Summary

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 Counterfeit”.

Grade Level

High school

NGSS Alignment

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

Objectives

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.

Chemistry Topics

This lab supports students’ understanding of

• Chemical changes
• Physical changes
• Molecular Structure
• Lewis Structure
• Polarity
• Chemical reactions
• Intermolecular forces

Time

Teacher Preparation: 30 minutes

Lesson: 45 minutes

Materials

For a class of 24, working in pairs

• 12 well plates
• 36 pipets
• 24 microspatulas
• Iodine, I₂ (s)
• Potassium iodide, KI (s)
• Water
• 2-propanol (isopropyl alcohol)
• Cyclohexane

Safety

• 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.

Teacher Notes

• This lesson accompanies the October 2019 ChemMatters article “Cash, Chemistry, and Counterfeit”, 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.