CHEMISTRY NOW: Chlorophyll Chromatography (Grades 5-8) Print

Objective:

Students will be able to design and carry out an investigation to separate the pigments from a leaf by paper chromatography. Students will identify a mixture by separating it into the different compounds.


Introduction Notes:

 

CHEMISTRY NOW: Chlorophyll Chromatography

 

Subject Area: Chemistry                                                        

Grade Level:  Middle School Chemistry                               

Lesson Title: Chlorophyll Chromatography

National Science Standards:                                                 

Science as Inquiry: 5-8                                                           

Science as Inquiry: 9-12                 

                            

Physical Science Standards:

Properties and Changes of Properties: 5-8                   

Structure of Atoms: 9-12

Structure and Properties of Matter: 9-12

Chemical Reactions: 9-12                                            

 

Suggested Prior Knowledge:  concepts of solutions, mixtures, separation of mixtures, solubility

Purpose: To give students an understanding of paper chromatography, and allow students to separate a mixture of pigments extracted from leaves.

Key Vocabulary:  

chromatography– method used to separate a mixture of compounds based on differing solubilities of the compounds in the solvent being used

solution – homogeneous mixture of two or more substances

solutesubstance dissolved in a solution (the substance there is less of)

solvent substance dissolving the solute in a solution (the substance there is more of)

absorbent solid material used in chromatography that will attract and absorb the compounds being separated

eluent (solvent) – material used in chromatography which carries the compounds to be separated through the absorbent

photosynthesisprocess by which plants convert energy from sunlight, water and carbon dioxide gas into sugar

chlorophyll one of many pigments used by plants to absorb energy from sunlight in the process of photosynthesis

Objectives:      

  1. Students will be able to design and carry out an investigation to separate the pigments from a leaf by paper chromatography.
  2. Students will identify a mixture by separating it into the different compounds.

Materials:  

-          safety goggles

-          chromatography paper or filter paper cut into strips

-          chromatography solvent (commercially available: 90% petroleum ether and 10% acetone)

-          pencil

-          ruler

-          wooden splint

-          250 ml Erlenmeyer flasks

-          50 ml graduated cylinder

-          parafilm or aluminum foil

-          stapler or tape

-          spinach leaf (fresh) or other leaves

-          penny (one-cent coin)

-          scissors

-          UV light source (optional)

Procedure: ­    

1. Discuss with students the process of photosynthesis, and the pigments that plants use to carry out this process. Plants use chlorophyll and other pigments to collect light energy. Review solutions and mixtures. Discuss with students how we can design an experiment that allows us to visually separate these pigments from a leaf. Begin with a leading question and follow-up:

 

    • - What makes leaves green or other colors? (pigments such as chlorophyll)
    • - What kind of experiment can we do to find out how many pigments are in a leaf?
    • - What kind of experiment could we do to compare leaves of different colors or from  different kinds of plants to see if they have different pigments?
    • - Do you think autumn leaves that are colored yellow or orange still contain green chlorophyll?
    • - What about multi-colored leaves? Do they contain the same pigments?

 

2. Lab safety protocols should be followed, and safety equipment used.

 

3. Work with students to develop a hypothesis or question to test and design an investigation to answer their question or test their hypothesis. Some possible topics include:

      1. a. Do all leaves have the same number and kinds of pigments?
      1. b. Do different-colored leaves contain different pigments?
      1. c. Do darker green leaves contain just green pigments?
      1. d. Do red or orange fall leaves contain green chlorophyll?
      1. e. How do the pigments compare in leaves from plants that prefer lots of sun to those that like the shade?

4. Guide students to develop an investigation that is valid and allows them to visually test their hypothesis or question. Be sure that proper safety procedures are followed and that they all wear goggles.

 

5. The following basic procedure can be used:

      1. a. Obtain the leaf or leaves to be tested.
      1. b. Obtain chromatography paper strips, pencil, ruler, penny, Erlenmeyer flask, wooden splint and scissors.
      1. c. Cut the chromatography paper into a long thin strip that is slightly narrower than the mouth of the flask and is long enough to extend from the bottom of the flask out the mouth. A rectangular strip about 15 cm long and 2 cm wide is generally good.
      1. d. Make a pencil line 1 cm in from one narrow end, as in the diagram below. This end will be the bottom of your strip.

 

 

 

 

 

 

 

      1. e. Place a piece of the spinach or other leaf over this line and use the edge of the penny to rub across the leaf along the pencil line drawn on the strip, so that the pencil line is now covered with pigments from the leaf. It is important that the chromatography strip contain a single, narrow, horizontal green line.
      1. f. Stand the strip of chromatography paper in the empty flask and staple it to the coffee stirrer so that it hangs freely from the stirrer into the flask but does not touch the bottom of the flask. It should hang very close (0.5 cm or so) to the bottom of the flask when the stirrer rests across the opening of the flask.
      1. g. Remove the chromatography strip from the flask and put enough solvent in the flask to completely cover the bottom of the flask to a depth of less than 1.0 cm. When you hang the strip back into the flask, it should touch the solvent but the solvent should be below the pencil line on the strip.
      1. h. Carefully place the chromatography strip in the flask so that the bottom of the strip is in the solvent yet the solvent level is below the pencil line.
      1. i. Allow the solvent to move up the chromatography strip. This is caused by capillary action. As the solvent is drawn up the strip, it will carry the pigments in the sample at different rates depending on the characteristics of the individual compounds. When the solvent level gets close to the top of the strip (not to the staple and stirrer yet), remove the strip from the solvent to stop the run and make a light pencil mark at the solvent top. It may also help to use a pencil to mark the separated bands on the strip in case the colors fade as the paper dries.
      1. j. Let the strip dry. You should be able to see the pigment spots for each pigment in the leaf separately.
      1. k. If you have access to an ultraviolet (UV or “black”) light, have students shine the light on their strips and record their observations. Some pigments will fluoresce under UV light.

 

6. Have the students record the data and observations from this experiment. They should record the color (be descriptive!) of each pigment and compare the distance each traveled. If they test different leaves, they can try to see if the leaves share some similar pigments and look for different pigments.

 

7. Have students draw a conclusion (or conclusions) about their results.

 

8. Students may present their results to the class in a slideshow or on a poster.

 

9. After students have drawn their conclusions, the teacher may want to lead a discussion about the similarities and differences found. Explain that different plants use different pigments depending on their needs (varying amounts of sunlight, leaf type etc).

Additional Resources:

         http://orgchem.colorado.edu/hndbksupport/chrom.html

         http://en.wikipedia.org/wiki/Photosynthetic_pigment

         http://biology.wsc.ma.edu/biology/courses/concepts/labs/pigments/

         http://en.wikipedia.org/wiki/Chlorophyll

         http://www.rpi.edu/dept/chem-eng/Biotech-Environ/CHROMO/chromintro.html

 

 

 

 

 

 

 

Student Worksheet for Chlorophyll Chromatography

 

 

Experiment Title: ___________________Date: __________Name: ______________

 

Student Hypothesis or Question:

 

____________________________________________________________________

 

Materials:

 

____________________________________________________________________

 

____________________________________________________________________

 

____________________________________________________________________


____________________________________________________________________

           

 

Procedure: (Include all safety procedures)

 

1. __________________________________________________________________

2. __________________________________________________________________

3. __________________________________________________________________

4. __________________________________________________________________

5. __________________________________________________________________

6. __________________________________________________________________

 

Data: Colors/Pigments Observed

 

Leaf 1

Leaf 2

Leaf 3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Diagram of strips:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Analysis of Data:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Conclusion:

 

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

 

 

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