Lesson 3 of 5
Objective: SWBAT track energy changes in an open calorimeter when mixing water of different temperatures.
In an effort to give a day for the concept of specific heat the chance to percolate and settle in students' brains, today we do a lab where it is irrelevant. This is a simple, open calorimeter with students mixing samples of water at different temperatures and masses.
Since we are so crunched for time at the end of the semester, I do not discuss heat loss due to being an open system, or heat loss to the calorimeter. If we had more time, I would love to delve into this further. This would make a great extension for an honors class to push them into error analysis.
The experiment is exceptionally safe and easy. All you need are:
- 400mL beaker
- Two styrofoam coffee cups
- Stirring rod
- Thermometer or temperature probe
- 100mL graduated cylinder
I construct the calorimeter as shown. The beaker provides a base so the cups don't tip over with the tall stirring rod and thermometer in them.
- 400mL beaker
- Hot plate
- Hot hands/Pac-man
For the room:
- Hot plate with large Erlenmeyer flask (2000mL) full of water
- Hot hand
The last piece of setup I decided on after the first class of the day. Tables went through the 300mL of hot water very quickly, and it took a long time to heat additional tap water. By having a large reservoir of hot water, the lab procedure was able to be run much faster. For safety, I poured this water out as it was a large, heavy set up.
This lesson connects to Science and Engineering Practice 3, plan and carry out investigations, Science and Engineering Practice 4, analyze and interpret data and Science and Engineering Practice 6, construct explanations. It also aligns with the Energy and Matter Cross Cutting Concept: Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system. This lesson continues our exploration of HS-PS3-4, plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system.
Pre-Lab and Safety
When students come in, I have them pick up a copy of the Calorimetry Lab and head to their seats. I am making sure the hot plates are all on and heating properly without boiling.
I show students the calorimeter set up and explain the basic set up of the lab. They will be mixing water of various temperatures and amounts in the calorimeter. I ask why we have two coffee cups in each other and students respond to hold the heat in. I show them why the set up is in the beaker by letting one fall over without the beaker.
I explain that they can use tap water for their other water source beyond the hot bath. I explain that we are missing ice for activity 3, so it is up to the students to figure out how to get temperatures different from the introductory activity.
Due to the hot water, I remind students to be wearing aprons and goggles, and then invite them back to the lab.
Students come back, and get their safety equipment on. I remind the class that steps one and two are done for them, and that they will need a timer/phone to time the water after mixing. Some students ask for a reminder about what the graduated cylinder looks like.
Most groups work just fine and record their data in the data table, or alongside the procedure steps and then transfer it to the data table.
This is fairly typical data, although many students found a final temperature exactly half way between the starting temperatures of the two samples. It may be lower due to the open nature of the calorimeter allowing heat to escape.
In investigation 2, students are allowed to vary the amount of each water sample to see how that affects the final temperature of the mixture. The only limitation is that they cannot exceed 200mL total (the capacity of the small coffee cups). This choice is evidence of continued growth along Science and Engineering Practice 3- planning and carrying out investigations. Students are given some restrictions, but get to plan their experiment within those restrictions.
Students struggle a bit at first, but once one or two groups figure out the criteria, the rest of the students quickly catch on or ask for help. In setups like the one below, I ask students which sample they expect the final temperature to be closer to. Every group expected a temperature closest to the larger sample, which tells me that they really have conceptualized how the sample size affects the energy stored in the sample.
For Investigation 3, students were to return to 100mL starting samples, but vary the temperatures of the starting conditions. Since we didn't have ice, students ran the tap to get it as cold as possible. Other groups decided to crank up the hot bath and get it as hot as possible.
This student sample data ends almost exactly at the average from the starting temperatures. Given more time and mixing, it likely would have stabilized at 54.5 degrees.
Allowing students to decide the starting temperatures provides an opportunity for a rich debrief, as each group will find similar results, with the temperatures stabilizing near the mid-point of the initial temperatures, regardless of the actual initial temperatures. Sharing this data as part of a class discussion the next day could help strongly cement the concept of conservation of energy within the system.
Analysis and Conclusions
Once students finished and cleaned up their station, they moved back to the front of the classroom to work on the analysis and conclusions. I thought the first question about whether or not the system was open or closed was a no-brainer, but students really over thought it, and locked up in what I refer to as "chemistry brainfreeze" where the mere connection to chemistry makes students lose all common sense.
When I prompted "Was the calorimeter open or closed?" they looked at me, paused and replied "Open?"
- "There was no lid?"
Students then got more comfortable and stopped overthinking the questions and did well on the next two, as evidenced below.
Students were less confident in their response to number 4, and since my 2nd period spent so much time waiting for water to reheat, didn't get to it. They generally recognized that the final temperature still ended up close to the middle of the two starting temperatures.
Student responses to question 5 were very inconsistent. They understood that the heat transferred from hot to cold, but did not always get to the molecular level explanations of high energy (hot) molecules colliding with low energy (cold) molecules and speeding them up. This was an area I knew I needed to follow up on in the following lesson.
The conclusions were also inconsistent. Many students referred to heat, temperature and molecular speed without really relating them clearly. This student could have used some stronger structure to the paragraph, but they had the gist of the relationships.
When they finished, students turned in their papers so I could grade and assess their understandings to plan the next lesson.