Summary

In this lab, students will use their scientific detective skills to determine the identity of pigments used in various types of pH test strips based on how they each interact with several solutions of different pH values.

Grade Level

High School

NGSS Alignment

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

• HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

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

• Indicate that molecular structure is related to chemical property (in this case, color).
• Explain how an indicator works.
• Analyze and interpret data in order to identify unknown substances.

Chemistry Topics

This lab supports students’ understanding of:

• Acids and Bases
• pH
• Indicators
• Chemical Properties
• Physical Properties

Time

Teacher Preparation: 45 minutes

Lesson: 60 minutes

Materials

• Thymol Blue pH Strips, 5 per student
• Methyl Red pH Strips, 5 per student
• Bromothymol blue pH Strips, 5 per student
• Phenolphthalein pH strips, 5 per student
• Lemon Juice, small bottle
• Tap water
• Baking Soda, small box
• Borax, small box
• 5 mL centrifuge tubes or test tubes, 5 per student
  • Note: centrifuge tubes were used as leak-proof containers because materials were shipped to students remotely

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, all material can be disposed by either pouring down the drain in the case of liquids (common household solutions) or throwing in regular trash in case of solids (pH strips).
• 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.
• Consult the SDS for the chemicals used:
  • Baking Soda
  • Borax
  • Lemon Juice
  • Thymol Blue
  • Methyl Red
  • Bromothymol Blue
  • Phenolphthalein

Teacher Notes

• To learn more about this lab, read the associated article, Pigment pH Puzzles published as a Resource Featured in the November 2021 issue of Chemistry Solutions.
• Background/Introduction (see PowerPoint): The PowerPoint slides included are meant to be an example of some of the topics that could be covered. The depth and breadth of the topics should be adjusted to the level of the students. We included a brief introduction followed by a short overview about chemical structures and bonding (line structures specifically). We then gave an introduction to pH (adding/removing H⁺) before presenting the activity. Following the activity we discussed some “real world” applications of pH such as ocean acidity, chemical leak detection and plant pigments such as cabbage, violets, and hydrangeas.

• Students will be given four centrifuge tubes (or test tubes) with different premade solutions labeled 1-4, and four types of test strips labeled A-D.  
• On the student handout students are provided with four chemical schemes and the color schemes for the different pigments used in the pH strips. Their task is to (a) figure out which scheme/structure corresponds to which strip, and (b) rank the solutions in order of their pH.
• We encourage teachers to complete the tests in advance with your own solutions, to ensure that you get the color change results that are expected.  
• An answer key has been provided for teacher reference (using the unknowns as labeled below), and can also be found in the PowerPoint document on slide 11.
• The activity is designed for students to figure out the puzzle (use their data to label all unknowns correctly), so providing limited instruction is recommended.
• The teacher should create 4 unknown test solutions, as described below. The test strips must also be organized and labeled.

• Tips and common pitfalls:
  • We only gave students a limited number of pH strips, so it was important to emphasize using them judiciously (dipping only the end – the other end can be used later if need be to double check) and taking detailed notes throughout the experiment.
  • pH strips are designed to be read while wet. They can change color as they dry which may be confusing to students, so it is important to record initial observations.
  • The color changes should be clear and different enough to easily complete the puzzle, but it is worth testing it first as the teacher as changes in concentration of the acid/base solutions may lead to varied results. In our experience, all of the students were able to solve the puzzle based on their observations as long as they did a good job keeping track of their data.