Solidifying Our Understanding of Physical and Chemical Changes

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Objective

YWBAT: Model and explain the different between a chemical and physical change

Big Idea

Sometimes students need the opportunity to reinforce their understanding of how matter changes. This lesson serves as a formative assessment strategy to assess students' abilities to explain the difference.

Overview

This lesson allows me to check students' understanding of physical and chemical changes. My students always sit with their assigned group, so if you don't already have desks configured like that it may help if you do this before class.  

Each group gets a different example of a physical or chemical change and create evidence-supported arguments within each group to determine what type of change is taking place. Students record their thoughts and observations on this sheet, which I then collect to assess their understanding and provide feedback.

Do Now

5 minutes

As students develop understanding of how matter changes, we need to constantly have them refer back to the chemical make-up of the matter they are dealing with. In order to determine if a physical or chemical change is taking place, I want my students to understand that chemical composition (molecular makeup) changes in a chemical change, which affects the properties and physical appearance of the new matter. Ultimately, students will be using the evidence collected from this activity and the lesson being described now to explain the Law of Conservation of Matter using CER. But first, students must establish a solid understanding of physical and chemical changes to determine what is happening to the chemical composition of matter when it is changing.   

To activate prior knowledge, I ask the following question as the Do Now:

What is the difference between a chemical and physical change?

Students record their answers in their notebooks and I have them discuss their thoughts with their groups. If there are disagreements, they have to work those out as they support their arguments with evidence from their own research and what is displayed on the handouts that each group will get.

Introducing Group Activity and Flow

3 minutes

I explain that today we are going to test our understanding of various changes that take place in matter to see if we can determine the type of change taking place and how we can support our ideas with evidence.

Each group gets a piece of paper that describes a type of physical or chemical change. As a group we discuss what is happening during the change and record our thoughts on this handout, which contains all of the materials needed to implement this lesson.

Rationale: I want to check for understanding here, so I am not requiring students to make their own tables. This is more about creating evidence-supported arguments.

After about 4-5 minutes, we switch clockwise and repeat the same process until we have gone through all 6 examples of changes.

We then review as a class and I collect responses to give feedback.

Students Analyze Examples, Present Arguments and Check for Understanding

35 minutes

As students are working together to determine what type of change is occurring to the matter in their example, I am circulating around to each group and checking for understanding.

Here is a video of students discussing what type of change tarnished pennies would be:

Students are not considering everything in the picture, which is preventing them from developing a deeper understanding of physical vs. chemical changes. Simply asking the question, "What about the sawdust?" helps students to determine that the wood was just being cut into smaller pieces of the same matter, therefore, further supporting their claim that it is a physical change.

If students are struggling to determine if a physical or chemical change is taking place, I generally ask them to explain what is happening to the matter--does it have the same chemical composition or a different one? Do any of the other properties change?  What do you notice on the reactants side compared to the products side?  How can we represent this visually with particulate modeling?

I do my best to let students figure it out for themselves and listen carefully to their arguments.  If groups cannot come to a consensus then I ask the probing questions above and walk away. This allows them time to reflect and refine their ideas.

Wrap-up

5 minutes

I first collect everyone's work and we review the concepts as a class.  I project the examples on the board and we try to determine if a physical or chemical change is taking place.  I require students to defend their ideas with evidence.

Homework:

I ask students to write reflections about how their understanding of physical and chemical changes have changed since the beginning of class.