In this lab, students will investigate how iodine interacts with various substances. They will use color changes to justify whether a chemical or physical change is taking place. This activity is referenced in the October 2019 ChemMatters article called “Cash, Chemistry, and Counterfeiting.”
This lab will help prepare your students to meet the performance expectations in the following standards:
- HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
- Scientific and Engineering Practices:
- Asking Questions and Defining Problems
- Analyzing and Interpreting Data
- Engaging in Argument from Evidence
By the end of this lab, students should be able to
- Express observations clearly.
- Confirm whether a chemical or physical change took place.
- Understand how intermolecular forces justify their observations.
This lab supports students’ understanding of
- Chemical changes
- Physical changes
- Molecular Structure
- Lewis Structure
- Chemical reactions
- Intermolecular forces
Teacher Preparation: 30 minutes
Lesson: 45 minutes
For a class of 24, working in pairs
- 12 well plates
- 36 pipets
- 24 microspatulas
- Iodine, I2 (s)
- Potassium iodide, KI (s)
- 2-propanol (isopropyl alcohol)
- Wear safety goggles and gloves during the investigation.
- Iodine should be contained in a fume hood. Do not breathe dust or fumes. Wear gloves when handling. Refer to SDS for additional information.
- Wear protective clothing when handing potassium iodide. Refer to SDS for additional information.
- 2-propanol (isopropyl alcohol) is a highly flammable liquid and vapor, keep away from heat sources, open flames and sparks. Refer to SDS for additional information.
- Cyclohexane is a highly flammable liquid and vapor, keep away from heat sources, open flames and sparks. Refer to SDS for additional information.
- Follow proper local disposal instructions for the organic solvents. Refer to SDS of specific material for additional information.
- This lesson accompanies the October 2019 ChemMatters article “Cash, Chemistry, and Counterfeiting,” but it can be used to investigate chemical vs. physical changes on its own.
- It is suggested that students read the article prior to completing the lab. Reading guides and additional activities are available for teachers to use in the associated Teacher’s Guide.
- In the article, it’s mentioned that when triiodide interacts with starch, a deep blue color is observed. That deep blue color is different from the deep blue/purple color of I2. In the presence of starch, I3- slips into amylose’s helical structure, and that results in a charge-transfer complex that is deep blue in color.
- In the investigation, students are asked to consider the results they would observe with acetone – you could certainly have them conduct this experimentally to test their hypotheses.
For the Student
The key ingredient in counterfeit-detector pens is triiodide (I3-), which produces a deep blue color when it reacts with starch in ordinary printer paper. No starch is present in the cotton/linen blend that makes paper bills, so no reacting results when the pen is run across a bill. In this activity, you will make I3- yourself and determine whether physical or chemical changes occur along the way.
Read the ChemMatters article “Cash, Chemistry, and Counterfeiting.”
*Note: The examples used below can be independent of the article–things that you have experienced.
- Identify three examples of chemical changes that take place in real life.
- Identify three examples of physical changes that take place in real life.
To determine whether chemical or physical changes take place by observing color changes.
- Wear safety goggles and gloves at all times during the investigation.
- When you’ve completed the activity, follow your teacher’s instructions of how to clean up.
- Observe and/or research each chemical you will use: I2, KI, water, 2-propanol, and cyclohexane. Record your observations in the table below.
- What state is each chemical?
- Is it polar, nonpolar, or ionic? (Hint: draw the Lewis structures of each chemical in the space below.)
- In a well plate, use a pipet to fill one well half way with water.
- Use a different pipet to fill another well half way with 2-propanal.
- Use a third to fill another well half way with cyclohexane.
- Add a small piece of I2 with a microspatula to each well and give the well plate a gentle stir to allow the I2 to mix/react with each solvent.
- Make observations. Did a chemical or physical change take place? Explain what happened in the data table below.
|Water + I2|
|2-propanol + I2|
|Cyclohexane + I2|
- With a clean microspatula, add a tiny amount of KI to the first two wells and observe what happens.
- Does a chemical or physical change take place? Record it in the table below.
- What happened in the solution to cause the change? Explain what happened in the data table below.
|Water + I2 + KI|
|2-propanol + I2 + KI|
- In reference to Procedure Part 2 of the procedures: What would happen if you added I2 to acetone? Explain. (Hint: You may need to do some research about the properties of acetone before you can answer this question!)
- In reference to Procedure Part 3: What do you think would happen if you added KI to the cyclohexane and iodine solution? Explain.
What can you conclude from your investigation about chemical and physical changes?