-associate to form spherical aggregates called micelles (see Figure 2). The nonpolar hydrocarbon tails in the soap molecules spontaneously arrange themselves toward the interior of the micelle, giving it a hydrophobic core that repels and thus excludes water. The ionic head groups are arranged on the outside surface of the micelle and are surrounded by water molecules. The ability of soap molecules to form micelles explains how and why soaps work. Dirt and grease are nonpolar, hydrophobic substances that are not soluble in water. If water alone were used for washing or cleaning, the hydrophobic dirt and grease molecules would not dissolve in the water. In soapy water, however, dirt and grease molecules become trapped or suspended within the hydrophobic core of a micelle. The soap thus disperses or breaks up the dirt particles and dissolves them in the water. The dirt-containing micelles are water-soluble and are rinsed away in the wash. The formation of micelles is also related to the emulsifying action of soaps—their ability to form stable mixtures or suspensions of two or more immiscible liquids.

The specific chemical formula of a soap depends on the fatty acids in the original triglycerides. Soaps prepared using different oils will contain different fatty acids, and will therefore have some slightly different properties, such as texture, color, hardness, lathering ability or fragrance. However, the characteristic properties of soap, such as its surfactant action, will be present regardless of the oil used to make the soap.

Experiment Overview

The purpose of this inquiry lab is to make soap and study its properties. The investigation begins with an introductory activity to prepare soap via saponification of a fat and oil with a solution of sodium hydroxide. The properties of the soap will then be investigated—its pH, texture and emulsifying action. The results provide a model for guided-inquiry design of an experiment using a recipe for preparing soap with different oils or fats.

Materials

Prelab Questions

1. Define the following terms.
   1. Saponification
   2. Surfactant
   3. Micelle
2. What is the principal safety hazard in this experiment?
3. Explain why soap made by primitive methods was likely to be very harsh.
4. If the saponification value of olive oil is 190 mg KOH per gram, what is the SAP value of olive oil when using NaOH to make soap? Hint: What is the ratio of the molar mass of NaOH to the molar mass of KOH?

Safety Precautions

Sodium hydroxide solution causes severe skin burns and eye damage. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Notify the instructor and clean up all spills immediately. Avoid contact of all chemicals with eyes and skin. All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any material in the lab and do not remove any remaining food items after they have been used in the lab. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Introductory Activity

Part A. Preparation of Soap

1. Tare a 50-mL beaker on the electronic balance. Place about 7 g of tallow (solid fat) into the beaker using a metal spatula. Use a toothpick to scrape the tallow off the spatula into the beaker, taking care not to get any tallow on the outside of the beaker.
2. Measure about 3 g of olive oil into the same 50-mL beaker.
3. Place the beaker on a hot plate at the lowest setting or inside a 250-mL beaker filled with hot tap water. Heat the contents of the beaker until the fat melts and the mixture of olive oil and fat forms a homogeneous solution.
4. Carefully remove the beaker from the hot plate or warm water bath.
5. Measure 5.8 mL of 6 M sodium hydroxide solution into a 10-mL graduated cylinder and carefully add the sodium hydroxide solution to the melted fat and oil mixture.
6. Return the beaker to the hot plate or hot water bath and heat gently to between 40–45 °C. While heating, stir for 5 minutes. Note: Do not allow temperature to exceed 45 °C. Remove from hot plate if necessary and continue stirring.
7. After 5 minutes, carefully remove the beaker and place into a larger, 250-mL beaker filled with cold tap water.
8. Continue stirring until the soap mixture gets thick—the product is ready to pour when the soap that drips back into the beaker from the stirring rod will trace a path (mark a trail) on the surface.
9. Label two small weighing dishes. Carefully pour the thickened soap solution from the beaker into the weighing dishes. Gently tap the dishes on the table to evenly distribute the soap in the dish.
10. Save the remaining soap in the small beaker for Part B.
11. Allow the soap in the weighing dishes to dry (cure) for several days. Describe the color, texture and appearance of the soap.

Part B. Properties of Soap

1. Add about 30 mL of distilled water to the leftover soap on the sides and bottom of the beaker. Carefully scrape the soap into the water as needed.
2. Heat the beaker on a hot plate at a medium setting and stir gently until the leftover soap dissolves. Cool to room temperature. In the data table, describe the appearance of the soap solution.
3. Measure the pH of the soap solution using either a pH meter or pH paper. Record the pH of the solution.
4. Place two test tubes in a test tube rack. Add 3 mL of distilled water to the first test tube and 3 mL of the soap solution to the second test tube.
5. Add one drop of olive oil to each test tube. Swirl or shake each test tube and then let sit for 5 minutes.
6. Describe the observation of this emulsification test in the data table.
7. When the soap bars in the weighing dishes are dry, measure and record the pH of the soap by placing a drop of distilled water on pH paper and pressing the paper onto the top of the soap bar. If desired, wait another week and repeat the pH test.

Guided-Inquiry Design and Procedure

Form a working group with other students and discuss the following questions.

1. Research frequently used fats and oils in soap recipes. How does the nature of various fats and oils affect the characteristics and quality of soap?
2. Explain why substituting the same quantity of one oil for another in a soap recipe may not have the desired results.
3. Most soap recipes call for superfatting—either adding more fats or oils than needed to react completely with the sodium hydroxide (usually 5% excess fat) or using less sodium hydroxide than required for the amount of oil in the recipe (sometimes called the lye discount). Explain the benefits of superfatting.
4. Write a step-by-step procedure for making soap using a different recipe than the one in the Introductory Activity. Include all necessary materials, glassware and equipment, safety precautions that must be followed, the concentration of sodium hydroxide solution, the order of mixing, timing, accuracy, how the properties of the soap will be tested, etc.
5. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12616_Student1.pdf