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Author: Janice VanCleave

Saturated and Unsaturated

Fats and oils are naturally occurring organic compounds called triglycerides. While made of basically the same components, fats and oils can easily be distinguished by the fact that, at room temperatures, oils are liquid while fats are solid. Each molecule of fat or oil is composed of two different types of building block compounds: glycerol and fatty acids. Both types of compounds are made of atoms of carbon, hydrogen, and oxygen.

In this project, you will study the difference between the molecular structure of saturated and unsaturated triglycerides. You will find out how to determine the iodine number of triglycerides and compare the degree of unsaturation of different oils with different iodine numbers. You will also study the effect of double bonds and length of fatty-acid side chains on the melting point of triglycerides.

Getting Started

Purpose: To use iodine to determine whether a triglyceride is saturated or unsaturated.


  • tap water
  • 2-quart (2-liter) cooking pot
  • heavy-duty aluminum foil
  • pencil
  • black permanent marker
  • 5 test tubes
  • 1-tablespoon (15-ml) measuring spoon
  • safflower oil
  • eyedropper
  • tincture of iodine
  • craft stick or stirring rod
  • heat mitten
  • test tube clamp


CAUTION: Keep the iodine out of reach of young children. It is poisonous and is for external use only. Tincture of iodine contains alcohol, which needs to be kept away from an open flame. Iodine will stain skin, clothes, and other materials.

  1. Fill the pot with 2 inches (10 cm) of water.
  2. Cover the pot with a piece of aluminum foil that has been folded in half.
  3. Use the pencil to make holes in the aluminum foil cover just large enough to stand the test tubes in.
  4. Use the marker to label the test tubes "C," "1," "2," "3," "4."
  5. Fill each test tube half full with oil. Make sure the height of the oil is the same in each tube.
  6. Use the eyedropper to add two drops of iodine to test tubes 1 through 4. Use the craft stick to thoroughly stir the contents of each test tube. Tube C will be the control.
  7. Record the initial color of the liquid in each test tube in the row for "0 minutes" in an Oil Data table like Table 29.1
  8. Stand the test tubes in the pot by inserting one tube in each hole in the aluminum foil.
  9. Turn the stove on to medium heat.
  10. Saturated and Unsaturated

  11. After 5 minutes, wearing the heat mittens, use the test tube holder to pick up each tube one at a time. Observe the color of the liquid in each tube and record it in the Oil Data table. Return the tubes to the pot of water.
  12. Repeat step 10 twice or until no further color change occurs in tubes 1 through 4.
  13. Compare the colors of the tubes recorded in the Oil Data table.


The addition of iodine to the pale yellow oil in tubes 1 through 4 turns the oil a pale reddish brown. At first small drops of iodine can be seen suspended throughout the liquid in these tubes. After heating, the reddish color of the liquid in the tubes slowly fades and the liquid returns to its original pale yellow color. The liquid in control test tube C does not change from its original color.

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