In this lesson, students take on the role of a forensic chemist who is tasked with investigating a suspicious drowning incident. Students will conduct a series of titrations on an evidence sample of water collected from the victim's lungs as well as on several water samples from local water sources (lakes, rivers, wells, etc.) Based on their findings, they will determine where the victim actually drowned.
This lesson will help prepare your students to meet the performance expectations in the following standards:
- 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. (example: higher acidity will neutralize faster thereby allowing for comparison matching)
- Scientific and Engineering Practices:
- Using Mathematics and Computational Thinking
- Analyzing and Interpreting Data
- Engaging in Argument from Evidence
By the end of this lesson, students should be able to:
- Calculate the molarity of acid in a water source.
- Analyze data to determine at what location a drowning occurred.
- Use correct laboratory procedures to perform a titration.
This lesson supports student understanding of:
- Acids and Bases
- Neutralization Equations
- Identifying an Unknown
Teacher Preparation: 20-45 minutes
Lesson: 80 minutes
- "Crime Scene” materials (optional, but highly encouraged)
- Examples: skeleton, blue material (to simulate water), plants, rocks, and elevated surface (to recreate a waterfall), etc.
Article from local newspaper describing the scenario
- These can be made online (example shown)
- For each lab set-up (per group of 2 students)
- Burette with clamp
- 0.1-M NaOH (100-mL)
- indicator (cabbage recommended but any universal indicator would suffice)
- “Standard water” (used to find when neutrality is reached)
- multiple beakers and flasks
- Multiple “Water Samples” of varying molarity (*DO NOT label with molarity value)
- Always wear safety goggles when handling chemicals in the lab.
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
- When working with acids and bases, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
- When diluting acids, always add acid to water.
- MSDS for Hydrochloric Acid and Sodium Hydroxide
- Teacher preparation time will vary depending on the availability of the various concentrations of acids, standard base, and indicator. Additionally, more time will be needed to create the mock crime scene.
- Teachers should plan for an additional 30-40 minutes of preparation time if the solutions need to be created specifically for this lab, and if cabbage indicator is being prepared.
- This lab can be used in one 90 minute block period, or divided to be used in shorter class meetings. For example:
- First class meeting (40-50 minutes): Introduction, safety, and initial testing.
- Second class meeting (40-50 minutes): Repeated testing, and data analysis.
- Optional period (40-minutes): Google Sheets, data input, interpret data/draw graphs.
- Setting the scene is vital to this lesson. Set up a “crime scene” in the corner of the class or in the hallway. Not only does this build excitement for a particular lesson but it also encouragers non-chemistry students to inquire as to what is occurring in your classroom. Caution tape can often be obtained from local construction companies, area police, or even the custodial staff at your school. But can also be found on Amazon.
- Teachers should update the student handout “Problem” section with the name of the lake in their specific drowning scenario. Blanks have been left in the current paragraph for this purpose.
- To make your own indicator:
- Submerge fresh red cabbage leaves into warm tap water.
- Place on a hot plate and heat until boiling.
- This solution, once allowed to cool, can be used as a universal indicator for many days.
- It is recommended that each lab group have their own solution.
- Refrigerate if using for multiple days.
- *Cover whenever possible to decrease the smell of boiled cabbage emanating throughout your classroom.
- Phenolphthalein could also be used, which is easier to see but doesn’t actually indicate neutrality, as it changes color at pH of 8.
- For the lab, I suggest that teachers give each group a water sample from just one source at a time and instruct the group to only use a portion of the sample to complete their experiments. This makes them cognizant of using chemicals sparingly, as once their sample is gone, there is no more for other groups to use.
- Students will complete multiple runs to verify their results. They will compare the molarity they found to the molarity of the water sample found in the victim’s lungs.
- The samples are actually acid samples, not samples collected from local water sources, as true water samples wouldn't have enough difference for students to notice. This can be discussed at the conclusion of the lab and can lead to discussion as to why different water sources have different acidity (ex: high ponds next to large farms vs. ponds in nearby mountains).
- While the students are performing the titration, be sure to circulate and encourage gradual titration, as students may be tempted to let the burette run continuously to save time.
- Students may also need help comparing the color of the final titrate to the control. Adding more indicator (within reason) should have no effect on concentration and should allow the color to more easily be seen.
- This lesson could be easily differentiated, depending on level of students, by creating "water" samples that are different by powers of 10 or by having similar molarities.
- Advanced students could be challenged to explain why the pond by the farm has higher acidity than the fresh water stream, use the spreadsheet to create their own equations for calculating molarity, develop graphs of trends, etc.
- If students are grouped by ability, give select groups the samples with lower molarity, as those are more difficult to correctly titrate.
- If students are grouped by mixed ability, encourage multiple trials and exchanging of duties so all students experience the mechanics of titration.
I suggest creating a Google Form (example shown) to collect student data and compile class data.
- A Google Sheet is automatically generated when students input data. (Example of shown in the Answer Key document).
For The Student
The process used to determine the molarity of an unknown sample is called a titration. By placing a known molarity of base (0.1M NaOH) into the burette and slowly adding it, drop by drop, to a known volume (10 mL) of an unknown concentration of acid (as indicated by an acid/base indicator), the volume of base that causes neutrality can be determined. Once the three known values have been obtained (Molarity of base, Volume of base, and volume of acid) the titration equation can be applied using the balanced equation:
Use the equation below to find the Molarity of an unknown sample:
Molarity of acid (MA) x Volume of acid (VA) = Molarity of base (MB) x Volume of base (VB)
- If it takes 50 mL of 3M NaOH to neutralize 100 mL of HCl, what is the concentration of the acid?
A body was found floating at _________ last week. Upon initial inspection, the body was identified as Bones E. Jones. The local police believe that that the victim may not have actually drowned at ___________ but instead was killed and transported to that location.
You have been commissioned (for free of course) to test the acidity of local water sources to attempt to match one type of water with that which was found in Bone’s lungs.
- Always wear safety goggles when handling chemicals in the lab. Wash your hands thoroughly before leaving the lab.
- Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
- When working with acids and bases, if any solution gets on your skin immediately rinse the area with water
- Obtain 10mL of a specified water sample of from a local area source.
- Add enough cabbage indicator to make a color change.
- Using 0.1M NaOH, carefully fill the burette to the top marking.
- In another beaker, obtain 10-20mL of tap water and add indicator to it. This will be the control sample to be able to see when neutrality is reached.
- To find the molarity of the suspected water source, slowly add drops of 0.1M base from the burette into the 10-mL of suspect water. Pause occasionally to stir the solution. Stop once the color of the suspect source matches the color of the control sample.
*Hint: the color may change over time. Neutrality is reached when the color remains matching for at least 10 seconds.
- Record your data in the table below and in the shared class document (if applicable).
- Repeat the experiment multiple times (at least three times) to assure accurate results.
- Using the equation above and the data collected form the class, calculate the molarity of each suspect water source. Compare the molarity values to the water sample found in Bones’ Lungs and determine the location of his demise.
|Trial||Water Source||Molarity of Base||Volume of water sample in beaker||Volume of Base used|
Using the equation given above and average data from the trials calculate the molarity of your source water.
*If available, use the class data found on the shared Google Document to calculate the molarity of each water source. Compare these to the water from Bone’s Lungs to determine where he was drowned.
- What is the average molarity for your water source? (Include the name of your water source).
- How does the molarity of your source compare with the molarity of the other water sources in your class? (If you used a shared Google Document, calculate other group’s molarities. If you did not use a Google Document, ask other groups about their results).
- What are THREE possible reasons groups that testing the same water source may have different calculated molarities? Avoid simply stating “math errors”. Try to think of systematic errors that may occur in experimental design.
- Why might water samples collected from different areas have different molarities? Think about environmental factors that may affect acidity levels.
Where was Bones drowned? Provide evidence from your reasoning. This should be a one sentence summary of what was learned.
How certain are you of your results?
Can you think of another forensic scenario where titrations could be used as part of the investigation or suggest another lab technique that would apply to this investigation?