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Primary Colors Lab -- Teacher Background

1 lab period for wet lab

1 class period for spectral analysis and preparation

Learning Goals:

Prepare samples for UV-visible analysis.

Collect data on a UV-visible spectrometer.

Analyze spectra of primary colors.

Analyze a spectrum of a secondary color.

Lecture Connection

Equipment/Materials:24 students working in pairs

Chemicals Needed	Equipment and Supplies Needed
blue food coloring solution (blue dye #1)* red food coloring solution (red dye #40) yellow food coloring solution (yellow dye #5) assigned food coloring solution	60 cuvettes UV-vis Spectrometer

* McCormack's brand food coloring was used in developing this laboratory experiment.

Solution Preparation: Standard food coloring straight from the bottle will be too concentrated to prodcue useable spectra. The table lists dilutions of different food colorings that will produce the maximum and concentrations readable by the spectromter. as well as a dilution that will produce a so-called 'ideal spectrum' somewhere in the middle of the range. You may find that because drop size from the food coloring bottles is inconsistent and somewhat difficult to control, the actual dilutions may vary from the specifics listed here. Distilled or tap water may be used to dilute the food coloring.

Food Coloring	Maximum Concentration	'Ideal' Concentration	Minimum Concentration	Peak Location/Absorbance: Maximum Concentration	Peak Location/Absorbance: 'Ideal' Concentration	Peak Location/Absorbance: Minimum Concentration
Red	1 drop/40 mL	1 drop/50 mL	1 drop/400 mL	524 nm/1.231 324 nm/.875 271 nm/.795	524 nm/.907 317 nm/.532 271 nm/.565	525 nm/.091 264 nm/.147
Blue	1 drop/30 mL	1 drop/40 mL	1 drop/425 mL	631 nm/1.556 413 nm/.329 313 nm/.448	631 nm/1.525 413 nm/.369 313 nm/.502	633 nm/.174 269 nm/.127
Yellow	1 drop/40 mL	1 drop/50 mL	1 drop/425 mL	411 nm/1.153 275 nm/.894	431 nm/.663 273 nm/.441	432 nm/.038 265 nm/.058
Orange**	1 drop each red and yellow/160 mL	1 drop each/90 mL	1 drop each/800 mL	424 nm/.845 269 nm/.845	424 nm/.695 273 nm/.515	424 nm/.079 269 nm/.043
Green	1 drop/25 mL	1 drop/40 mL	1 drop/420 mL	632 nm/1.31 415 nm/.987 271 nm/.655	632 nm/1.317 415 nm/.890 271 nm/.655	631 nm/.053 419nm/.035
Purple**	1 drop each red and blue/ 60 mL	1 drop each/ 120 mL	1 drop each/ 800 mL	631 nm/1.598 542 nm/1.426 524 nm/1.333 411 nm/1.182 321 nm/1.219	631 nm/1.583 532 nm/1.334 413 nm/.686 316 nm/.920 277 nm/.771	631 nm/.033

** These solutions were made by mixing 2 individual food dyes. Because two drops were used, the amount of water used was doubled.

Sample Data: We have included sample food coloring spectra as a reference. Please keep in mind that different brands, different colors, and different techniques will affect the exact wavelength and absorbance values.

The following six spectra represent 'ideals' from the primary and secondary colors.

The follow spectra represent maximum and minimum concentrations of the labeled food colorings.

At the extremes, the spectra curves start to 'break up'. They become more jagged. The peaks are less distinct. At the maximums (highest concentrations), the peak may become a plateau, a flattened curve with no identifiable peak.

Safety: Students should follow general safety procedures regarding the handling of glassware.

Disposal: All materials are nontoxic. Although solutions may be flushed down the drain, it is good training for students to empty their cuvettes into labeled waste or used-materials containers. This procedure will encourage good disposal habits when more hazardous materials are used in the lab.

Suggestions:

Before beginning, you may want to review the wavelength ranges for visible light.
For secondary colors that are mixed in the classroom, ask students to compare the absorbance values for the peak wavelengths and explain the differences. For example, a higher concentration will produce a higher absorbance.
Introductory demonstration: Prepare a series of 5-8 dilutions with food coloring (Suggestion: Undiluted coloring, maximum concentration, ideal concentration, 1 drop/100 mL, 1 drop/200 mL, 1 drop/300 mL, minumum concentration). Scan these samples ahead of time, printing the spectra. Seeing the spectra with the dilutions will illustrate for the students which samples are too concentrated, etc. This comparison will also highlight the sensitivity of the spectrophotometer: Students may be surprised to see that a dilution with color barely visible to the eye will produce highly usable spectra.
Molarity Exercise: Relate the dilutions to the concept of the molarity. Assume some starting dilution is 1.0 M and prepare a series of dilutions from this 'stock' solution. You may choose a simple series (.1M, .01M, etc.) or a less predictable series. Students can calculate the molarity of the remaining dilutions using the formula M(initial)V(initial) = M(final)V(final), where M = molarity and V = volume.
Discovery: Some independent discovery aspects may be added to this lab in several ways. (a). Have students create their own dilutions for each food coloring, keeping a record of each dilution they make. They probably will start with a far too concentrated dilution and need to dilute
Lecture Connections Questions

The following problems relate concentration and absorbance. They may be used to assess individual student understanding of spectroscopy, for small group problem solving, or for whole-group discussion.

Given the following spectra taken from chemical samples, what would you predict for a spectra if the two chemical samples were mixed?

a.

b.

c.

d.

Troubleshooting:

To correct poor spectra, try one or more of the following suggestions:

1. Blank the machine. Listen for the click that signals the blank has been read. Re-scan samples.

2. Be sure the cuvette is clean and dry, and inserted properly into the holder. (Moisture or fingerprints on the clear sides of the cuvette can interfere with the machine's reading.)

3. If the spectra are showing either jagged peaks or flattened plateaus instead of distinct, smooth peaks, the solution may be too concentrated. Dilute the solution and re-scan the sample.

4. You may also view the Spectra Library of Common Mistakesfor further comparison.

Maine State Learning Results (MSLR): This lab encourages skills listed in the following performance indicators under content area for secondary scores:

E. The Structure of Matter:

J. Inquiry and Problem Solving

Students will apply inquiry and problem-solving approaches in science and technology. Students will be able to:

Make accurate observations using appropriate tools and units of measure.

Verify , evaluate, and use results in a purposeful way. This includes analyzing and interpreting data, making predictions based on observed patterns, testing solutions against the original problem conditions, and formulating additional questions.

Demonstrate the ability to use scientific inquiry and technological method with short-term and long-term investigations, recognizing that a there is more than one way to solve a problem. Demonstrate knowledge of different strategies.

K. Scientific Reasoning

Students will learn to formulate and justify ideas and to make informed decisions. Students will be able to:

3. Develop generalizations based on observations

4. Determine when there is a need to revise studies in order to improve their validity trough better sampling, controls, or data analysis techniques.

L. Communication

Students will communicate effectively in the application of science and technology. Students will be able to:

3. Make and use appropriate symbols, pictures, diagrams, scale drawings, and models to represent and simplify real-life situations and to solve problems.

4. Employ graphs, tables, and maps in making arguments and drawing conclusions.

7. Use computers to organize data, generate models, and do research for problem solving.

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