In the Heat of the Summer: Boiling Point Test
Lesson 3 of 6
Objective: Students will use the concept of boiling point to see if evidence from a crime scene matches evidence found in the possession of four suspects.
In this lesson students will continue a series of lessons in which they try to solve an attempted murder mystery using the packet In the Heat of the Summer Forensics Lab. This unit began with a glass density lesson. In today's lesson students read about how someone tried to burn a professor's house by putting an accelerant on it. The suspect was foiled, but in his or her haste to escape the perp dropped the accelerant. Police identified the boiling point of the accelerant and now are analyzing unknown liquids found in four suspects' possession.
This lesson aligns to the NGSS Science Practices Analyzing and interpreting data, and Planning and carrying out investigations because it requires students to carry out an investigation about boiling point and then interpret their data. While it does not focus on the intermolecular forces that account for different boiling points, it does introduce the idea so that later in the year when students study this relationship they will already have a specific experience to refer back to.
Going into this lesson students have already studied boiling point using this lesson.
To do this lab students will need the following:
- safety goggles
- hot plate
- 250 ml beaker
- 4 test tubes
- 4 mystery liquids ( I used isopropyl alcohol, ethyl alcohol, each with a drop of food coloring, and two different colored waters, but other types of alcohols could be substituted for the waters)
The unidentified liquids found in the suspects' possession, and the liquid found at the crime scene, are as follows:
Ms. Marian Goldbloom and crime scene: ethyl alcohol
Mr. Howard Johnson: isopropyl alcohol
Mr. James Hilton: red salt water
Dr. Diana Jackson: blue salt water
Here is the Teacher Guide: Master Data Sheet, which is a document which summarizes the evidence for each suspect for all of the lessons in this unit.
Do Now: Students read read the Investigation Log and Investigation notes for June 28 from the In the Heat of the Summer packet so that they can ground their work today in the larger purpose, which is to develop an evidence-based argument over the course of an investigation. The argument's thesis is that a District Attorney should or should not prosecute a suspect based on the evidence. The reading also is useful for student engagement. Students enjoy trying to solve a mystery.
Activator: I use cold call (randomly calling on students) for the following questions, and get answers like the ones below. I do this to check for understanding, to publicly have some fun, and to help students who started class slowly to catch up.
What was the attempted crime? Someone tried to start Dr. Rodriguez's house on fire
How was the perp thwarted? D. Rodriguez discovered the crime when he awoke.
What evidence do police have? An accelerant whose boiling point is 78.4oC.
What did an investigation of the suspects reveal? each had an unknown liquid in their possession
Mini-lesson: Students conducted a boiling point test already in this lesson and so the purpose of the mini-lesson is to review boiling point. I start by asking the class "How do you find the boiling points of various liquids?"
They explain that last class we had three different liquids, one in each test tube. The test tubes sit in a water bath that they heat on a hot plate. They recorded the temperature and noted the coolest temperature that each liquid began to boil. They then matched the temperatures to known values for liquids to identify the unknown liquids.
I then suggest that there were some challenges with that lab, and ask for feedback about that.
Students note that the bubble were hard to see the test tubes, and suggested that if we separate out the test tubes it might be easier to see. Some students suggested using a bigger beaker. One suggested that students need to pay better attention to the test tubes because they produce really small bubbles. A conversation about how to pay better attention yielded the fact that constant monitoring at eye level is important.
I note that some students missed the boiling point and that if this happens eventually the material will stop boiling. I also note that I added a drop of dye to each of the solutions in an effort to make the bubbles more apparent.
Student Activity: Students set up and conduct the boiling point test for the liquids found from the four suspects. I have poured each liquid into its own container and labeled them with the name of the suspect. I encourage students to not crowd the various stations where the materials are set up, and I monitor the lab for safety and to make sure the set-up is flowing smoothly.
Catch and Release Opportunities: Once students' water baths have heated up I begin to help students to detect the bubbles. Here is a video in which I help a student monitor her boiling point. I know that Mariana Goldbloom's liquid should boil first because it is ethyl alcohol, followed closely by Johnson, so I do what I can to help students see bubbles if there are any. When students get a boiling point they record it on the page in the packet where there is a space for boiling point data.
However, I should note that this test is weak as an indicator of boiling points. It would be more effective to heat the test tubes directly in a test tube heater and closely monitor the temperature. I base this on the fact that student data varies widely. Of course, I am not concerned about the data's imprecision; if anything, I am grateful for it because when students go to analyze the results of all their tests, I want them to discern between strong data and weak data, and the results from this test, with their inaccuracy and imprecision, serve this purpose well. Here is a boiling point class data compilation to emphasize this point. Note that many students reported that the test tubes that had colored water in them boiled at really low points. I do not reveal the materials because I want students to make their arguments based on their data and their class data.
In this portion of the lesson we discuss our data. Here is a boiling point data analysis video excerpt that shows me talking about the data. As you can see, I am leaving room for ambiguity in reaching a conclusion from this test because the data is ambiguous. I want students to base their ultimate argument (who dunnit?) based on strong evidence. In subsequent classes students will conduct other tests where the data is much more precise and accurate, and that will give them a comparison by which to evaluate how strong their data is from each test.
For homework students are asked to write a paragraph for the boiling point test as outlined in the last page of the In the Heat of the Summer packet. Here is a Boiling Point Paragraph Exemplar of student work for the Boiling Point section of the Forensics Report that students will turn in at the end of the unit.
What I learned from reading the student work is that it is really important to show students exemplars of what is expected. With that said, however, I am quite pleased with students' ability to make an evidence-based argument. Clearly, this student did not make an argument for the guilt of any one suspect because the data was not convincing.