CHEMISTRY NOW: What Color Is Your Flower? (Grades 5-8) Print

Objective:

Students will investigate the pigments in flower petals. Students will design and carry out an investigation to determine how many pigments there are in red flower petals. Students will analyze the pigments in red flower petals and determine if different red flowers use the same or different pigments.


Introduction Notes:

CHEMISTRY NOW:What Color Is Your Flower?

 

Subject Area: Chemistry                                                        

Grade Level:  Middle School Chemistry                                

Lesson Title:  What Color Is Your Flower?

National Science Education Standards:                                                                                         

Science as Inquiry: 5–8                                                          

Properties and Changes of Properties of Matter: 58                                                                          

 

Suggested Prior Knowledge: concepts of molecules, mixtures, solutions, and chromatography

 

Purpose: To separate the pigments in red flower petals and determine if all red flowers contain the same pigments

Key Vocabulary:  

absorbent—material that will attract and absorb another substance; used in chromatography to absorb the compounds being separated

capillary actionthe ability of liquid to flow against gravity where liquid spontaneously rises in a narrow space such as a thin tube, or in porous materials such as paper or in some non-porous materials such as liquefied carbon fiber

chromatography—method used to separate substances in a mixture, based on the solubility and polarity of the substances

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

pigment—a coloring matter in animals and plants

solute—substance dissolved in a solution

solution—homogeneous mixture in which a solute is dissolved in a solvent: the solute cannot be filtered out and does not settle out; these do not exhibit the Tyndall effect

solvent—substance dissolving the solute in a solution

Tyndall effectscattering of light by minute particles in a mixture that results in a visible light beam showing in the mixture

Objectives:      

  1. Students will investigate the pigments in flower petals.
  2. Students will design and carry out an investigation to determine how many pigments there are in red flower petals.
  3. Students will analyze the pigments in red flower petals and determine if different red flowers use the same or different pigments.

 

Materials:  

- Safety goggles

- Red flower petals from several different freshly cut red flowers (from home gardens or a florist)

- Chromatography paper

- Chromatography solvent (isopropyl alcohol)

- Wooden coffee stirrers

- Stapler

- Scissors

- Pencil

- Penny coin

- Large test tube

- Colored pencils

Procedure: ­    

1. If not previously taught, begin this lesson with a discussion on why something appears to be red (or green or blue or…). A red flower appears red because the red pigments absorb all of the light we can see except for the red light. Petals thus reflect red light to our eyes. The light receptors (called cones) in out eyes receive the red light and our brains interpret the color as red.

 [Please note that special care and consideration should be taken with students who are color deficient (color blind). See notes in the procedure.]

 

2. Follow this up with a discussion of the colors of flowers. In this investigation, students will investigate the pigments in red flower petals.

 

  • - What is in flowers that makes their petals appear to have color?
  • - Do all red flower petals contain the same pigments?
  • - Do red flower petals only contain red pigments?

 

3. Discuss with students the concept of chromatography. Introduce the idea that students will work in cooperative groups to design and carry out an investigation into the pigments in red flower petals. They will separate the pigments and try to determine if all red flower petals contain the same pigments. Begin with a leading question, and follow up:

 

  • - How does chromatography separate compounds?
  • - How can we design an investigation to determine if all red flower petals contain the same pigments?

 

4. Work with students to design a valid investigation similar to the one outlined in this lesson plan. Guide them to develop a hypothesis and identify the variables they will test. Work with students to determine what will be their control and what will be the independent and dependent variables in their experiments. Students should determine a list of materials they will need to carry out their investigation, and they should develop an appropriate data table.

 

5. Ensure that students outline the steps of their procedure carefully. To ensure that they can compare their results, they should be certain that as many of their materials and conditions as possible remain constant. This means that the temperature, type of solvent, size of paper, and where the pigments are placed on the paper should remain the same in each trial. Some leading questions include the following:

 

  • - How will you get the pigments onto the chromatography paper?
  • - How will you determine if the pigments are the same compound?                                    
  • (Rf values can be calculated; see calculations section below)

 

6. Lab safety equipment should be used, and safety protocols followed. Chromatography solvent is volatile and flammable; it should only be used in a well-ventilated laboratory where there are no open flames. Goggles should be worn at all times.

 

7.  General procedures that could be used: (Special note: Color deficient students may benefit from having a chromatographic strip with similar colors labeled on it to use as a comparison. If you notice a particular student mislabeling the colors, have the school nurse test this child for the ability to see colors.)

 

a. Obtain 2–3 petals from several different red flowers. Each student group should have at least 2 petals from 3 different red-flowered plants.

 

b. Obtain chromatography paper, pencil, ruler, penny, large test tube, wooden splint and scissors.

 

c. Cut the chromatography paper into a long thin strip that is slightly narrower than the mouth of the test tube and is long enough to extend from the bottom of the test tube out the mouth.

 

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. Label each strip (using pencil) at the top so that you can easily identify each strip later.

 

 

 

 

 

 

 

e. Place a piece of the red flower petal over this line and use the edge of the penny to rub across the petal along the pencil line drawn on the strip so that the pencil line is now covered with pigments from the petal. It is important that the chromatography strip contain a single, narrow, horizontal red line.

 

f. Stand the strip of chromatography paper in the empty test tube and staple it to the coffee stirrer so that it hangs freely from the stirrer into the test tube but does not touch the bottom. It should hang very close (0.5 cm or so) to the bottom of the test tube when the stirrer rests across the opening.

 

g. Remove the chromatography strip from the test tube and put enough solvent in the test tube to completely cover the bottom to a depth of less than 1.0 cm. When the strip is put back in the test tube it should touch the solvent but the solvent surface should be below the pencil line on the strip.

 

h. Carefully place the chromatography strip in the test tube so that the bottom of the strip is in the solvent yet the solvent level is below the pencil line. See the diagram below.

 

 

i. The solvent will move up the chromatography strip as a result of 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.

 

j. Let the strip dry. You should be able to see the pigment spots for each pigment in the flower petal separately.

 

8.  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. They should also use colored pencils to draw a colored picture of the bands for their lab write-ups or presentations. If the student groups test different petals, they can try to see if the petals share some similar pigments and look for different pigments.

 

9.. (Optional calculations) Students can also calculate Rf values for each separated pigment to help them determine if similar-looking pigments are the same compound:

 

Rf = the distance traveled by the pigment

   the distance traveled by the solvent

 

10. Have students draw a conclusion (or conclusions) about their results. Relate results to the topic of solutions.

 

  • - Why do some pigments settle out of solution first?
  • - Why do some pigments travel all the way up the chromatography strip?

 

11.  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, different pollinators, etc).

 

12. After the students have finished their investigations, they may present their findings to their classmates and compare their results with those of the other students.

 

13. Some follow-up ideas include the following:

 

  • - Do flowers of other colors also contain some of these same pigments?
  • - Are any of these pigments able to change color in varying environments (such as acidic or basic conditions)?
  • - How would chromatography be used in TV crime shows?
  • - Why would this process help determine if a Rembrandt painting was real?

 

Additional Resources:

http://www.flinnsci.com/Sections/Safety/safety.asp

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

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

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

 

 

 


 What Color Is Your Flower?

 

Experiment Title: _____________________Date: __________Name: ______________

 

Student Hypothesis or Question:

 

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Materials:

 

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Safety Precautions:

 

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Procedure:

 

Wear safety goggles for all lab work.

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Data and Observations:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Analysis of Data:

_______________________________________________________________________

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Conclusions:

_______________________________________________________________________

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