In this lesson students will conduct a brief mini-lab to reconnect to the idea that molecules are moving. They will then work on designing an experiment that measures the reaction rate between calcium carbonate and hydrochloric acid when either temperature, concentration, or surface area are manipulated.
This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs because the experiment students design will examine one of these variables. Students will have to relate what they see at the macroscale by explaining what is occurring at the nanoscale.
It aligns to the NGSS Practice of the Scientist of planning and carrying out investigations because students will be start to plan their experiment during this lesson.
It aligns to the NGSS Crosscutting Concept of Stability and change because rates of change—in this case the production of carbon dioxide—can be quantified and modeled.
In terms of prior knowledge or skills, students should have a basic understanding of atoms and molecules, and chemical reactions.
The materials needed for this lesson include the following: food coloring, different temperatures of water (hot, room temperature and cold).
Do Now: Students begin class by reading the Dissolving Rate Mini-lab directions and highlighting what they will be doing.
I reason that this is a good way to start class because this is the first day of a new unit. I want them to have something to do that relates to today’s class.
Activator: Once I have tended to administrative duties like attendance, I invite students to come and chat with me in a huddle. I tell them that I am pleased with how our nuclear unit ended, and I tell them that now we will be going back to chemistry that involves chemical reactions. I ask students to explain the lab back to me that we are conducting today.
I have chosen this approach because it is a nice way to get students to lead the class. A diversity of voices in the classroom makes for a more positive and inclusive learning environment. I add a few details: they can use the microwave if they want to get hotter water than what comes out of the tap, and they can use ice to cool water if they want it to be colder.
Mini-lesson: I tell students that before they do the lab I would like them to watch a brief video. We watch it and then I ask them to tell me what they saw. They note the word globule and we practice saying the new word. They then note that water molecules are moving, and that this movement is what makes the fat molecules move.
This instructional choice reflects my hope that students will make a connection to what is happening at the nanoscale to what they are about to see at the macroscale in their mini-lab.
Student Activity: At this point students run the lab. As this mini-lab video shows, the temperature differences in the water lead to dramatic and visually appealing results. During this time I walk around and interact with the different lab groups and make observations. I check to see how well students are following the procedure, and I try to guess which cup contains which temperature of water. I am usually correct. One group does not know which cup is which, and they claim my guess was wrong, but I touch the cup and prove that my visual observation is correct! Some students accidentally bump their lab bench, which makes the water move visibly. I note that this is considered an experimental error, and errors like this do happen.
I want students doing this work because I want them to see for themselves that there is motion in a cup of water even if they do not normally see it.
After the mini-lab we debrief the experience. As evidenced by the mini-lab debrief video, students are making a connection between temperature and the rate that the food coloring dissolved—hotter temperatures lead to a faster rate. However, students were less sure about why this was. I note this and know that in a future lesson in this unit students will read more about kinetic energy and temperature, and so I do not force the answer. My expectation is that in a future lesson we will come back to the idea of molecules in motion, also known as kinetic molecular theory.
To wrap this lesson up I let students know that I will be attending the National Science Teachers Association annual conference in Chicago for the next two class days.
I explain that their major assessment for this unit will be a lab report that reports on the results of an experiment they do. The purpose of the experiment is to see how a variable affects the rate of a reaction. I show them their Reaction Rate assignments that has their names next to one of three variables—temperature, concentration, or surface area and ask them to record their variable on their Reaction Rates Experiment.
I ask them to read the directions and make note of any questions that they have. They do not have any. I tell them that they should first work on the Content Reading Questions, and that they can then use the information from the reading questions to inform their choices for Part 1 of the lab report (the experimental design). I note that when I return we will critique their experimental designs using the LT 2 Reaction Rates Experiment Grading Sheet, which will serve as their lab report rubric.
Ending class this way allows me to keep students moving forward while I am at a conference.