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Saturated and Unsaturated Oils and Effect of Temperature on Oil's Viscosity

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Author: Janice VanCleave

Oils and fats are easily identified: Oils are liquid at room temperature; fats are solid. Oils may appear to be alike, but their chemical makeup can be very different.

In this project, you will experimentally differentiate between saturated and unsaturated oils. You will test the degree of unsaturation of different oils and determine the effect of temperature on the breaking of chemical bonds in unsaturated oils. You will also compare ability of saturated and unsaturated oils to flow, and determine the effect of temperature on the oils' viscosity.

Getting Started

Purpose: To test for the presence of an unsaturated oil.

Materials

  • 2-quart (2-liter) cooking pot
  • water
  • metric measuring cup
  • safflower oil
  • baby-food jar
  • eyedropper
  • tincture of iodine
  • spoon
  • stove
  • timer

Procedure

CAUTION: Keep the iodine out of reach of small children. It is poisonous and is for external use only.

  1. Use the measuring cup to pour 25 ml of safflower oil into the baby-food jar.
  2. Use the eyedropper to add five drops of iodine to the oil. Stir with the spoon.
  3. Record the time as zero and describe the contents of the jar in a data table such as the one shown on page 38.
  4. Fill the pot with 2 inches (5 cm) of water and place it on the stove.
  5. Stand the jar of oil in the pot.
  6. Heat at a medium temperature.
  7. Record an observation of the contents of the jar every two minutes for ten minutes.
  8. Turn off the heat. Leave the jar in the pot of water to cool before disposing of the oil.

Fatty Acids Saturated and Unsaturated

Results

The addition of iodine to the pale yellow oil turns it slightly reddish brown, and small drops of iodine can be seen suspended throughout the liquid. As the oil is heated, it returns to its original pale yellow color.

Why?

Oil molecules are polymers (large molecules containing small single molecules linked together). Fatty acids are the monomers (small molecules linked to form polymers) that bond with glycerol to make oil molecules. Fatty acids are made of carbon, hydrogen, and oxygen. The carbon atoms in the acids bond together. The carbons connect by one or two bonds. Molecules of oil with single bonds between the carbon atoms are said to be saturated. Molecules of oil with two or more double bonds are referred to as unsaturated. The more double bonds in a molecule, the more unsaturated it is.

In a saturated molecule, each carbon atom is connected to four atoms, as illustrated by the following ethane gas molecule:

In an unsaturated molecule, each carbon atom is connected to fewer than four other atoms. Ethene, for example, is an unsaturated molecule in which each carbon atom is connected to three atoms:

Note: The line between each symbol represents a single bond between the atoms. The double lines represent a double bond.

In the presence of other chemicals, such as iodine, one of the multiple bonds breaks and the iodine atoms attach to the carbon atoms until four atoms are bonded to each carbon atom. In this experiment, iodine is responsible for the amber color of the tincture of iodine solution. Heating the safflower oil aids in breaking the multiple bonds between the carbon atoms. Iodine loses its color as it connects to the oil molecules.

Oils combine with iodine in proportion to the number of double bonds the oils contain. The combination of iodine with ethene provides a simple illustration of how the iodine bonds with an unsaturated molecule:

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