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After teaching my high school chemistry students about properties of mixtures, and different methods of separating them, I wondered, Can I assess students’ understanding of chemistry by using hands-on activities?

In class, we’d learned about heterogeneous and homogeneous mixtures, as well as their inherent properties, and how mixtures could be separated based on those properties. Through a series of hands-on lab activities, students were able to explore topics such as chromatography, Tyndall effect, magnetism, boiling, and crystallization. It seemed only natural for the assessment to follow a similar hands-on pathway.

I decided that I would create a Mixture Separation Challenge as an inquiry activity, and let student teams devise their own methods of separating a mixture that I provided. Students worked in small groups of two or three in order to complete this challenge, which allowed each student to have a voice in the procedure, whereas larger groups might have hindered that collaboration. The groups were teacher-selected, to maximize time spent on-task, and to foster collaboration and communication.

A mystery mixture

I created an odd mixture of sand, iron filings, salt, and aquarium gravel, but did not tell my students what the mixture was composed of. I gave each student group a cupful of the mixture, and encouraged them to make notes of their observations about it. Many students chose to investigate the composition of the mixture using a wooden stick, spoon, or magnifying glass. Students were also allowed to touch the mixture or shake the container.

After the student groups examined the mixture, I had them share their initial observations with the whole class. Their observations included: black flakes, white powder, and rocks. To my surprise, students were also able to observe that some of the sand was coarse, while some was fine. After sharing their observations, students provided guesses as to what materials they thought were in the mixture.

Each group of students had the opportunity to share their ideas about the composition of the mixture, and afterward, I explained the actual composition. The aquarium gravel and the sand were the easiest parts of the mixture for students to identify; on the other hand, many groups were not able to visually identify the black flakes as iron filings, or the white powder as salt. Knowing the composition of the mixture was important, because this information would help them develop experimental procedures for separating the mixture. This understanding also allowed students to apply their prior knowledge and familiarity with the properties of each component.

Conducting the hands-on activity

Figure 1. Student completing the hands-on separation of the mixture

The goal of the activity was to have students apply their content knowledge to the separation task. Prior to starting, the only prompt that I provided students was, separate the mixture. We quickly discussed what needed to take place in each group prior to actually conducting the experiment.

In response, students shared thoughts such as, “Figure out what to do,” or, “Determine what materials to use.” I provided students with lab sheets on which to record their procedure, results, and reflection. When each student group felt comfortable with the plan they had in place, I had them get started.

Because this was an inquiry challenge, students needed the opportunity to explore and test out their own ideas. This meant that in my role as facilitator, I had to refrain from interjecting. Each student-developed procedure had merit: it was developed from a collaborative conversation, including student reflection about observations and using sound scientific content knowledge (such as that a magnet should be able to remove iron). The lab was conducted in a lab setting where it was safe to investigate variables. This allowed student groups to feel comfortable in testing their ideas through trial-and-error, iterating on their lab procedures and design process as needed.

I set up a variety of materials that students could use in their inquiry, including water, filters, forceps, hot plates, beakers, cups, magnets, ziploc baggies, and wooden stirrers. Pro tip: Provide access to a portable work surface, like a plastic lunch tray, to contain the mess. But make sure students don’t let the tray come in direct contact with the hot plate! Because there is value in giving students some autonomy, I also allowed them to brainstorm what type of materials they might need to use.

If you’re providing a basic set of materials, it is important to be open to adding in additional materials. Often students are more creative than we give them credit for, so if a student group thinks of a process that involves another item or piece of equipment that is available, encourage them to experiment! For example, I provided standard magnets for my students to use; however, one group asked about using a neodymium magnet instead, since we had used it for a previous lab. I suggested that this group use the standard magnets that I provided, but then repeat the procedure using the neodymium magnet, so they would have a standard of comparison.

As student groups worked on devising their procedures, I circulated around the lab. This gave me the opportunity to ask each group questions about their experiment. In my role as facilitator, I asked open-ended questions, mainly to be sure that students had a clear plan before they started their experiment. These included questions like, “What lab equipment do you plan to use?” and “Why?” Other questions referred to a specific step in their procedure, such as, “What do you think will occur as a result of that step?” I also reminded each group of safety procedures that they needed to follow when conducting their experiment. Safety protocols included: wearing safety goggles, making sure students knew how to safely use a ceramic-top hot plate, using thermal gloves when handling hot glassware, and not placing hot glassware directly on the countertops. Pro tip: Make sure students have prior experience using a hot plate.

Figure 2. Components of the mixture separated.

Each group approached the challenge directly, with varying degrees of success (Figures 1 and 2). As the teacher facilitating the activity, it was important for me to guide students in reflecting on the process itself. Every group demonstrated success, in that they showed they understood different ways that mixtures could be separated. When I checked in with each team at the beginning of the activity, I could informally assess their understanding of mixtures, as well as which component of the mixture would be removed during each step of the challenge. For example, students who chose to add water to their mixture were able to explain that the water would assist in removing salt through boiling. 

The student lab sheet was important in guiding students’ reflections on their procedure, so that the teams could see which processes were successful in separating individual components, and which procedures needed to be modified. Students completed the reflection and lab sheet as a group, and this provided a way for them to collaboratively discuss their experiment as a prototype.

For example, one group used a magnet to separate the iron filings. This was successful; however, the iron stuck to the magnet. They suggested putting the magnet inside of a plastic bag instead of directly placing the magnet in contact with the iron. Another group added water to their mixture in order to dissolve the salt. However, they were not able to successfully remove the iron with the magnet, because the entire mixture was wet. They understood what needed to be done in order to separate the mixture, and they were able to reflect that they should have removed the iron first.

To wrap up the activity, I allowed each group to informally share their ideas with the class. This was conducted through a lab “gallery walk.” This helped all students in the class to be able to visualize the results, and to see which parts of the mixture each group was able to separate. As a class, we traveled from table to table, circling around each team’s table while the group explained their procedure and shared out results.

In concluding the activity, it is important to make sure that students realize that each group will have different results. On the student lab sheet, I provided the opportunity for reflection, so that students could share which parts of their procedure were successful.

Drawing on the engineering design process, I viewed the original procedure as a prototype. So, as a part of their final reflection, students were encouraged to provide a description of how their procedure might be modified in order to more thoroughly separate the mixtures. If students began to conduct their experimental procedure, but then realized that they needed to change their plan, they were allowed to do so. The iterative process, and the integration of prototyping a design, allowed students to learn that there are many different types of engineers, including chemical and materials engineers.

Varying outcomes

Each student team should be able to achieve varying degrees of success in separating the mixture. When evaluating the activity, remember that the “failure” of any particular group’s experiment doesn’t necessarily mean that those students didn’t understand how to actually separate the materials. For example, if students use a magnet to separate the iron filings, that will work much better when the iron filings are dry. If students add water to the mixture, it becomes more difficult. The students still understood that a magnet was needed; the error was only in their experimental design.

I used this activity with high school students, but it could be easily modified for use in middle school. The materials are easily accessible and safe to use for middle school students. However, if a hot plate is used, the teacher may want to facilitate that station, and directly supervise the students’ activity.

Students really enjoyed having the autonomy to design their own experiment, and the freedom to conduct the lab in a safe and supporting environment. Students were also able to demonstrate their content knowledge while applying appropriate experimental procedures. As a result, this challenge served as an assessment, an inquiry-based engagement activity, and a way to implement the engineering design process.

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