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Glass Density Evidence
From ChemPRIME Sometimes identifying one type of glass or glass fragment at a crime scene may be critical in solving a crime, but the largest database [1] includes 304,107 glass compositions, so identifying one of them can be daunting. Although chemists can identify glass by more conclusive methods involving elemental analysis, density may still be used as a screening method, as FBI documents indicate FBI Glass Density. Density can be used to identify smashed glass at the scene of a crime. [2]. Densities of dozens of glasses are known [1]. The most common type of glass is ordinary window or bottle glass. It's called soda-lime glass because it is made of sodium carbonate (Na2CO3) and CaO (calcium oxide or lime) in addition to silica (sand or quartz, (SiO2). Pyrex (the Corning brand of borosilicate glass, about 70% silica, 10% boron oxide (B2O3), 8% sodium oxide, 8% potassium oxide, and 1% calcium oxide).Glass densities are determined by an ASTM standard method involving flotation in liquids [3], but the density of glass fragments can also be measured by water displacement, as described below. The ASTM flotation method is interesting. A layer of "heavy" bromoform is first added to a cylinder. It has a density of 2.889 g/cm3 at 15 °C, so most common glasses will float on its surface. Next, a layer of 20% ethanol in bromoform is carefully floated on the bromoform. Ethanol is light, having a density of 0.789 g/cm3, so when it's mixed with bromoform, the resulting solution is just a bit denser than bromoform. Layers of 40%, 60%, and 80% ethanol in bromoform, followed by 100% ethanol are added, and the layers are allowed to sit overnight. They diffuse into one another, giving a gradually changing density from 0.789 to 2.889 g/cm3. When glass pieces are added, they float at the level where the density matches. A density gradient column can be created with sugar solutions in dyed water [4], as shown in the figure to the left. 
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Skill One: Measuring
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THE CRIME
On April 1, 2018 (yes, last night), Maize High School's front office was broken into by an intruder!
Dr. Botts' office was ransacked and several items of value to him - his pet eagle, his favorite comb, and his best red hoodie - were stolen. He wants them back!
On the trail of the criminals responsible for this, Disciplinary Officers Sonya Tice, Kevin Frye, and Craig Broadbent bring in three suspects for questioning:
On April 1, 2018 (yes, last night), Maize High School's front office was broken into by an intruder!
Dr. Botts' office was ransacked and several items of value to him - his pet eagle, his favorite comb, and his best red hoodie - were stolen. He wants them back!
On the trail of the criminals responsible for this, Disciplinary Officers Sonya Tice, Kevin Frye, and Craig Broadbent bring in three suspects for questioning:
1) Student Emma S., who - being a lover of the outdoors and a kindhearted girl - always loved that pet eagle and always wanted to see it flying free
2) Student Ethan, who always wants to catch Dr. Botts' hair uncombed for funny photos for Instagram
3) Student Cayden H., the Maize High student who needs something warmer to wear on his trips to Iowa
CRIME SCENE INVESTIGATION: EVIDENCE COLLECTION
GROUP WORK (30 min.)
Glass was found at the crime scene. Samples of the glass are located at the lab stations, along with tweezers for safe handling.
REMEMBER LAB SAFETY! YOU MAY NOT TOUCH PIECES OF BROKEN GLASS WITH YOUR HANDS!
1) Create a data table on your own paper or in a MS Excel document on your student computer.
2) With your assigned crime scene investigation (CSI) group, measure at least 10 samples of the glass.
(NOTE: Though slightly less accurate, due to bromoform being toxic if inhaled, we will use the traditional water displacement method today.)
2) Student Ethan, who always wants to catch Dr. Botts' hair uncombed for funny photos for Instagram
3) Student Cayden H., the Maize High student who needs something warmer to wear on his trips to Iowa
CRIME SCENE INVESTIGATION: EVIDENCE COLLECTION
GROUP WORK (30 min.)
Glass was found at the crime scene. Samples of the glass are located at the lab stations, along with tweezers for safe handling.
REMEMBER LAB SAFETY! YOU MAY NOT TOUCH PIECES OF BROKEN GLASS WITH YOUR HANDS!
1) Create a data table on your own paper or in a MS Excel document on your student computer.
2) With your assigned crime scene investigation (CSI) group, measure at least 10 samples of the glass.
(NOTE: Though slightly less accurate, due to bromoform being toxic if inhaled, we will use the traditional water displacement method today.)
Skill Two: Graphing Density,
Finding Slope & the Equation of a Line
CRIME SCENE EVIDENCE: ANALYSIS
NOT GROUP WORK (30 min.)
1) You will now need to make a trend line for your data.
2) Once you have a line graph, find the slope of that line and record it next to your line graph.
3) Record the equation of that line on your document. *NOTE: Do not leave "X" and "Y" in your equation. Instead, replace these with the variables on the "X-axis" and on the "Y-axis".
4) Next, look at the evidence found on the three suspects:
- Emma S. had glass with a density of 7.2 g/cm3 on her person.
- Ethan had glass with a density of 4.28 g/cm3 on his person.
- Cayden has glass with a density of 2.23 g/cm3 on his person.
CONCLUSION
Claim: Write a brief summary of who you think committed this crime.
Evidence: Explain the evidence you have from the data you collected and the trendline you made to support your claim.
Reasoning: If you are certain of your claim, tell the reason you are certain. State clearly who should be arrested and why. (NOTE: If you are not certain, include possible sources of error and make suggestions for improvement.)
For a 24 point SUMMATIVE GRADE, you will need to submit:
1) Data Table (1-2-3-4)
2) Graph (1-2-3-4)
3) Equation of the trendline (1-2-3-4)
4) Claim (1-2-3-4)
5) Evidence (1-2-3-4)
6) Reasoning (1-2-3-4)
1) Data Table (1-2-3-4)
2) Graph (1-2-3-4)
3) Equation of the trendline (1-2-3-4)
4) Claim (1-2-3-4)
5) Evidence (1-2-3-4)
6) Reasoning (1-2-3-4)