Biology. Fats: Chemistry and Identification

Introduction: Fats are present in living organisms. These chemical make up certain parts of your body. Fats are often stored when present in excess and also serve as an energy source. Fats are an important part of our diet.

Objective: In this lab, you will

  1. learn that all fat molecules are made up of two kinds of smaller molecules, glycerol and fatty acids.

  2. use structural formulas and models of glycerol and fatty acids to determine how these molecules join to form fat molecules.

  3. learn how to use the solubility test to tell if a substance is a fat.

  4. learn how to use the brown paper test to tell if a substance is a fat.

Materials: scissors - paper models - dropper - labels - test tubes - test tube rack - olive, corn or peanut oil - water - brown paper - unknown substance X, unknown substance Y and unknown substance Z - lighter fluid - test tube stoppers

Part A. Pre-Lab Questions

To better understand the chemistry of fats, it is helpful to study first the small molecules which join to make up fat. Fat molecules are made up of two small "building blocks", or chemical molecules. These molecules are called glycerol and fatty acids.

Glycerol.  Figure 1 shows the structural formula of glycerol.

1. What elements are present in glycerol?

2. Are there any elements in glycerol that are not in carbohydrates?

3. What is the molecular formula for glycerol?

4. Are there two times as many hydrogen atoms as oxygen atoms in glycerol?

Fatty Acids. The second kind of molecule which is part of a fat is a fatty acid. Many different fatty acids exit, but all contain

hydrocarbon chains with a carboxyl functional group. Some fatty acids are considered "saturated" where all the carbons are singly bonded and saturated with hydrogen atoms. Other are "unsaturated" where the carbons contain one, two or more double bonds. Hydrogen atoms are removed to form these double bonds. As a result, these unsaturated fatty acids contain kinks. These kinks prevent the fatty acids from packing together and therefore they are typically less dense. Stearic, oleic and linoleic acids examples of fatty acids. Figure 2 shows that structural formulas for these three fatty acids.

5. What elements are present in all fatty acids?

6. a. What is the molecular formula for stearic acid?

    b. What is the molecular formula for oleic acid?

    c. What is the molecular formula for linoleic acid?

7. How many oxygen atoms are present in each fatty acid?

8. What functional group is present in all fatty acids?

Combining Glycerol and Fatty acids to form fats

A fat molecule consists of one glycerol molecule and three fatty acid molecules joined.

·         Cut out the glycerol and fatty acid paper model molecules and attempt tot construct a fat molecule.

9. Will the fat molecule fit together as pieces in a puzzle?

·          Remove three -OH ends from the glycerol and three -H ends from the fatty acids. Now join the molecules to form a fat.

10. a. How many glycerol molecules are needed to form a fat molecule?

      b. How many fatty acid molecules are needed to form a fat molecule?

·          Join the left over -H and -OH ends from your model

11. What chemical substances is formed when the -H and -OH ends are joined?

Production of a fat molecule is a chemical reaction. A chemical shorthand way of expressing the formation of a fat is as follows:

    Glycerol + Fatty acid molecules --> Fat  +  Water molecules

12. How many water molecules are formed when one fat molecule is produced?

13. A change in the type of fatty acid molecules results in a different type of fat molecule. What molecule remains unchanged in all fats?

Part B. Identification of Fats

Two different tests can be used to determine the presence of a fat, the solubility test and the brown paper test.

Solubility Test on Known Fats

·         Label two test tubes #1 and #2

·         Add 10 drops of lighter fluid and 5 drops of oil to tube #1

·         Add 10 drops of cold water and 5 drops of oil to tube #2

·         Mix the contents of each tube by gently swirling the tube with contents.

·         Wait one minute and then examine the contents of the two tubes. Write your observations in the lab report.

A positive solubility test for fats is that the fat dissolves in lighter fluid and not in water.  A negative test is that is doesn't dissolve in lighter fluid and may or may not dissolve in water.

Brown Paper Test for Fats

·         On separate pieces of brown paper, rub one drop of oil and one drop of water. Oil is a fat and water is not.

·         Allow the paper to dry for a few minutes.

·         Examine the pieces of paper to check for a translucent spot. Record your results in the lab report

A positive brown paper test for fats is the formation of a translucent spot. A negative test is no translucent spot exists.

Testing Unknown Substances for Fats

  Perform the lighter fluid solubility and brown paper test on each of the following substances

        (a) substance X

        (b) substance Y

        (c) substance Z

   Record your observations and indicate whether the unknown materials contain fats by identifying positive or negative tests for fats.

Biology. Fats: Chemistry and Identification                                                        Names:

Part A. Pre-Lab Questions

1. What elements are present in glycerol? ______________________________________________________________

2. Are there any elements in glycerol that are not in carbohydrates? _________________________________________

3. What is the molecular formula for glycerol? __________________________________________________________

4. Are there two times as many hydrogen atoms as oxygen atoms in glycerol? _________________________________

5. What elements are present in all fatty acids? __________________________________________________________

6. a. What is the molecular formula for stearic acid?______________________________________________________

    b. What is the molecular formula for oleic acid?_______________________________________________________

    c. What is the molecular formula for linoleic acid?_____________________________________________________

7. How many oxygen atoms are present in each fatty acid?_________________________________________________

8. What functional group is present in all fatty acids? _____________________________________________________

9. Will the fat molecule fit together as pieces in a puzzle?__________________________________________________

10. a. How many glycerol molecules are needed to form a fat molecule? ______________________________________

      b. How many fatty acid molecules are needed to form a fat molecule?_____________________________________

11. What chemical substances is formed when the -H and -OH ends are joined? ________________________________

12. How many water molecules are formed when one fat molecule is produced? ________________________________

13. A change in the type of fatty acid molecules results in a different type of fat molecule.

      What molecule remains unchanged in all fats?________________________________________________________

Part B. Identification of Fats

Analysis

1. Name the types of molecules and number of each type needed to form a fat molecule. ______________________________

_____________________________________________________________________________________________________

2. List two ways that a fatty acid molecule differs from glycerol. _________________________________________________

_____________________________________________________________________________________________________

3. Explain why grease on clothing will not come out with cold water. _____________________________________________

____________________________