Honors Chemistry Lab 3-1 Absorption Spectroscopy Name ______Per.______Background Information The color of light is associated with a specific wavelength, frequency, or energy. Colored solutions are colored because they absorb certain wavelengths of light while allowing other wavelengths of light to pass through (transmit). As observers, we see the wavelengths of light that are not absorbed. The spectrophotometer is an instrument designed to measure the color (wavelength) and the amount of light that is absorbed or transmitted by an aqueous sample of a substance. When a sample of matter is exposed to a beam of light, some wavelengths (colors) of that light will be absorbed by the sample; other wavelengths will be transmitted. When the amount of light absorbed by a sample of matter is graphed as a function of wavelength, the result is called an absorbance spectrum. Since every substance has an electron configuration that is unique to that particular substance, every substance exhibits a unique spectrum. Absorption spectra are useful to scientists for two reasons. First, they can be used for the identification of unknown substances. Second, they are useful to determine the amount of a substance present or its purity. In this experiment, you will learn how to operate a spectrophotometer. You will use the spectrophotometer to analyze a colored solution to determine which colors of light are absorbed by the solution.

Procedure Part A: Color vs. Wavelength of Visible Light **Use the spectrophotometers set up in the middle of the lab table to do part A.**

Part B: Food Coloring Absorbance **Use the spectra-viz and labquest at the end of the lab table to do part B**

Pre-Lab Questions 1. In general, between what wavelengths is visible light found (look at the first data table). ______

2. Why do you need to run a “blank?”

3. Why do the outsides of the cuvettes need to be clean and dry?

4. What is the relationship between absorbance and transmittance?

5. Predict the color(s) of light that will be transmitted most by each color of solution. Color of Solution Color(s) Absorbed Most Color(s) Transmitted Most Red Blue Green Yellow

Data Table and Graphs Part A: Color vs. Wavelength of Visible Light Wavelength nm) 650 600 550 500 450 400 350 Color on wood surface

Part B: Food Coloring Absorbance Label the axes and sketch a graph for each colored solution and the mixture. Honors Chemistry Color of Solution: Color of Solution: Color of Solution:

Data Analysis 1. The most important points of interest on the graph are where the color had the highest absorbances and lowest absorbances. Identify these wavelengths for the two solutions and the mixture and record below. 2. For each solution, use your observations from Part A to determine the color of light that is associated with the highest and lowest absorbance.

Color Highest Absorbance Lowest Asorbance Color Associated with Color associated with (nm) (nm) highest absorbance lowest absorbance

3. Was your prediction in the pre-lab questions correct? Give evidence (numerical proof) that your prediction was supported or not supported.

4. Explain, in terms of absorbance and transmittance, why your colored solution appears (to your eyes) to be the color it is.

5. If an absorption spectrum of a colored solution has more than one absorbance peak, what does this indicate about the colored solution?

6. The absorption spectrum of chlorophyll a indicates strong absorption of red and blue wavelengths. a. Explain why leaves appear green.

b. Would you recommend using light that emits red wavelengths to grow plants? Explain your answer. Procedure Honors Chemistry Part A: Color vs. Wavelength of Visible Light **Use the spectrophotometers set up in the middle of the lab table to do part A.**

1. Set the wavelength dial to 650 nm. 2. Check that the sample compartment is empty. Close the compartment lid and then use the left front knob to adjust the instrument until the instrument reads 0% transmittance. 3. Obtain the “blank,” a cuvette filled with distilled water. Wipe off the outside of the cuvette with a Kimwipe. Holding the cuvette just below the rim, place the cuvette into the sample compartment. Close the lid. 4. Using the right front knob, adjust the transmittance until the instrument reads 100% transmittance. 5. Remove the blank cuvette. Place the cuvette containing the wooden chip into the sample compartment. The slant of the wood must face toward the wavelength knob (toward the incoming beam of light). The angle of the wood will reflect the light up into your eye, so that you can see the color that is actually being produced by the instrument. Look into the sample compartment and record the color you see in the data table. 6. Observe the color of the light appearing on the chalk for all the wavelengths listed in the data table. If the color is too faint, cup the opening with your hands to help you see the color better. Every person will see slightly different colors because we all have different eyes. If you cannot see a specific wavelength, look at your electromagnetic spectrum handout to determine the color.

Part B: Food Coloring Absorbance **Use the spectra-viz and labquest at the end of the lab table to do part B**

1. Obtain 4 cuvettes. Fill one with distilled water. Fill 2 with colored solutions (pick two!) found on the common lab equipment table. Mix the same two colors in the 3rd cuvette – half and half. Record the color of your solutions. 2. Place the blank into the compartment. 3. Remove the blank and place one of the colored solutions into the compartment. Sketch the graph of absorbance v wavelength on one of the axes in the data table. 4. Repeat step 3 with the other two colored solutions.