In this lesson students will take notes about the differences between physical and chemical change. They will then look at a number of processes involving either physical or chemical change and use evidence to decide which kind of change occurs. They will then make "slime" as an example of chemical change. This lesson corresponds to the NGSS Practice of Engaging in argument from evidence because students have to defend their choices when they describe something as physical or chemical change. Students will not need prior knowledge for this lesson.
Materials list for the hands-on activity for each student:
30 g white glue
4 grams borax
food coloring (optional)
At the start of class students are tasked immediately with a Do Now: Read Procedure for Making Slime and draw a cartoon using the Slime Lab Cartoon graphic organizer to show you understand the procedure. This serves two purposes. First, it engages students--making slime is fun. Second, it helps to center everyone around a task that will be useful later on in the class while I settle students and take attendance. Here is an example of student work in response to this prompt.
In order to give students some context for the differences between chemical and physical change I give a brief lesson using Chemical vs physical change slides. Students record their notes in a Chemical vs physical change notes organizer.
One of our main missions at our school is to prepare students for college. I go through the presentation at an ordinary pace, and I encourage students to keep up as best they can, and to not get discouraged if they fall behind in note-taking because they will have a chance to review everything after the presentation. What they need to understand is that they will probably not have that luxury of reiteration in college, and they therefore need to get faster at taking notes. It is a skill that is partially based on practice and partially based on learning how to process and reduce notes by using abbreviations, arrows, and other time saving devices.
After discussing the points in the slides, I ask students how they did. I post the Chemical vs physical change notes organizer answers so they can do quality control on their notes.
Once students have their notes, I ask them to use their notes and their prior experiences to decide whether each of the items listed in the Physical or Chemical Change practice exercise is physical or chemical change. I explain that some of the examples might be difficult for them to decide, and so they will need to record their thinking in the evidence column so that we can go to correct the work they will have a record for how they were thinking about the example.
Students work on this for about 10 minutes individually, and then I ask them to compare answers at their tables. After they have discussed their answers, I post the Physical or Chemical Change Answers on the Elmo and ask the class which examples were most perplexing. Bleaching hair is a good example--many students are not aware that this process involves chemically altering the hair. I also ask students to share some of their evidence with the class, and chart indicators of chemical and physical change. This example of student work shows a good use of evidence to justify the student's answers.
After this exercise I release students to make slime. The different materials are set up in several locations around the room so there is room for students to access the materials without me having to do too much in the way of lab setup. They have about 15 minutes to make the slime and clean up, so how they use their time is really important. I warn students that if they have not made their two solutions after seven minutes then it is too late and they should move into clean-up mode. Clean-up is simple--they can flush the liquids down the drain, and most students take their slime home with them in reused plastic shopping bags I provide for that purpose. Student use warm water and a scrub brush to clean out their plastic cups. Students do not make any written observations for this lab.
This is a pretty tight class when clean-up and play time is factored in, so I keep the debrief short. However, it is important to re-emphasize the attributes of chemical and physical change. When new substances are created, chemical change has occurred. When no new substance has been created, this is typically physical change. I remind students that even though phases can appear quite different from one another, they still have the same chemical composition, and therefore phase change is a physical change. When asked if the creation of slime is a chemical or physical change, students recognize that this is an example of chemical change because two liquids combine to form a material that feels solid.