Classifying Evidence of a Chemical Reaction

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Objective

SWBAT articulate specific signs that a chemical reaction has occurred based on the evaluation of their data from prior lab investigation.

Big Idea

Four observable indicators of chemical change are temperature change, color change, formation of a gas, and formation of a solid.

Why This Lesson?

This lesson provides follow-up to the first lesson in this unit, What is a Chemical Reaction?, and addresses two specific Science and Engineering Practices (SEPs).

In this lesson, students are using their observations from the previous day's investigation to construct explanations (SEP 6) about what constitutes evidence of a chemical reaction.  Students are also engaging in argument from evidence (SEP 7) as we determine as a class what does classify as evidence of a chemical reaction.

There are no specific Performance Expectations (PEs) that are addressed by this activity, however, several PEs do expect students to show a deeper understanding of chemical reactions (particularly HS-PS1-4, HS-PS1-5, and HS-PS1-7).  This lesson builds a basic understanding of what evidence we use to evaluate whether or not a chemical reaction occurs when two or more substances are mixed together.

Warm-Up

5 minutes

While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books.  I use warm-ups to either probe for students' prior knowledge about the day's upcoming lesson or to have them bring to mind and review what they should have learned previously.  (To read more about Warm Up and Reflection Books, please see the attached resource.) 

Today's Warm-Up: "How can we tell if a chemical reaction occurs when we mix two substances?"

In this case, the warm-up is asking students to recall the previous day's lesson in which they completed an investigation to determine whether or not chemical reactions occurred when mixing different combinations of chemicals.  It is also preparing students to evaluate what they determined to be evidence of a reaction in order for us as a class to create a list of possible signs a reaction occurs.

If time permits, I walk around with a self-inking stamp to stamp the completed warm-ups indicating participation, but not necessarily accuracy.  On days when there is too much business keeping, I do not stamp.  Students have been told that warm-ups are occasionally immediately checked and other times not.  At the end of each unit, Warm-Up/Reflection Books are collected and spot-checked.  

 

This student response includes reference to "shape change," which is a physical change.  The responses from my class were similar to this one, indicating that I need to clearly explain the differences between physical and chemical changes.

Again, this student includes change in texture and color, both of which could be chemical or physical changes.  Right now, I am satisfied that my students are thinking about things that could possibly indicate chemical reactions.  I will need to address their obvious misconceptions/incorrect ideas as the lesson progresses.  The answers to this warm-up, however, indicate that students are thinking back to yesterday's lesson (where they coupled different solutions to look for evidence of a reaction) and applying their observations to answer today's warm-up.

Class Discussion

15 minutes

After stamping the warm-up, I ask for volunteers to share what they wrote.  I have an established protocol for discussion warm-up answers.  Students raise their hands to share, and once there are at least five hands in the air, I make a mental note of who volunteered and begin calling on students.  I do not always just call on volunteers and will sometimes call on a student who has not shared recently.  My reasoning for doing this is to make sure that the same students are not always the ones who are sharing answers.  This strategy also allows students to volunteer specifically when they feel confident about a particular answer.  Because I do vary whom I call upon, I am still holding students accountable to share sometimes even if they are not enthusiastically volunteering.

As students share, I ask them to only share one thing that they think is evidence of a reaction.  I do this so that as many people as possible can share instead of allowing one student to provide all of the answers.  I document their answers on the white board so everyone can see.  As we add examples, I check with the class to add anything missing, until everyone agrees all examples are included on the board.

As answers are collected, I do not comment on the validity of their answers.  I simply record.  Today's list included:

  • bubbling/fizzing
  • cloudiness
  • color change
  • temperature change
  • pH change
  • disappearing 
  • mass change
  • solid forming

I revisit this list as I proceed in the demonstrations today.

 

 

Demo & Class Discussion

15 minutes

In order to emphasize signs of a chemical reaction, I ask students what served as evidence of a chemical reaction in yesterday's lesson.  They should respond that they observed cloudiness (or a solid forming) and color changes.

Students wear safety goggles, as do I, since we will be passing around the demonstrations.

I present the following four demonstrations.  For each demonstration, I stand in the middle of the room, in between the two sides of tables that students sit at.  I begin by stating what I am mixing.  After mixing, I ask students to note the evidence of a chemical reaction, and then I refer to their list on the board.

First, I mix about 50 mL of copper sulfate with 50 mL of sodium hydroxide.  The combination creates a dark aqua precipitate.  Students should note the formation of a solid and the color change as both signs of a reaction.  

Next, I mix a couple of spoonfuls of ammonium nitrate with water.  The combination results in a cold solution that often has condensation forming on the outside of the beaker.  As this beaker is passed around, students should note the temperature change.

To continue investigating temperature change, I mix a couple of spoonfuls of calcium chloride with water.  The combination results in a warm solution.  As this beaker is passed around, students should note that a temperature change can be increasing or decreasing.

Last, I put a couple of spoonfuls of baking soda in a small plastic cup.  I put about 150 mL of vinegar in a zipping sandwich bag.  Before zipping the bag, I set the plastic cup inside, careful to keep the baking soda from mixing with the vinegar until after I completely close the bag.  Then, I dump the cup and allow the reactants to mix.  As the reaction creates gas bubbles, the bag itself begins to blow up.  Students should note the formation of a gas.

We revisit the student created list on the board.  I address all of the signs that students submitted and we decide what we just witnessed and what we can include in our final list.  I give counterexamples for things that are not considered evidence of a reaction (i.e. adding vinegar to water changes the pH of the solution, but the chemicals are not reacting so pH change does not make our final list).

Student Reflection

5 minutes

In student's Warm-Up/Reflection Books, students should spend about 3-5 minutes writing a response to the day's reflection prompt.  Prompts are designed to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking.  The responses also allow me to see if there are any students who are missing the mark in terms of understanding.  The collection of responses in the composition books can also show a progression (or lack thereof) for individual students.  

Today's Reflection Prompt:  "Describe an everyday real-life process that you think might be a chemical reaction and explain why (what evidence do you have?)."

Desired student responses should include a justification that includes evidence of any of the following:

  • solid forming
  • gas forming
  • temperature change
  • color change

Sample student work:

This student sites temperature changes as evidence that cooking involves chemical reactions, but has missed the idea that the temperature changes should not be because we are externally manipulating it.  I will need to emphasize that the temperature change we would consider evidence of a chemical reaction would NOT be caused by heating or cooling--heating or cooling would be caused by energy flow (addition of thermal energy or removal of it), and not necessarily a chemical reaction.  

This student mentions bubbling, but is referring to boiling water, which is a physical change again.  As I reviewed the student answers today, I realized that understanding chemical and physical differences can be fairly confusing for students and they need a context for understanding what is causing the changes in order to understand what the changes indicate.