Summary

In this lesson students explore the properties related to color and how those properties vary with changes in concentration. This lesson introduces the use of a spectrophotometer to measure wavelength and absorbance in colored solutions as well as the use of Beer’s Law to determine an unknown concentration.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS4-4: Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter.

AP Chemistry Curriculum Framework

This lesson plan supports the following unit, topic and learning objective:

• Unit 3: Intermolecular Forces and Properties
  • Topic 3.13: Beer-Lambert Law
    • SAP-8.C: Explain the amount of light absorbed by a solution of molecules or ions in relationship to the concentration, path length, and molar absorptivity.

Objectives

By the end of this lesson, students should be able to

• Contrast hue and intensity as aspects of color and relate them to wave properties.
• Measure wavelength and absorbance spectra of solutions using a spectrophotometer.
• Compare the absorbance spectra of different colored solutions.
• Identify the absorbance spectrum of a mixture as a combination of the spectra of the individual chemicals.
• Complete serial dilutions.
• Compare absorbance spectra of solutions to identify any differences as concentration of the solution changes.
• Create a graph of concentration versus absorbance and apply Beer’s Law to determine the unknown concentration of a solution.

Chemistry Topics

This lesson supports students’ understanding of

• Solutions
• Beer’s Law
• Concentration
• Physical Properties
• Spectrophotometry

Time

Teacher Preparation: 20 minutes

Lesson:

• Engage: 20 minutes
• Explore: 60 minutes
• Explain: 30 minutes
• Elaborate: 20 minutes
• Evaluate: 20-40 minutes

Materials (per group)

• Liquid food coloring
• FD&C Food Dyes: Blue #1, Blue #2, Red #3, Red #40, Green #3
• Water
• 10 mL graduated cylinder
• 10 test tubes (at least 10 mL in volume)
• Test tube rack
• Disposable pipets
• Stirring rod
• Vernier SpectroVis or SpectroVis Plus Spectrophotometer
• Cuvettes

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption

Teacher Notes

• Engage:
  Interactive Demo:
  
  • Hold up containers of different food coloring solutions and ask students to state the color of the solution.
    • Have a few different colors, but have two that would be generally called “blue”. Change the color (not the concentration) so that the two are visibly different blues. This can be accomplished by adding other colors, like red, to the blue, but not going so far as to turn it into another recognizable color.
      • When you get to the 2^nd blue, act surprised and say something along the lines of, “But you said this other one was blue and they are clearly not the same color…”
      • Initiate a discussion about how else you might be able to classify colors. Examples are shade, hue, intensity, combining other colors, like green-blue…
        
  • Hold up the same containers, one at a time. Remind students that the light in the classroom consists of all colors of the rainbow. Ask them which color(s) they think are being absorbed by the solution. You may want to have them record these predictions in a notebook.
    • Next, take a sample of one of the solutions and put it into a cuvette. Be sure it is one that will only have one peak. If you are using FD&C dyes, this will be true of any of these. If you are using store-bought food coloring, avoid green. Also, be sure it is dilute enough to give a smooth peak. This should be tested out before class. You will need to show students your computer screen. This is best if you can project it onto a screen or use a SmartBoard to show it.
      • Tell students you are putting the solution into an instrument that will show them which visible colors are being absorbed and which are not.
      • (You should have the SpectroVis already calibrated and ready to go before class.) Insert the cuvette into the chamber and click Collect and then Stop to show the absorbance spectrum of the solution.
      • Use the colored background to help students make sense of the plot and how it shows what color ranges are and are not absorbed by the solution.
      • Hold up a different color solution and ask them to predict what the spectrum will look like.
      • Show them the spectrum of that color and further discuss that the range of colors most absorbed by the solutions are the ones that are NOT seen by the eye.
  • Explain, or reinforce, if this is prior knowledge, that you can only see color because of light. Be sure to clarify the following points in your discussion:
    • Only light in the visible range (ROYGBIV) is perceived by humans as color.
    • If you see a color, it is actually the light that is colored.
      • In opaque, colored objects, some light gets absorbed and the rest gets reflected. The light that gets reflected is what reaches our eyes. So, a red piece of paper is red because the colors that get reflected are perceived by our rods and cones as being red. This may be red light or other combinations that blend into red.
      • In transparent, colored objects (like solutions), some light gets absorbed and the rest gets transmitted. The light that gets transmitted is what reaches our eyes. So, a blue solution is blue because the colors that do not get absorbed are perceived as blue. This means that light in the range of blue, and possibly some other colors, did NOT get absorbed by the solution.
  • Ask the students to read and underline new ideas from the background.
    • Then discuss the information from the background to ensure they understand that they are going to study some factors that make a color look like it does.
• Explore:
  Students will identify peak wavelengths in the absorbance spectra of different food coloring based solutions. They will also complete a serial dilution of a food coloring solutions to generate data showing how absorbance varies with concentration.
  • Tip: You can test the food colorings you are using to determine which will give the best spectra so as to pre-select the one that gives the clearest spectrum for dilution.
  • Note: The procedure given is for use with the Vernier SpectroVis Plus or SpectroVisSpectrophotometer, so you may need to alter the instructions if you are using a different type of machine.
  • Skills to review include: how to use a spectrophotometer and any related interface, the safe handling of glassware, how to complete a serial dilution, and how to accurately read a meniscus.
  • Assign each student group a pair of FD&C dyes (two blues or two reds)
• Explain:
  After completing Part A and B, students will be able to identify differences in the wavelengths absorbed in various food coloring solutions and also identify those peaks in a solution made from a mixture of food colorings. After completing Part C students will be able to identify that the wavelength doesn’t change, but absorbance does, as the concentration of the solution is decreased through dilution.
  • Skills to review include: how to identify the wavelength of a peak on an absorbance spectrum; how to identify which regions of a spectrum (colors) were absorbed or transmitted to give a solution a certain color; how to construct a scatter-plot graph; how to draw a line of best fit on a graph; how the best fit line describes the relationship between variables.
    
• Elaborate:
  • Students are asked to consider real-world applications of both using a spectrophotometer for identification of an unknown and for determining the concentration of an unknown solution in the Analysis section.
  • Skills to review include: Extrapolation and interpolation of data when using a graph to determine a value for one variable if given the other variable.
• Evaluate:
  This activity is structured so that students will demonstrate their findings by recording data as well as evaluating their results through both a graph and questions that support synthesis of the concepts as well as error analysis.
  • The conclusion asks them to consider the color of a mixture as being intensive or extensive.They should conclude that the wavelength(s) responsible for the color of the mixture is intensive, but the intensity of the color (concentration of each color-absorbing species) is extensive.

• Preparation of materials:

• Pure FD&C food dyes may be purchased from any science supply company. They can be purchased as solids or as pre-made solutions, similar in concentration to the food coloring from a grocery store. Blue #2 has a short shelf-life and will eventually degrade and look yellow. Blue #2 can be kept for about a year with success, but it will degrade shortly after that. To avoid accelerated degradation, you should keep this away from light. All other colors are shelf-stable.
• Food coloring may be purchased from the grocery store.
  • Liquid food coloring works better than gel, and red-yellow-green-blue is better than neon colors.
• Food coloring can be used directly from the bottle.
• Food coloring will dye skin and clothing, so have students use gloves if this is an issue.
• Tip: Do a test run first to make sure your initial concentration isn’t too high – absorbance values above 2.0 are much noisier than those at lower values. The reference photo below shows the color intensity that worked well in sample data collection.

• Sample Data and Anticipated Results:
  Part A:
  • Spectra for all four food colorings (red, yellow, green and blue) are shown.
  • The line color corresponds to the color of the dye that was used.
  • An overall spectrum is included so that you can see how each dye absorbs less in the region of the spectrum that corresponds to the dye color (for example, note the dip in the blue portion of the spectrum found in the analysis of blue dye.

Part B:

• Shown below are the individual spectra for yellow, blue and green dyes, shown in the color of the dye used.Worth noting is how the spectrum of the green dye includes the peaks found in both the yellow and blue dyes, effectively making it the sum of the blue and yellow dyes.
• The spectrum of the mixture made by mixing blue and yellow together is shown in black.
• Note that the peaks of the mixture match those of the green solution, indicating that the manufacturer likely makes its green dye by combining its yellow and blue.

Part C:

• Shown below are the spectra for the green dye solution and its first four dilutions.
• Note that as the solution is diluted, the peak wavelength stays the same while the absorbance of that wavelength changes.
• It is possible to see that as the concentration decreases by 50% with each dilution, the absorbance decreases by the same amount.
• Notable is the first dilution, which has as an absorbance in the 2.0 range and therefore is less dependable.

Data Section:

• Below is the Beer’s law graph for the dilutions above.
• This graph shows the data point that has an absorbance close to 2 but it has not been included in the graph plotted from the data.
• Tip: If student-generated graphs don’t produce a straight line (even after removing high absorbance points), it is likely due inaccurate dilutions.
• Using a volumetric pipet to measure both the solution and the water added will greatly improve the quality of data obtained.

• Additional Background information
  The following information may be helpful:
  • The perceived color of a solution is a function of both the wavelengths absorbed and the amount of absorbing molecules within the solution.
  • As the amount of food coloring in the solution increases, the absorbance will increase because there are more molecules to absorb the same amount of light. This will lead to a solution that appears more intense in color. The wavelength best absorbed by the solution, however, remains unchanged as the composition of the solution is the same.
  • From this we can see that color must be considered as composed of two different properties: wavelength, which is intensive; and absorbance, which is extensive.