What Is A Chemical Reaction?

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Objective

SWBAT determine if a reaction has occurred when two substances are mixed and provide lab evidence for that determination.

Big Idea

When two substances are mixed, a chemical reaction may or may not occur; there are specific things that we can look for to determine if a reaction occurs or not.

Why This Lesson?

Several Science and Engineering Practices (SEPs) are addressed by this lesson in which students mix to chemicals together and record observations in order to determine if those chemicals have reacted with each other.  First, students are planning and carrying out investigations (SEP 3).  After recording their observations, students are analyzing and interpreting data (SEP 4).  Finally, students must determine if the data they have collecting means a chemical reaction has or has not occurred between the two mixed chemicals.  During class discussion, students have the opportunity to engage in argument from evidence (SEP 7), using their own observations as that evidence.

While there are no Performance Expectations PS1 (PEs) that directly assess whether or not students can identify chemical reactions, there are several that do directly assume that students understand what happens during a chemical reaction.  For example, HS-PS1-4 asks students to understand that chemical reactions deal with changes in bond energies.  HS-PS1-5 asks students to explain how concentration or temperature changes effect rates of reaction.  HS-PS1-7 asks students to prove mathematically conservation of mass during a chemical reaction.  All three of these PEs then depend on a student knowing what occurs during a chemical reaction and a perfect introduction to get students thinking about what does occur involves direct observation of what is and what is not a chemical reaction.

Pre-Lab

5 minutes

Before students arrive to class, the following should be set up so that students can get right to work on their investigation.  I have long tables with chairs instead of student desks which serves well for students to work on this investigation in their seats.  I have students work at their tables in groups of about 3 for this activity because it allows for high engagement.  Mixing things to see what happens sparks an excitement that I want students to directly experience as much as possible, hence the smaller than usual groups.

For each lab group:

  • Two well-plates that are 4 x 3
  • Labelled dropper bottles of: barium nitrate
  • sodium hydroxide
  • copper sulfate
  • potassium iodide
  • silver nitrate
  • iron nitrate
  • hydrochloric acid
  • 3 safety goggles

The concentrations of the chemicals do not need to be high--I typically use anything in the range of 0.1M - 1M.

 

As students enter the class, they see the goggles, dropper bottles, and well plates on their tables.  I tell students to put on the goggles as they enter the room.  I also tell them to wait for further instructions before touching the rest of the equipment.  I have them take out something to write with and tell them to keep everything else off of the tables (bags, etc.).

When class begins, I hand out LAB - Observing chemical reactions to each student.  I explain that each table will work as a group to determine if a chemical reaction occurs when they combine two of the chemicals.  I show them that the well plates should be arranged in the same orientation as the layout of the observation tables and demonstrate combining the first two chemicals in the matching individual well using a document camera attached to an LCD projector so that everyone in the room can see.  

I emphasize that the chemicals should not be touched and that any spills should be cleaned up immediately with paper towels and water.  I tell them the story about how I went to prom with silver nitrate stains on my arm and had to wear make-up to cover the blotch on my face from an AP Chemistry mishap that happened the week before prom when I was in high school.

I explain that students should record all observations in the corresponding observation table.  I point out that even if nothing happens, that in itself is an observation, and should be recorded.  I ask if there are any questions, answer them, and allow students to work independently to complete their investigation.

Lab Work

40 minutes

Students work in groups of 2-4 depending on how many people are sitting at their respective tables.  This allows us to start in with lab investigation quickly without depending on students to choose groups or to move into lab stations without wasting time.  Since this investigation is time consuming and I want students to have enough time to thoughtfully analyze what their observations mean, it is crucial to start in with student work as soon as possible while still making sure they understand the expectations of today's activity.

Once pre-lab instruction concludes, I tell students to put on their goggles (already provided at the ends of their tables) and begin investigating.  As students work, I walk around to supervise and catch any glaring mistakes they may be making in their procedure, as well as ask them questions that help me gauge whether they understand what they are doing and what they are looking for in their observations.

Questions I ask (and anticipated answers from students):

  • Do you think any of these combinations reacted?  (Students should indicate some obvious combinations like sodium hydroxide and silver nitrate, sodium hydroxide and iron nitrate, copper sulfate and sodium hydroxide, and silver nitrate and hydrochloric acid.)
  • How do you know? (Students should point to clear changes that they observe, i.e. color change or precipitate forming--I don't expect them to use the word "precipitate," but they should notice a cloudiness.)
  • Are there any that didn't react? (Students should indicate that most of the combinations did not result in any noticeable change that might indicate a reaction had taken place.)
  • How do you know? (There was no change!)
  • Are there other indications of a chemical reaction that we aren't observing today?(We are not measuring temperature changes today and those might be indicative that a chemical reaction is occurring.)

 

Here is a short video of one conversation I had with a lab group as they finished the first set of chemical reactions:

 As students finish up their investigations, I guide them to clean up: washing out and drying the well plates in the designated sinks, replacing chemical dropper bottles in the appropriate storage tray, returning goggles to the goggle bin, and completing cleaning their tables with cleaning wipes.  Any groups finishing well before the rest are directed to start reading the back side of their lab handout and thoughtfully answering the questions.

Class Discussion

10 minutes

Once the lab groups have cleaned up, or when there is about 10 minutes left of class (whichever comes first), I direct students to settle down and I ask if they liked the lab.  I do this because I know this is the first wet lab that we have done this year and I want to acknowledge their enthusiasm for it.  I ask if they were able to observe any chemical reactions and expect a resounding yes.  I also expect students to point out that not every combination resulted in anything interesting happening.  In fact, more often than not, the combinations did not result in any observable changes.

I ask students to think about what they observed and determine if there were clearly combinations that did show something interesting.  I point out that the analysis question on the back of their lab handout asks for them to list which combinations seemed to produce chemical reactions AND to explain WHY they think those combinations did react.  I specifically ask them to use their observations as evidence because students need to know that scientific evidence is observational.  As conclusion, I ask students to synthesize what they learned today to see if they can come up with a set of parameters to look for when trying to determine if chemicals have reacted or not.  (In tomorrow's lesson we discuss today's results and look at demos of other reactions that yield results other than precipitates.  We will also discuss the limitations of some evidence in conclusively determining whether or not a chemical reaction has occurred.)

I have students turn in this handout today at the end of the class period because I want students to stay focused during class and think deeper about what the lab means before they are too far removed from what they did.

 

This student's work is an exemplar of diligent observation recording:

 

Here is an example of student work where I should have caught that she was not recording what the changes were, just that there were or were not changes:

 

Here is a sample of student work very similar to most of the responses I received:

 

 

I deliberately do not give my students a lot of background information about what to look for as evidence of a chemical reaction.  Some students have prior knowledge on the topic from middle school or 9th grade integrated science and I want to probe what they may already know.  I do this for two primary reasons: 1. I want to know how much time I might need to spend on this topic to make sure my students understand (and I also want to spend less time if they need less), and 2. I want to identify any misconceptions that might exist in my students' thinking.  Here are a wide variety of samples from my students: