Chemistry in miniature! This micro-chemistry demonstration shows students how to differentiate between physical and chemical changes using seven toxic solutions. As students analyze and interpret data (SP4), about the properties of substances before and after the substances interact to determine if a chemical reaction has occurred (MS-PS1-2), they recognize that the macroscopic patterns students observe are related to the nature of microscopic and atomic-level structure (CCC).
This lesson is an introductory demonstration that assumes students have a basic understanding of properties of matter (Mystery Substances - Properties of Matter Investigation Part 1 and Part 2), atomic structure and chemical formulas. Additionally, this lesson is best used in conjunction with a subsequent student driven inquiry-based lesson on the topic of chemical reactions to further explore this sophisticated concept.
In order to ENGAGE students in this lesson, students view Flaming Gummy Bear:
The first viewing is always exciting! Students view the video a second time with the purpose of:
Determine if you are observing a physical or chemical change. Provide evidence for your claim.
In this video, students are observing a physical and chemical change. Discuss with the class how the melting of the potassium chlorate is a physical change. It is not necessary for students to fully understand this reaction on a molecular level. The purpose of this discussion is primarily for students to differentiate between a physical and chemical change.
Challenge students to recall that a physical change is a change that can be observed without changing the substance into a new substance. In this case, the substance is melting, or changing form, but is still potassium chlorate. Eventually, the potassium chlorate (KClO3) decomposes (breaks down) into potassium chloride (KCL) and oxygen (O2). This is the first chemical change (chemical reaction). The heat and oxygen allows the sugar in the gummy bear to combust (burn) which is a secondary chemical reaction. Challenge students to recall that chemical changes result in new substances being formed. To finalize the engagement stage of the lesson, students generate a list of clues of chemical reactions observed in the video or that they've previously observed.
Teacher Note: This lesson is based on demonstrations adapted from the Lab Aids Kit: Identification of Chemical Reactions (Kit #84). This kit includes all of the materials required to complete the lesson as a demonstration or student investigation. Due to the nature of the chemicals, expense and introductory nature of the lesson, I chose to conduct this lesson as a guided demonstration to provide students guidance in terms of accurate note-taking, support making accurate observations and class discussion to link the collected data to the claims students make about whether physical or chemical changes occur. This kit could certainly be replicated by buying individual components used during the exploration.
The EXPLORE stage of the lesson is to get students involved in the topic so that they start to build their own understanding. To help students explore physical and chemical changes, students participate in an active demonstration in which seven known solutions are combined with each other (21 possible combinations). This video explains how to set up the demonstration:
The first part of this exploration is completed in Data Table 1 of Identification of Chemical Reactions Demonstration Student Handout as shown here:
The second part of this exploration is completed in Data Table 2 of Identification of Chemical Reactions Demonstration Student Handout. Students record data as seven known solutions are mixed into all possible combinations. After each of the known solutions are mixed, each of those solutions is mixed with the unknown solution.
Teacher Note: Demonstration of Chemical Reactions Teacher Key
The EXPLAIN stage provides students with an opportunity to communicate what they have learned so far and figure out what it means. This stage of the lesson presents a great place for a quick formative assessment. To explain what they have learned, students complete the final explanation on Data Table 2 of Identification of Chemical Reactions Demonstration Student Handout. This explanation requires students to analyze their data in order to identify the unknown solution and provide data-based evidence to support their claim. Discussion about the data analysis process and how students were able to accurately identify solution #5 as the unknown solution can provide significant insight into whether students understand the difference between physical and chemical changes. Probing questions such as these encourage students to develop their thinking:
How did you determine the identity of the unknown solution?
What evidence can you provide that supports your claim?
How do you know the unknown solution isn't solution #____?
What types of physical changes did the unknown solution undergo?
What types of chemical changes did the unknown solution undergo?
How could you tell the difference between physical and chemical changes of the solutions?
The EVALUATION stage is for both students and teachers to determine how much learning and understanding has taken place.
Due to the introductory nature of this lesson, this lesson is best used in conjunction with a subsequent student driven inquiry-based lesson on the topic of chemical reactions such as Physical and Chemical Changes Investigation.
At the conclusion of this activity, a check-in quiz may be beneficial to check for understanding before moving on. A quick assessment related to physical and chemical changes is here: Properties and Changes of Matter Quiz.