Summary

In this lab investigation, students will explore how the color of curcumin, the molecule responsible for the color of turmeric, shifts in response to pH and temperature changes. Students will use their results to propose conditions for its use as an indicator in smart food packaging designs to detect spoilage.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
• Scientific and Engineering Practices:
• Asking Questions and Defining Problems
• Analyzing and Interpreting Data
• Constructing Explanations and Designing Solutions

AP Chemistry Curriculum Framework

This lab supports the following units, topics, and learning objectives.

• Unit 7: Equilibrium
• Topic 7.9: Introduction to Le Châtelier’s Principle
• 7.9.A: Identify the response of a system at equilibrium to an external stress, using Le Châtelier's principle.
• Science Practice 2: Question and Method
• Skill 2.F: Explain how modifications to an experimental procedure will alter results.

Objectives

By the end of this lab, students should be able to:

• Determine how changes in pH and temperature affects the equilibrium position in a chemical reaction.
• Determine if an equilibrium reaction is endothermic or exothermic based on how it adjusts to temperature changes.
• Use results of testing to propose conditions for a commercial application of curcumin.

Chemistry Topics

This lab supports students’ understanding of:

• Equilibrium
• Le Châtelier's Principle
• Establishing Equilibrium

Time

Teacher Preparation: 30-40 minutes

Lesson: 90 minutes

Materials

For teacher preparation:

• Organic turmeric curcumin powder, less than one bottle
• Ethanol, 95%, 750 mL per class of 24 working in groups of 2
• Hydrochloric acid to dilute for 100 mL of 0.010M per class section
• Sodium hydroxide pellets or solution to dilute for 100 mL of 0.010M per class section
• Distilled water

For 24 students working in groups of 2

• 72 test tubes (6 test tubes/group), at least 15-mL capacity
• 12 test tube racks
• 1500 mL curcumin stock solution
• 100 mL 0.01 M HCl
• 100 mL 0.01 M NaOH
• Hot plate
• Ice
• Thermometers
• 24 beakers, 400-600 mL size (cold and hot water bath for each group)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.
• Keep ethanol away from open flames as it is highly flammable.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
• When working with acids and bases, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• When diluting acids, always add acid to water.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Students should wash their hands thoroughly before leaving the lab.
• Safety Data Sheets:
• Ethyl Alcohol Denatured
• Hydrochloric Acid Solution 0.1 M - 2.4 M
• Sodium Hydroxide Solution Less than 0.3 M

Teacher Notes

Prerequisite knowledge for this lab is:

• Chemical equilibrium is a dynamic condition in which the rate of the forward reaction equals the rate of the reverse reaction, and the concentration of the reactants and products remain constant.
• Each reaction at equilibrium, at a specific temperature, has a certain K_eq (equilibrium constant) which represents a ratio of product to reactant concentrations at equilibrium.
• If you have not/will not introduce the equilibrium constant in a quantitative way, you should remove question #2 from the Prelab Questions, as it deals with values of Q vs K, and question #3 from the Analysis, as it relates to an equilibrium constant value.
• The equilibrium position refers to the relative amounts of all reactants and products in a chemical reaction once it has reached a state of dynamic equilibrium.
• LeChâtelier’s Principle states that an equilibrium system will respond to stressors by reacting in the direction that minimizes the stressor.
• More accurately, changing the conditions will cause a change in the rate of either the forward or the reverse reaction. This change is temporary until the new rate of one reaction catches up to the new rate in the other and they again reach equilibrium.
• When the change causes the forward reaction to be temporarily faster than the reverse, we call this a forward shift or a right shift. The opposite is called a backward shift or a left shift.

Preparation of the curcumin stock solution:

• For each class, make 1500 mL of curcumin solution (If you do not have a 2-L beaker or flask, you can break this up into multiple batches by scaling the amounts. The amount of turmeric does not need to be exactly as written here. It is more important that any batches to be used within a single class have the same amount. See “Solvent notes” below.)

•  Add 1.25 g of turmeric powder to 750 mL of 95% ethanol.
• There will be some residue left undissolved. This is from the other components of your turmeric.
• You do not need to filter it out, as it will not affect your results.
• It will settle quickly, so you can simply decant the turmeric/ethanol solution into another container and then dispose of the residue.
• Add 750 mL distilled water to the ethanol/turmeric solution to make ~1.5 L solution. (Note, total ethanol and water volume when mixed will be less than the sum of the two volumes, as they are not additive.)

Potential adjustments, enhancements, or extensions (Please read the “Notes on the Science” section below for reasoning!)

• If you do not have an extended lab period, it is suggested that you either assign for homework or take about 15 minutes at the end of the class before you want to start the lab to complete pages 1-3 of the student lab handout.
  • Pages 1-3 contain the background and pre-lab questions, as well as information to help students understand the task, which has an open-ended component to it.
  • On lab day, you should review the goals of the investigation and ensure students know what they are trying to find out through experimentation.
  • A reasonable expectation is that students complete their experimentation during the allotted class period (~40-50 minutes) and have all documentation completed.

• You might choose to assign the Analysis Questions for homework to be submitted or reviewed during the next class period.
  • The day after completing the lab, student groups should work together to review their documented results and compose a thorough conclusion.

• Based on your own expectations, you should clarify to students whether both partners should record the data and whether the conclusions will be written by the group or by each person individually.
  • It is a good idea to reinforce that students may not get the same results as other groups, as there are a lot of different ways to cause a color change.

• As a follow-up, or to increase the complexity of the investigation, you could also have students experiment on varied ratios of water/alcohol for the solvent. That would certainly be a step in the overall research as scientists investigate different materials in which to embed the curcumin. The ratio used for this lab was chosen for the best student experience (maximize color change and allow room for testing a range of pH changes, as described in the Notes on the Science below.)

• To make the pH more quantitative, you can use pH paper or pH probes. The lab is written in the most efficient form, using “added drops” of acid or base as a proxy for pH, to allow for maximum experimentation time within a single class period.

• As a possible extension, students could think about if they were to cook a dish with turmeric. One could design an experiment to see whether and how temperature and pH might interact to affect curcumin’s color while cooking! This could be tricky with effects of all the different oils, fats, etc. that would affect pH, but it would be an interesting undertaking!

Extra background information about the related science in this lab can be found in the downloadable version of the Teacher Guide. This section is intended for teacher reference only! This information is outside the scope of a high school or AP chemistry course but can aid the teacher in guiding the students toward productive investigations.