CHEMISTRY NOW: What Color Is Your Flower? (Grades 9-12) Print

Objective:

Students will investigate the pigments in flower petals. Students will design and carry out an investigation to determine how many pigments are in various colored flower petals. Students will analyze the pigments in flower petals and determine if any of these pigments change color in acidic or basic conditions, and thus can act as acid–base indicators.


Introduction Notes:

CHEMISTRY NOW:What Color Is Your Flower?

 

Subject Area: Chemistry                                                        

Grade Level:  High School Chemistry                                    

Lesson Title:  What Color Is Your Flower?

National Science Education Standards:                                                                                         

Science as Inquiry: 9–12                                                           

Structure and Properties of Matter: 912

Chemical Reactions: 912                                                                             

Physical Science, 912                                                                                                              

 

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

 

Purpose: To separate the pigments in flower petals and determine which of these pigments exhibit acid–base indicator properties

Key Vocabulary:  

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

acid—substance that has a pH less than 7 and produces hydrogen ions in solution. (It might be useful to connect to a lesson on water that would help students visualize where hydrogen and hydroxide ions come from.)

base—substance that has a pH greater than 7; produces hydroxide ions in solution

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 (a kind of solvent; see definition of solvent below)—material used in chromatography that carries the compounds to be separated through the absorbent

indicator—compound that changes color in different conditions (In this case, we are looking at pH indicators, which are compounds that give different colors in solutions of different pH; we will use In to represent the Indicator ion.)

pH—measure of the acidity of a substance: a pH of less than 7 is acidic (lower number = more acidic), and a pH of greater than 7 is basic; in general the scale goes from 1 (very acidic) to 14 (very basic); pH = −log[H3O+]

pigmenta coloring matter in animals and plants

solute—substance dissolved in a solution

solutionhomogeneous mixture of two or more substances

solvent—substance dissolving the solute in a solution

strong acid/base—an acid or base that completely dissociates into ions (H+ or OH and other ions) in solution

weak acid/base—an acid or base that only partially dissociates (ionizes) in solution

Objectives:      

  1. Students will investigate the pigments in flower petals.
  2. Students will design and carry out an investigation to determine how many pigments are in various colored flower petals.
  3. Students will analyze the pigments in flower petals and determine if any of these pigments change color in acidic or basic conditions, and thus can act as acid–base indicators.

 

Materials:  

- Safety goggles

- Flower petals from several different freshly cut flowers (from home garden or a florist)

- Chromatography paper

- Chromatography solvent (isopropyl alcohol)

- Wooden coffee stirrers

- Stapler

- Scissors

- Pencil

- Penny coin

- Large test tube

- Colored pencils

- Vinegar solution (acid)

- Ammonia solution (base)

Procedure:

1. Begin with a discussion of the colors of flowers. In this investigation, students will investigate the pigments in flower petals. Students will need to separate the pigments (chromatography) and then try to determine if any of the separated pigments could be used as an acid–base indicator. Hydrangeas, for example, will produce different colored flowers depending on whether the soil is acidic or basic.

 

  • - Why do flowers have colored petals?
  • - Do all flower petals contain the same pigments?
  • - Do flower petals only contain one color of pigments?
  • - Why are some flowers, such as morning glories, often seen in different colors, sometimes on the same plant?
  • - How can flowers from a plant change color?

 

2. Discuss with students the concept of chromatography. Introduce the idea that students will work in cooperative groups to design and carry out investigations into the pigments in flower petals. They will need to separate the pigments in order to study these compounds. Begin with a leading question, and follow up:

 

  • - How does chromatography separate compounds?
  • - How can we design an investigation to extract the pigments from a flower so that we can study these compounds?

 

3. 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.

 

4. 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 onto 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)
  • - How will you determine if these compounds exhibit acid-base properties?

 

5. Lab safety equipment should be used and safety protocols. 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.

 

6. General procedures that could be used:

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

 

b. Obtain chromatography paper, pencil, ruler, penny, large test tube, wooden coffee stirrer, 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 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 pigment from the petal. It is important that the chromatography strip contain a single, narrow, horizontal 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 or the sides of the test tube. 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. Allow the solvent to 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 solubility 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.

 

7. 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 color in the bands for their lab write up or presentations. If they test different petals, they can try to see if the petals share some similar pigments and look for different pigments.

 

8. (Optional calculations) Students can 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.

 

9. Once students have extracted the different pigments from their flowers, they can cut up the chromatography strip and test these extracted pigments for color changes.

 

a. For each colored pigment separated from the chromatography paper, cut the pigmented section into two pieces, each with some of the colored spot on it.

b. Record the original color of the spots, then place a drop of acid solution on one and a drop of basic solution on the other and observe any changes.

c. Record observations.

 

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

 

  • - Do any of the pigments change color when tested with the acidic or basic solutions?
  • - What color is the pigment in acidic solution? In basic solution?

 

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 their classmates.

 

13. Some follow-up ideas:

 

  • - Can growing these flowers in acidic or basic soil result in different flower colors?
  • - What are some implications of what was learned today?
  • - How could this information be used in everyday life?

 

Natural Flower Indicators

Dahlias

Daylilies

Geraniums

Hibiscus

Hollyhocks

Hydrangeas

Blue Iris

Morning Glories

Purple Mums

Pansies

Peonies

Petunias

Poppies

Red or Pink Roses

Violets

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

http://findarticles.com/p/articles/mi_ml1200/is_n3_v149/ai_17868041/

 

 

 

 


What Color Is Your Flower?

 

Experiment Title: _____________________Date: __________Name: _____________

 

Student Hypothesis or Question:

 

_____________________________________________________________________

 

Materials:

_____________________________________________________________________

 

_____________________________________________________________________

 

_____________________________________________________________________

 

 

Safety Precautions:

 

_____________________________________________________________________

 

_____________________________________________________________________

 

_____________________________________________________________________

 

Procedure:

 

Wear Safety Goggles for all lab work.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

 

Data and Observations:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Analysis of Data:

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

 

 

Conclusions:

 

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

 

 

Close NBC Learn

FILTERING

If you are trying to view the videos from inside a school or university, your IT admin may need to enable streaming on your network. Please see the Internet Filtering section of our Technical Requirements page.

DVDs AND OTHER COPIES

Videos on this page are not available on DVD at this time due to licensing restrictions on the footage.

DOWNLOADING VIDEOS

Subscribers to NBC Learn may download videos and play them back without an internet connection. Please click here to find out more about subscribing or to sign up for a FREE trial (download not included in free trial).

Still have questions?
Click here to send us an email.

Close NBC Learn

INTERNATIONAL VISITORS

The Science of the Olympic Winter Games videos are only available to visitors inside the United States due to licensing restrictions on the Olympics footage used in the videos.

FILTERING

If you are trying to view the videos from inside a school or university, your IT admin may need to enable streaming on your network. Please see the Internet Filtering section of our Technical Requirements page.

DVDs AND OTHER COPIES

The Science of the Olympic Winter Games is not available on DVD at this time due to licensing restrictions on on Olympic footage.

DOWNLOADING VIDEOS

Subscribers to NBC Learn may download videos and play them back without an internet connection. Please click here to find out more about subscribing or to sign up for a FREE trial (download not included in free trial).

Still have questions?
Click here to send us an email.

Close NBC Learn

Choose your product

NBC Learn K-12 product site
NBC Learn Higher Ed product site

For NBC Learn in Blackboard™ please log in to your institution's Blackboard™ web site and click "Browse NBC Learn"

Close NBC Learn

If you have received a new user registration code from your institution, click your product below and use the "Register now" link to sign up for a personal account.

NBC Learn K-12 product site
NBC Learn Higher Ed product site

For further assistance, please contact our NBC Learn Support Team and we'll be happy to assist you.

Start Your Free
day
Day Trial!