Experiment: Solubility and the Percent of Oil in Peanuts

OBJECTIVE: In this experiment, you will determine the solubility of a variety of solutes in several solvents. The observed solubility will be related to the structure and intermolecular forces of both the solute and solvent. On the basis of these tests, you will then devise and carry out an experimental method for the determination of the percent oil in peanuts.

BACKGROUND: The extent to which one substance will dissolve in another depends on the structural properties of both substances. One such property is the degree of polarity in a molecule. For example, water is a highly polar molecule which exhibits strong intermolecular forces called hydrogen bonds. A rule of thumb is that "like tends to dissolve like." Thus water will tend to dissolve other polar substances, including salts. This has great biological significance since most biochemical reactions occur in an aqueous medium. However, some molecules have both polar and nonpolar ends, and it is sometimes difficult to predict the solubility of these substances in various solvents. Some of these substances have great practical use. For example, detergents are molecules which can dissolve in water, but they also have a nonpolar portion which dissolves in oil. Water solutions of detergents can therefore form emulsions with oils and fats.

In this experiment, you will test the solubility of sodium chloride, glycerine, citric acid, naphthalene, sugar (sucrose), monosodium glutamate (MSG), gelatin, and vegetable oil in the following solvents: hexane, water, ethanol, and acetone. The last four solutes represent, respectively, a disaccharide, an amino acid (in salt form), a protein, and a lipid, all of which have biological significance.

Many chemical analyses, especially those involving the separation of substances, depend on the appropriate choice of solvent. Foods such as peanuts are mixtures of chemicals which have properties similar to those of the pure substances above. Peanuts contain, among other things, proteins and oil. One method for separating these components is based on solubility differences.

PRELABORATORY EXERCISE

1. Draw Lewis structures for hexane (C6H14), water (H2O), acetone (C3H6O) and ethanol (C2H6O). Consult your text or a handbook if necessary.
   
2. Arrange the liquids in question 1 in order of increasing polarity. Which ones can form hydrogen bonds?
   
3. Draw one isomer each for hexane, acetone and ethanol. Is each isomer as polar as the original structure? Explain.
   
4. Obtain from a text or handbook the formulas and structures for the following solutes which you will test in this experiment: salt, sugar (sucrose), glycerine, citric acid, naphthalene, monosodium glutamate, and oleic acid (a typical unsaturated vegetable oil). Record a few physical properties of each solute.
   

PART I: SOLUBILITY TESTS

MATERIALS Solvents: hexane, water, ethanol, and acetone
Solutes: sodium chloride, glycerine, citric acid, napthalene (moth flakes), sugar, monosodium glutamate, gelatin, and vegetable oil
Test tube rack, 13 x 100mm test tubes, rubber stoppers (#00), spatulas

HAZARDS: Hexane, acetone and ethanol should be used only in a well-ventilated room. These solvents may be poured in the fume hood. Avoid contact of these solvents with the skin.

PROCEDURE

1. Label four test tubes as follows: H (for hexane), W (for water), E (for ethanol) and A (for acetone).
   
2. Half-fill each test tube with the appropriate solvent. Stopper the test tubes immediately because some of the solvents are volatile and have strong odors. AVOID POURING THESE SOLVENTS ON YOUR HANDS.
   
3. Using a spatula, add a pinch (or 3 drops) of any one of the solutes to each of the four test tubes. Ask your teacher to demonstrate a pinch of solute. Be sure to add similar amounts of solutes to the different solvents.
   
4. Stopper each test tube and shake vigorously for 30 seconds, making sure that you hold the stopper in tightly. Record the solubility of each solute as SOLUBLE, SLIGHTLY SOLUBLE, or INSOLUBLE. If two liquids are immiscible (one insoluble in the other), two distinct layers will form. However, when only a few drops of liquid solute are used, small droplets of the solute may adhere to the inside walls of the test tube after shaking, and the two layers may not be obvious.
   
5. Repeat the above procedure with each of the other solutes. Wash the test tubes when necessary and be sure to clean your spatula before you sample a new solute.
   
6. Predict what would happen if you add water to an acetone solution of moth flakes. Try it and record your observations?
   
7. Predict what would happen if you add hexane to a solution of water and sugar. Try it and record you observations.

DISPOSAL: All test tubes containing hexane or naphthalene should be emptied into an organic waste container. The other solutions may be poured down the drain.

QUESTIONS

1. Group the eight solutes in categories according to which ones have similar solubility properties. Are there any substances which are difficult to classify?
   
2. Compare these groups you have chosen in terms of the physical properties you listed in the prelaboratory exercise. Are there many or a few physical properties in common?
   
3. Label the groups of solutes you have formed either as NONPOLAR, SLIGHTLY POLAR, POLAR, or IONIC. Justify your decisions.
   
4. Give explanations for the observations you made in steps 6 & 7.

PART II: THE PERCENT OIL IN PEANUTS

In this part of the experiment, you will design and carry out a procedure for determining the percent oil in peanuts. Keep in mind that peanuts contain oil (similar to the vegetable oil in Part I) and insoluble matter like protein (a polymer synthesized from amino acids).

MATERIALS
12 shelled, dry-roasted peanuts; Mortar and pestle; Balance
Filter paper and funnel (buchner funnel and aspirator may also be available)
Choice of solvents

PRELABORATORY EXERCISE: Design a detailed procedure for determining the percent oil in peanuts. Discuss the choice of solvent, procedure and calculations.

PROCEDURE AND REPORT: After your teacher has approved your procedure, carry out the analysis. Record all observations and any difficulties or changes in procedure (prior approval is needed). Show all calculations in your report. Discuss any limitations in your experimental method as well as any sources of error.

DISCUSSION: The number of solvents and solutes may be increased or decreased in this experiment, depending on the materials and time available. However, a sufficient number of each should be used so that generalizations can be made about solubility. Solvents can conveniently be dispensed from dropping bottles or plastic wash bottles. Liquid solutes can be dispensed from small plastic dropping bottles (squeeze type). Solid solutes can be dispensed from vials.

Sample Data: Part I

A solute is considered soluble if it completely dissolves in the solvent. When water is added to an acetone solution of naphthalene, one liquid phase is formed and naphthalene precipitates from solution. When hexane is added to a solution of sugar in water, two liquid layers form and there is no precipitate. The structures of the solutes are shown below:

Part II: In one experiment, several massed peanuts were mashed in a mortar with a pestle. Hexane was added to the mortar and thoroughly mixed with the mash. The mixture was transferred to a gravity filter, and the filtrate was collected in a pre-weighed beaker. The residue in the filter was washed with a few small portions of solvent. The solvent was allowed to evaporate from the beaker and a viscous oil remained. The beaker and oil were massed. The protein/carbohydrate residue was dried and massed.

The following data were collected:
Mass of peanuts = 7.23 g
Mass of empty beaker = 68.11 g
Mass of beaker + oil = 70.80 g
Mass of protein/carbohydrate residue = 4.16 g

Calculated Values:
Mass of oil = 2.69 g
Mass lost = 0.38 g
% oil = 2.69/7.23 x 100% = 37.2% (based on oil recovered)
% oil = 3.07/7.23 x 100% = 42.5% (based on residue)

Reference: This experiment is a modification of an experiment written by Lois Williams, Horace Mann School, Bronx, NY

Submitted by Bob Cairo, Princeton Chemistry Institute, 1988. Modifications by Mark Case, CHEM 6 Team TORCH Binder, 1995.