SC 130 Physical Science laboratory 13: Flower colors, acids, and bases

Notes for spring 2008: The chromatograph appears to have failed, despite prior work suggesting it could be done. The only difference was the use of a rock pounder in the functional chromatograph and the use of a mortar and pestle in class. Not enough crushing? The upshot is that pens or MM's would work better and this section of the lab was effectively not done.

Need more substances to test next term: ammonia, cream of tartar, acetone, drano...Could use more droppers.

Post-lab marking notes: Students remain unclear on acids versus bases. That concept in and of itself appears to be sufficient for the laboratory. Drop the chromatograph. Label the "known" substances more clearly. "Lime. Citrus fruit. Sour. Known acidic solution. Baking soda. Bitter. Known basic solution." Test more substances. Dilute bleach to see if oxidizing effect can be reduced. Test neutral fluids such as rubbing alcohol, vegetable oil. Dilute drain cleaner. Look for and acquire other substances such as cream of tartar, baking powder, coralline lime powder.

This laboratory explores the color components of flowers and the sensitivity of these colors to acids and bases.

Flower petal color constituents

In this section of the laboratory you will perform a paper chromatography to attempt to separate the pigments that comprise a colored flower petal.

In a mixture, the substance dissolved in another substance is called the solute. The substance doing the dissolving is called the solvent. If you dissolve sugar in water, the sugar is the solute and the water is the solvent.

For this laboratory, you will grind flower petals to attempt to release the pigments in the petal. The petal solution placed in a small vial of rubbing alcohol, and the alcohol will soak up into filter paper placed in the bottle.

The rubbing alcohol (solvent) is the mobile phase of the chromatography system, whereas the paper is the stationary phase. These two phases are the basic principles of chromatography. Chromatography works by something called capillary action. The attraction of the alcohol to the filter paper (adhesion force) is larger than the attraction of the alcohol to itself (cohesion force), hence the alcohol moves up the paper. The flower petal pigments will also be attracted to the paper, to itself, and to the alcohol differently, and thus a different component will move a different distance depending upon the strength of attraction to each of these objects.

The solvent travels further up the filter paper than the sample pigment

To measure how far each component travels, we calculate the retention factor (Rf value) of the sample. The Rf value is the ratio between how far the component travels and the distance the solvent travels from a common starting point (the origin). If one of the sample components moves 2.5 cm up the paper and the solvent moves 5.0 cm, then the Rf value is 0.5. You can use Rf values to identify different components as long as the solvent, temperature, pH, and type of paper remain the same. In the image the light blue shading represents the solvent and the dark blue spot is the chemical sample.

When measuring the distance the sample traveled, you should measure from the original location of the top of the liquid in the vial.

To calculate the Rf value, we use the equation:

Rf = distance traveled by the sample component ÷ distance traveled by the solvent

In our example, this would be: 2.5 cm ÷ 5.0 cm = 0.5

Note that an Rf value has no units because the units of distance cancel. Procedure:

  1. Go outside and find some flower blossoms with colored petals.
  2. Gather a few of these blossoms and bring them back to the laboratory.
  3. Using the petals from one blossom, gring the petals in a clean mortar with a small amount of alcohol.
  4. Put the ground petals along with a little more alcohol in a small bottle along with a string of filter paper.
  5. Allow this to sit for the duration of the period.
  6. Note whether any pigments have migrated and/or separated on the paper chromatograph.
  7. Calculate the Rf value for any pigments seen.

Sensitivity to acids and bases

This section of the laboratory explores whether flower petal pigments are good indicators of acids and bases, and if so, which flowers work best. To produce our flower petal test solutions the petals will be boiled in water.

Procedure:

  1. Rinse out a test tube.
  2. Add one dropper full of a flower petal solution to a test tube.
  3. Write down the color of the flower petal solution in the test tube.
  4. Rinse out the dropper.
  5. Add a couple drops of lime juice (acidic) to the test tube.
  6. Agitate the test tube to facilitate mixing.
  7. Take note of and write down any color changes that occur.
  8. Rinse out the test tube again.
  9. Again add one dropper full of a flower petal solution to a test tube.
  10. Rinse out the dropper.
  11. Add a small amount of baking soda (basic) to the test tube.
  12. Agitate the test tube to facilitate mixing.
  13. Take note of and write down any color changes that occur.

Repeat the above procedure for the different flower petal solutions in the laboratory. Make a table [t] of your results.

Once you have found flower petal solutions that change color for acids or bases, use them to analyze [a] whether the following are acids or bases (additional compounds might be made available on lab day):

Conclusion [c]

[c] Summary of findings. Discuss what, if any, pigments were the same in more than one flower. Discuss what flowers worked best for detecting acids. Discuss what flowers worked best for detecting bases.