Materials Included In Kit

Additional Materials Required

Prelab Preparation

• Iron(III) chloride solution, 0.5 M: Add 13.5 g of iron(III) chloride hexahydrate (FeCl₃•6H₂O) to about 50 mL of distilled or deionized water and stir to dissolve. Dilute to 100 mL with water.
• Magnesium chloride solution, 0.5 M: Add 10.1 g of magnesium chloride hexahydrate (MgCl₂•6H₂O) to about 50 mL of distilled or deionized water and stir to dissolve. Dilute to 100 mL with water.

Safety Precautions

Sodium hydroxide solution causes severe skin burns and eye damage. Wear protective gloves and clothing and wash thoroughly after handling. Keep base neutralizer on hand to clean up any spills. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. 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

• This experiment may be completed within a typical 2-hour lab period.
• Remind students to be very careful when adding the solid fat into the beaker. If fat is left on the outside of the beaker, it will become very slippery when heated. Fat 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.
• Homemade soaps may be tinted and scented by adding a few drops of food dye and perfume or cologne while stirring the soap solution and before pouring it. Many students enjoy bringing in a favorite scent to create a personalized soap. If students have not followed directions carefully, the soap may contain excess sodium hydroxide solution and thus be irritating to the skin. Use your judgment in deciding whether students should be permitted to take their soaps home.
• The ability of surfactants to form micelles in water can be tested by measuring their effect on the surface tension of water. The critical concentration of soap that reduces the surface tension of water is generally very low.
• This experiment offers many opportunities for a cooperative class project to investigate how the nature of fats and oils will affect the characteristics and quality of soap. Tallow is known to give a hard soap that cleans well but does not give a good lather. Oil-based soaps give good, sudsy lathers and also act as emollients (moisturizers). Frequently used oils and their properties include castor oil, which gives a rich and soft conditioning soap; coconut oil, which gives a creamy, fluffy lather, even in cold water; olive oil, which gives a gentle soap that is good for sensitive skin; and palm kernel oil, which gives a smooth-textured soap. Some oils, such as avocado oil, contain proteins and vitamins that act as natural preservatives or antioxidants in soap.
• See the Supplementary Information in the Further Extensions section for information about the composition of naturally occurring fats and oils and also the structures of natural fatty acids. Compare the composition of fats and oils with what is known about their health effects.

Further Extensions

Supplementary Information

Table 1. Natural Fatty Acids

Table 2. Composition of Triglycerides—Naturally Occurring Fats and Oils

*Other fatty acids that may be present in amounts less than 1% include C₆, C₈, C₁₀ and C₂₀ saturated fatty acids; C₁₂, C₁₄ and C₁₆ monounsaturated acids; and C₂₀ polyunsaturated acids.

Answers to Prelab Questions

1. Define the following terms: triglyceride, saponification, surfactant, micelle.

   Triglyceride: A compound formed from glycerol and three fatty acids. It is a triester—the fatty acids are attached via ester linkages to the glycerol backbone. Fats and oils are natural triglycerides.
   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.
   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.
   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.

2. What is the principal safety hazard in this experiment? Explain why soap made by primitive methods was likely to be very harsh.

   The manufacture of soap requires a strong base, sodium hydroxide. Sodium hydroxide solution is caustic and corrosive and can cause severe skin burns. 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.

3. Olive oil has a saponification value of 190 mg KOH per gram. This means that it takes 190 mg of potassium hydroxide to react completely with 1 g of olive oil.
   1. What is the mole ratio for the reaction of a triglyceride with KOH? (See Equation 1.)

      One mole of triglyceride reacts with three moles of potassium hydroxide.

   2. Divide 0.190 g of KOH by its molar mass to calculate the number of moles of KOH that will react with 1 g of olive oil.
      {14051_PreLabAnswers_Equation_2}
   3. Use the answers to (a) and (b) to determine the number of moles corresponding to 1 g of olive oil. Divide 1 g by this number of moles to calculate the average molar mass (g/mole) of olive oil.
      {14051_PreLabAnswers_Equation_3}

Sample Data

Answers to Questions

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

   The homemade soap is similar in appearance to commercial soap. However, the surface of the homemade soap is slightly dry. Commercial soaps come in all colors.

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

   The soap solution is basic—pH 10.5. A commercial soap solution has a pH of 9.5.

3. How do the reactions of the soap solution with calcium, iron and magnesium ions mimic what happens when soap is used in hard water? Write the equation, including the product formula, for the reaction of soap with calcium ions. Describe common observations around the home due to the reaction of soap with hard water.

   Hard water contains high concentrations of calcium and magnesium ions. One of the problems with using soap in hard water is that the soap will not lather well and may form solid precipitates with calcium and magnesium ions. A common observation of using soap with hard water is the “soap scum” left in sinks and tubs.

   {14051_Answers_Equation_4}
4. Compare the results of the emulsification test of olive oil with water and with the soap solution. Explain in terms of the ability of soap to form micelles.

   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. The emulsion did not separate into two layers. A mixture of water and oil separated into two layers immediately after being shaken. Soap molecules form micelles, which are able to dissolve or entrap oil molecules within their hydrophobic core.

5. Explain why most soaps contain glycerol and describe the properties that glycerol adds to soap.

   Glycerol is a byproduct of the soap-making process. Unless it is specifically removed from the mixture by adding a solvent, the glycerol remains in the soap. Glycerol is a viscous liquid and is an excellent moisturizer, forming a layer that keeps moisture in.

6. Draw the general structure of a triglyceride containing both saturated and unsaturated fatty acids. Circle and label the glycerol backbone, an ester functional group and an unsaturated fatty acid residue.
   {14051_Answers_Figure_3}
7. Most homemade recipes for making soap use about 5% excess fat. Explain the benefits of using excess fat to make soap.

   Using excess fat has two main benefits in soap-making. Excess fat helps control the pH of soap. If there is excess fat, then all the sodium hydroxide that is added will be used up in the reaction. Without excess base, the soap will be less harsh or irritating to the skin. Excess fat also gives soap a smooth feel. Too much fat, however, will leave the soap greasy.

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
• Triglycerides
• Saponification
• Surfactants

Background

Legend has it that the word soap comes from Mount Sapo, a hill in Rome that was the site of animal sacrifice. According to the legend, animal fat and ashes washed down the mountain with the rain, producing sudsy river water that was used to wash clothes. It’s an interesting story, but there is no factual evidence for this legend. The earliest written reference comes from the Roman historian Pliny the Elder in the first century CE. Pliny described the preparation of sapo from goat fat and wood ashes and attributed the invention to the Gauls, who used it for hair treatment 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 BCE.

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 soapmaking 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 manmade product. Saponification involves the reaction of triglycerides—natural fats and oils—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). 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 solvated or 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, and the dirt-containing micelles 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.

Experiment Overview

The purpose of this experiment is to make soap and study its properties. The soap will be prepared 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, emulsifying action and solubility in hard water.

Materials

Prelab Questions

1. Define the following terms: triglyceride, saponification, surfactant, micelle.
2. What is the principal safety hazard in this experiment? Explain why soap made by primitive methods was likely to be very harsh.
3. Olive oil has a saponification value of 190 mg KOH per gram. This means that it takes 190 mg of potassium hydroxide to react completely with 1 g of olive oil.
   1. What is the mole ratio for the reaction of a triglyceride with KOH? (See Equation 1.)
   2. Divide 0.190 g of KOH by its molar mass to calculate the number of moles of KOH that will react with 1 g of olive oil.
   3. Use the answers to (a) and (b) to determine the number of moles corresponding to 1 g of olive oil. Divide 1 g by this number of moles to calculate the average molar mass (g/mole) of olive oil.

Safety Precautions

Sodium hydroxide solution causes severe skin burns and eye damage. Wear protective gloves and clothing and wash thoroughly after handling. Notify the instructor and clean up all spills immediately. Avoid contact of all chemicals with eyes and skin. Exercise care if you remove the prepared soap from the lab—it may be irritating to skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the lab.

Procedure

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. The fat is messy to work with—try not to get any 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. Carefuly remove the reaction 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. Heat gently on a hot plate or in a hot water bath to about 40 °C. While heating, stir for 5 minutes.
6. After 5 minutes, carefully remove the reaction beaker and place into a larger, 250-mL beaker filled with cold water.
7. 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.
8. Label two small weighing dishes. Carefully pour the thickened soap solution from the reaction beaker into the weighing dishes.
9. Allow the soap to dry (cure) for several days. Describe the color, texture and appearance of the soap.

Properties of Soap

1. Add about 40 mL of distilled water to the leftover soap on the sides and bottom of the beaker. Gently 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 three test tubes in a test tube rack and label them A–C. Pour about 3 mL of the soap solution into each test tube.
5. Add 5 drops of calcium chloride solution to test tube A, 5 drops of iron(III) chloride solution to test tube B and 5 drops of magnesium chloride solution to test tube C. Swirl each test tube to mix the contents. Describe the color and appearance of the mixture in each test tube.
6. Place two clean test tubes in the 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.
7. Add one drop of olive oil to each test tube. Swirl or shake each test tube and then let sit for 5 minutes.
8. Describe the observations of this emulsification test in the data table.
9. When the soap bars are dry, measure and record the pH of the soap by placing a drop of water on pH paper and pressing the paper onto the top of the soap bar.

Student Worksheet PDF

14051_Student1.pdf