1. Lab Session 5
What is the relationship between a substance’s color and the light it absorbs?

2. The Challenge
A local business produces colored salt solutions for use in baths and spas. Your team of chemists has been hired to conduct research and provide suggestions for new product colors.
At this time, the business wants to better understand the relationship between salt solution color and the absorption or transmission of visible light.
How can you determine how the color depends on the type of light that is absorbed or transmitted?

3. Visible light is a type of electromagnetic radiation.
A substance’s color depends on the interaction of incoming visible light and the electrons that comprise its atoms and molecules.
Visible light is a type of electromagnetic radiation.
Relevant wavelengths.

4. Color ABSORPTION SPECTROSCOPY
The actual color of a substance depends on the types of visible light that it absorbs and transmits.
The main challenge for this lab is to characterize the relationship between the visible light absorbed/transmitted and the color of substances when dissolved in water (aqueous solutions)
For this purpose, we will use a very powerful analytical technique:
ABSORPTION SPECTROSCOPY

5. Absorption Spectroscopy
This technique is based on the analysis of the types and amount of electromagnetic radiation absorbed by a chemical substance.
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6. How would the spectrum would look like?
Let’s Think
NaNO3 is a white solid that dissolves in water producing a colorless solution.
If you were to measure the visible light absorbed by the NaNO3 solution in the visible range,
How would the spectrum would look like?

7. Absorption Spectrum What would you expect to see for a: Green solution
Blue solution
Why would we want to know the spectrum?
These are the types of questions we want to answer in today’s experiment.
Very small or no absorbance in this range.

8. Spectrometers
A spectrometer is an optical instrument that allows us to measure the amount of light that hits the detector as a function of wavelength.
In order to use this instrument, we first need to prepare the aqueous solutions of the substances we want to analyze.

9. Verify your results with the different groups in the class.
Calculations
To face your challenge, your group needs to prepare 20.0 mL of M solutions of the following substances:
Co(NO3)2.6H2O Al(NO3)3.9H2O
Ca(NO3)2.4H2O Cu(NO3)2.5H2O
Ni(NO3)2.6H2O
In your groups, calculate the mass in grams that you need to prepare each of these solutions.
Verify your results with the different groups in the class.

10. Prepare 20 mL of 0.1 M solutions of each of the substances to analyze.
Preparing Solutions
Prepare 20 mL of 0.1 M solutions of each of the substances to analyze.
Available resources:
Powdered salts;
Spectrometer.
Volumetric material.
Balances.
You have 20 minutes

11. Your Predictions
Before facing your challenge, it is important that you make predictions about what you expect to see when generating the absorption spectra of each of the solutions.
In your groups, predict the shape of the absorption spectrum that you expect to see for each of the solutions.

12. Your Challenge
Your work in the lab should allow you to answer the following questions:
What is the relationship between a substance’s color and the type of light it absorbs and transmits?
How does light absorption varies with the concentration?
Can the color of a mixture of substances be predicted by the nature of the individual substance’s absorption spectrum?

13. Your Real Challenge
Do the necessary experiments to answer the three questions just discussed.
Available resources:
0.1 M solutions of colored substances;
Spectrometer.
Volumetric material.
You have 60 minutes

14. Claims and Evidence
Based on the results of your experiments, present your major claims and the evidence that you have to support the answer to the challenge questions.
Is the group answering all of the questions?
Are their claims clear?
Is the evidence reliable?
Is the evidence appropriate to support the claims?
How do you explain your results?

15. Final Reflections What did you learn from doing your experiment?
How would you improve what you did?
How have your ideas changed as a result of this lab?
What do you not completely understand?
What new questions do you have?

16. Your Report
Beginning questions (2 p): What questions guided your explorations?
Safety Considerations (2 p): What did you do to stay safe in the lab?
Procedures and Tests (2 p): What experiments did you do to answer your questions?
Data, calculations, and representations (6 p): What observations did you make? What data did you collect? What calculations and representations helped you make sense of the data?

17. Your Report Claims (2 p): What can you claim to answer your questions?
Evidence and Analysis (6 p): How did you interpret your results to support your claims?
Reflections and additional questions (10 p): What did you learn? What do you not completely understand? How have your ideas changed as a result of this lab? What new questions do you have? How would you improve what you did?

18. How can we use light to detect substances?
Lab Session 6
How can we use light to detect substances?

19. Your Challenge
Sparklers are a mixture of chemical substances that emit light of different colors when heated.
Your task is to use emission spectroscopy to detect the presence of certain chemical elements in sparklers of different colors.
What chemical elements are responsible for the color of different sparklers?