Name: __________________________________________________ Period:__________

Tests for Organic Compounds

 

Organic chemistry is the study of carbon containing compounds. There are four main classes of organic compounds: carbohydrates, lipids, proteins and nucleic acids.

 

Purpose: To test food for the presence of glucose, starch, lipids, and proteins.

 

Background: Scientists test for the presence of organic compounds using substances called indicators. Indicators do just what their name says; they indicate whether or not an organic compound is present. A color change of an indicator is usually a positive test for the presence of an organic compound.

Obtain ALL substances to be tested as directed by your teacher BEFORE beginning the testing section of this lab!!

 

Part A- Testing for Carbohydrates:

Carbohydrates are the major energy source for living organisms. In this lab, we will be using indicators to test for the presence of monosaccharides (simple sugars), and polysaccharides (complex carbohydrates).

 

Test for Simple Sugars or Monosaccharides (mono = one sugar, di = two sugars)

§  Indicator: Benedict’s Solution-light blue/green in color to begin with. Turns orange/red in the presence of simple sugars in a hot water bath.

 

1.      Put on your goggles.

2.      Label nine test tubes 1-9 and add 5 drops of the following solutions to each:

Tube 1: apple juice                  Tube 4: corn oil           Tube 7: unknown A

Tube 2: gelatin                                    Tube 5: cracker                       Tube 8: unknown B

Tube 3: milk                            Tube 6: water              Tube 9: unknown C

3.      Add 5 drops of Benedict’s solution to each tube.

4.      Carefully place all nine test tubes into the hot water baths for 2-3 minutes.

5.      Remove each test tube from the water bath using the test tube clamps.

6.      Allow tubes to cool in the test tube rack and record the color changes in the data table. Use the example below to indicate the presence or absence of a simple sugar. Record the symbols and the results in your data table.

Color Change

Symbol

Result

Blue/green

-

No simple sugars present

Red/orange

+

Large amount of simple sugar present

 

7.      Allow test tubes and solutions to cool. Wash test tubes out with soap and water. Turn upside down in the test tube rack to dry.

 

 

 

 

Test for Complex Sugars or Polysaccharides (three or more sugars)

§  Indicator: Iodine-normally a rusty orange in color. In the presence of starch (a complex carbohydrate) it turns black (positive response).

 

1.      Place a small sample (5 drops) of each of the following substance into each well of a micro-well plate.

Well 1: apple juice                                    Well 4: corn oil                       Well 7: unknown A

Well 2: gelatin                               Well 5: cracker                        Well 8: unknown B

Well 3: milk                                  Well 6: water               Well 9: unknown C

2.      Place 2-3 drops of the iodine into each well.

3.      Record the color changes in the data table and indicate which substances contain starch in the symbol and results columns of the data table.

4.      Wash ALL the wells with soap and water. Dry thoroughly!

Part B- Testing for Protein

§  Indicator: Biuret Reagent-normally a light blue in color. In the presence of protein it turns purple/violet (positive response). The extent of the color change depends on the length of the protein chain.

 

1.      Place a small sample (5 drops) of each of the following substance into each well of a micro-well plate.

Well 1: apple juice                                    Well 4: corn oil                       Well 7: unknown A

Well 2: gelatin                               Well 5: cracker                        Well 8: unknown B

Well 3: milk                                  Well 6: water               Well 9: unknown C

2.      Place 2-3 drops of the Biuret reagent into each well.

3.      Place the micro-well plate over a piece of white paper to help see the color changes because they are very slight.

4.      Record the color changes in the data table and indicate which substances contain protein in the symbol and results columns of the data table.

5.      Wash ALL the wells with soap and water. Dry thoroughly!

 

Part C- Testing for Lipids (fats, oils or waxes)

§  Brown Paper Bag-turns semi-transparent upon application of a lipid after the substance has dried.

 

1.      Obtain a brown paper bag strip and divide it into nine square numbered sections.

2.      Place one drop of each of the following substances on the appropriate square:

Square 1: apple juice              Square 4: corn oil       Square 7: unknown A

Square 2: gelatin                     Square 5: cracker        Square 8: unknown B

Square 3: milk                                    Square 6: water           Square 9: unknown C

3.      Allow each substance to dry for a 1-2 minutes.

4.      Hold the brown paper strip up to the light. If the spot is transparent (positive response), the substance contains lipids. Record if any substances contain lipids in the symbol and results columns of the data table.

5.      Throw the brown paper strip into the garbage.

 

Name: _______________________________________________ Period: ________________

Data Collection and Analysis:

Simple Sugar Test: Benedict’s Solution

Testing Substance

Color Change

Symbol

Results

Apple Juice

 

 

 

Gelatin

 

 

 

Milk

 

 

 

Corn oil

 

 

 

Cracker

 

 

 

Water

 

 

 

Unknown A

 

 

 

Unknown B

 

 

 

Unknown C

 

 

 

 

Complex Carbohydrate Test (polysaccharides): Iodine

Testing Substance

Color Change

Symbol

Results

Apple Juice

 

 

 

Gelatin

 

 

 

Milk

 

 

 

Corn oil

 

 

 

Cracker

 

 

 

Water

 

 

 

Unknown A

 

 

 

Unknown B

 

 

 

Unknown C

 

 

 

 

Protein Test: Biuret reagent

Testing Substance

Color Change

Symbol

Results

Apple Juice

 

 

 

Gelatin

 

 

 

Milk

 

 

 

Corn oil

 

 

 

Cracker

 

 

 

Water

 

 

 

Unknown A

 

 

 

Unknown B

 

 

 

Unknown C

 

 

 

 

Lipid Test: Brown Paper Bag

Testing Substance

Translucent Spot?

Symbol

Results

Apple Juice

 

 

 

Gelatin

 

 

 

Milk

 

 

 

Corn oil

 

 

 

Cracker

 

 

 

Water

 

 

 

Unknown A

 

 

 

Unknown B

 

 

 

Unknown C

 

 

 

 

Analysis:

1.      Which substances contained simple carbohydrates?

 

2.      Which substances contained complex carbohydrates?

 

3.      Which substances contained proteins?

 

4.      Which substances contained lipids?

 

5.      What type of organic compound was in Unknown B?

 

6.      What kind of food could unknown B possibly be?

 

 

7.      Did any of the substances contain more than one organic compound? If so which one?

 

8.      In order for these substances to be organic compounds, what element must they contain?

 

9.      What was the purpose of testing water in each of these tests?

 

 

10.  A student is running a test for simple sugars. She places 5 mL of each substance into different test tubes and adds five drops of Benedict’s solution to each tube. None of the tubes change color. Can she safely say there are no simple sugars in any of the substances? Why or Why not?

 

 

11.  Biuret reagent will turn your skin brownish/purple. Explain why this happens.

 

 

12.  Your brown paper lunch bag has a large transparent spot on the bottom. Suggest an explanation for this.

 

 

13.  What could you conclude if a positive test for any of the organic compounds occurred in the well/tube containing only the water?