In the Heat of the Summer: Density Test

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Objective

Students will be able to conduct a density test as they work towards building a body of evidence in an attempted murder mystery.

Big Idea

Density is a tool that forensic scientists use to match glass samples to the scene of a crime.

Introduction

In this lesson students begin a series of 5 tests related to physical and chemical properties. After conducting these tests, students will be asked to identify a suspect in an attempted murder case. The first test is a density test, in which students test the density of broken glass, which purportedly is found on a number of suspects after a break-in at the victim's home. Their job is to match the glass to the scene of the crime, if they can.

This lesson aligns to the NGSS Science Practices 3, Analyzing and interpreting data, and 5, Using mathematics and computational thinking because students gather mass and volume data and then perform the density calculation which they will then compare to a known value.

This lesson assumes prior knowledge about density; they have already studied density in this lesson. Students have also been asked to read the following sections of In the Heat of the Summer prior to today's class:  

  • Introduction
  • Investigation Log for June 21
  • Motives section of the Investigation notes
  • Potential Evidence from the June 21 incident

For this lesson, students use the following:

  • samples of broken glass, scavenged from old windows
  • graduated cylinders
  • electronic balances
  • tweezers for handling broken glass shards

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/Activator

10 minutes

Students walk into class and find the following task on the board:

Do Now: Think Pair Share: At your table, think about who you think is most likely the perpetrator in In the Heat of the Summer. Discuss with members of your table. Do you have consensus?

My goal with this unit is engagement. Even prior to this unit students are beginning to talk about the characters and the lab. Student interest is building, and I want to take advantage of the buzz. 

 

Activator: I ask for students to share their conversations. But then, while being careful not to dampen their enthusiasm, I explain that so far all we have are possible motives—we do not have any facts. Over the next several days we will conduct tests to see if we can discover enough evidence that will link the murder attempts to one of the four suspects, and we will use our evidence to form an evidence-based argument.

I remind students dramatically that glass was found on each of our four suspects, and that today’s test is a density test that students will use to see if there is a connection between the evidence gathered and the 4 suspects.

I then ask students to turn and talk at their table with this prompt:

"How do you find the density of an irregularly shaped object?"

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: Students have already studied density at this point in the year. However, in order to be successful, I find it prudent to have students review the key concepts.

I ask students to report out from their turn and talks; they ideally explain the following:

  • We need mass and volume measurements in order to find an object's density
  • Mass can be obtained from an electronic balance
  • The displacement method is needed to find the volume of irregularly-shaped objects such as glass.

 

Here is a video of a student explaining how to calculate density.

I point out that the glass pieces may be smaller than 1cm3 and so using a several pieces for a sample will probably yield more accurate results.

I also point out that for safety reasons, no student will touch glass with their bare hands. Rather, they will use tweezers to handle the broken glass, which has many sharp edges.

Application

25 minutes

Students conduct the density tests, and compare their results with another group, recording their data and the other group's data. A useful teacher resource that gives an overview of glass forensics as well as a procedure for conducting a glass density test can be found at this website.

The goal for this part of the lesson is for students to start to gathering evidence, but then also to think critically about their evidence. After they have gathered evidence and recorded it in their evidence tracker found in the In the Heat of the Summer packet, they will compare it to another group's data. Due to the inaccuracy of the graduated cylinders that can accommodate glass pieces (I use a 100 ml graduated cylinder due to its sufficiently wide mouth) students may follow the same procedure but get somewhat different results. I think this is in keeping with how professional science works -- scientists are limited in what they can do by the tools they have at their disposal. 

Students who use a larger glass sample are more likely to get an accurate density than students who do not because a half milliliter difference in volume will be less significant in a large sample. Further conversation about the limitations of the data can be found in the debrief section of this lesson. Here is video in which I discuss with a student about the limitations of using just a little bit of glass.

 

Catch and Release Opportunities: Throughout the lab I monitor for safety and correct unsafe behaviors such as rough-housing or handling glass with hands. I also record the various densities that students are recording and  discuss discrepancies in the data with the groups that have them because I want to be sure that students recognize that if they are getting different results then they should not feel over-confident about the accuracy of their data. I also remind students that precision is about getting consistent results, and I suggest that they conduct their test a few times to see if they get consistent results. 

Debrief

10 minutes

At the end of class I project student data that they have entered into a spreadsheet using a document projector. A student volunteer makes a brief presentation to explain what the density is of each of the four glass samples. The student also explains whether any of them match up to the density found at the scene of the crime (Dr. Rodriguez’s home).

The densities found may be far enough apart and match the crime scene glass for the first student. However, I will have a few students in mind who have varying densities, and I will encourage alternative viewpoints on the densities of the different glass samples. I will remind students again, with a somewhat ominous tone, that measurement error might arise in this test due to student mistakes but also due to the limitations of their equipment. For example, our graduated cylinders are only accurate to 1 ml, for example. 

I ask students to record their feelings about their data in light of the conversation and in light of how their data compared to their comparison group and the groups that presented.

This last step is important; at the end of the unit students have to write a report that names the person who should be charged with attempted murder—if there is enough evidence pointing to a specific culprit. The report also must and, that discuss any uncertainty the student has about his or her findings that might make the District Attorney’s task of proving guilt in court difficult. 

Learning that data that can be replicated is probably more accurate than data that can't is one of the scientific practices associated with this unit.

Here is a density data debrief video filmed later in the week in which I review the data discrepancies inherent in this lab. Students take a while to understand how important this is, and I keep discussing it throughout the subsequent forensics lessons. For the rest of the year I really want students to base their conclusions solidly in evidence.

For homework students are asked to write a paragraph for the density test as outlined in the last page of the In the Heat of the Summer packet.