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We recently developed our “Pigment pH Puzzles” activity and introduced it during Pathways to Science, a two-week summer enrichment program for high school students interested in STEM fields. This particular activity was designed as an hour-long session for a group of 25 high school students to complete in a remote environment. Due to the COVID-19 pandemic, the activity needed to be safe to package in kits and send home to students; however, it can be used as written for in-person instruction as well.

As candidates for Ph.D.’s in chemistry, our desired learning outcomes were to communicate some basic chemistry concepts, including the scientific method, in a fun and engaging way that was also safe and relatively inexpensive.

Background and motivation

Prior to high school, students are often exposed to the basic concept of acids and bases, as well as examples of each (lemon juice, baking soda, etc.) — yet usually, much less emphasis is placed on how different molecules are affected by pH changes. With this in mind, we wanted to highlight that:

  • molecular structure is related to chemical property (in this case, color),
  • pH can affect molecular structure,
  • pH therefore affects different molecules and their properties, and
  • we can use this to our advantage (such as using different chemical indicators).

In addition to communicating these chemical concepts through an engaging and hands-on exercise, we designed a puzzle, encouraging students to use critical thinking, problem solving, and experimental design skills.

We want to share this activity with other educators, in hopes that they can incorporate it as a lab in a chemistry class or, as we did, as an activity in an enrichment program. We believe the activity could also be done with middle school students, although it might take them a little bit more time to work through the puzzle.

Overview of the activity

Figure 1. Each student received an activity kit that included “mystery solutions,” pH strips, and universal indicator strips.

First, we presented a short PowerPoint presentation introducing the topics of chemical structure and how it influences color, water ionization, and pH. The amount of time spent on each topic and the amount of detail will vary greatly depending on the prior knowledge of the participating students. Since we had a group with widely varying backgrounds and only a short amount of time, we gave a brief overview, presenting just enough information for the students to be able to complete the activity. (The PowerPoint slides are included as part of the teaching resource for teachers to use in their own classrooms.)

After covering the background, we briefly explained the objectives and procedure, which are discussed below. We then split up the group into breakout rooms in Zoom, each with about five students and one graduate student volunteer. In these rooms, the students worked individually on the puzzles for about 20 minutes before coming back together to discuss their findings. Although we completed this in a remote environment, the same process could be easily accomplished in person, in the classroom.

As shown in Figure 1, the puzzle consists of four different solutions that we prepared in advance: tap water, a solution of Borax, diluted lemon juice, and a solution of baking soda. We distributed these solutions in centrifuge tubes for leak-proof shipping, and labeled them 1-4, respectively. We also included five different pH strips: methyl red, bromothymol blue, phenolphthalein, thymol blue, and a universal indicator, labeled A-D and U. These materials were acquired from Thomas Scientific.

Solving the puzzle

The task for students was to identify which pH strip (A-D) corresponded to which pigment (we gave them the chemical structures, names, and color/pH correlations), and also to sort the four solutions from most acidic to most basic. We gave each student a worksheet to assist them in making their assignments.

In designing this activity, we wanted to create a puzzle for the students to work through themselves, so we left the instructions open-ended. However, we did design the worksheet in a way that helped the students draw their conclusions. Students generally either selected one of the strips to test by dipping it into each solution, or selected a solution and then tested each of the strips in it. They took notes about the observed color changes for each solution-strip combination. Using their collected data in combination with the color change information on the student handout, they were able to determine the identity of each test strip.

Generally, the phenolphthalein strips (C in our activity) were the easiest to identify. They should change from white to pink only for the Borax solution (2 in our activity). This also helped the students identify which solution is the most basic. Methyl red (A) never turns blue in any of the solutions, bromothymol blue (B) never turns red in any of the solutions, and thymol blue (D) changes to a different color in each solution. The solution in which both the methyl red and thymol blue test strips turn red is the most acidic (lemon juice, or 3 in our activity). The students used these and other observations (as shown in Table 1) to determine the identities of each test strip and to sort the solutions by relative acidity/basicity.

Test Strip A Test Strip B Test Strip C Test Strip D
Solution 1 Orange Light green No change (white) No change (yellow)
Solution 2 Yellow Blue Change (pink) Greyish blue
Solution 3 Red No change (yellow) No change (white) pink
Solution 4 Yellow Blue No change (white) Green

Table 1. Expected results for test strips A-D when dipped in each mystery solution.

Once all the students had discussed their findings with each other, we introduced the universal pH indicator and discussed how it is a combination of various pH-dependent pigment molecules. We followed this discussion with some “real world” applications of pH, such as ocean acidification, chemical leaks, and plant pigments (associated slides are included for reference). This concluded our presentation, and we left a few minutes at the end for questions and feedback on the activity.

Potential modifications

As mentioned above, we only had one hour for this activity, and were working with a very diverse group of students. If this activity were to be implemented with a different audience or experience level, there are many modifications that could be made. Some examples could include asking the students to guess what each of the liquid solutions are based on their pH, the students’ prior knowledge, and other observations (such as smell or additional experiments). During our activity, several students were able to correctly guess some of the solutions. For more advanced students, a discussion of pKa might be appropriate as well.

Additionally, the level of background chemistry and introduction can be modified based on student level. This activity provides an excellent opportunity to explain how color change results from a change in the chemical structure. Looking at the structures of the indicators, students can see that the molecules have two important parts. One is a chromophore, containing various rings and double bonds, and the other is an acid/base that can lose or accept H+. Upon gaining or losing an H+, the number of rings and double bonds changes, and that causes the color change.

Potential pitfalls

We only gave the students a limited number of pH strips, so it was important to emphasize using them judiciously (dipping only the end) and taking detailed notes throughout the experiment. In addition, pH strips are designed to be read while wet, and can change color as they dry. This may be confusing to students, so it is important for them to record their initial observations somewhat quickly. If too much time passes, many of the strips will turn yellow, which can be misleading. The initial color changes of the wet strips are clear and different enough to easily complete the puzzle, but teachers may find it helpful to test the reactions first, 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.


Students should wear safely goggles while handling chemicals, and wash their hands thoroughly before leaving the lab. When working with acids and bases, if any gets on a student’s skin, they should immediately alert you and flush their skin with water. All liquid materials in this activity are common household solutions and can be disposed down the drain; the solid materials (pH strips) can be thrown out with regular trash.


We asked the students to rank this activity on a scale from “I didn’t enjoy this activity at all” (which received no votes) to “Loved it! Please bring them back next year!”, which received 28% of the votes. Over all, 60% of students either loved it or really enjoyed it, which we found to be a very positive response. When compared with all of the enrichment activities the students participated in over the two-week program, this activity exceeded the average responses. Full data can be found in the table below. A few things to note about this feedback: the students attended their choice of eight enrichment sessions that encompassed a range of activities, from lectures, to demonstrations, to hands-on activities. The students also filled out the survey at the very end of the entire program, so influences such as recency bias could be a factor.

Feedback Session
I didn't enjoy this activity at all. 0 0.00 10 0.02
It was just okay. 2 0.08 76 0.12
It was pretty good. 8 0.32 215 0.33
I really enjoyed it! 8 0.32 191 0.30
Loved it! Please bring them back next year! 7 0.28 151 0.23
Total 25 1.00 643 1.00

Table 2. Feedback from students about the activity.


We developed a new activity in the form of a puzzle to teach both the concepts of pH as well as how chemical structure can determine physical properties. We led and presented our session virtually, but it can easily be used in the same form in in-person settings. This activity requires only safe, cheap, and available materials, making it accessible to a large range of groups.

Photo credit:
(article cover) mkabakov/Bigstock.com