Identifying an Unknown Metal

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Objective

SWBAT perform a calorimetry lab to determine the specific heat of an unknown metal sample and identify it from their lab data.

Big Idea

Intensive properties of matter can be used to identify chemicals.

Background

Due to running out of time at the end of the semester, this is the final lesson in the Calorimetry unit.  Ideally, I would love to do a calorimetry of liquids lab as well.  This lesson builds on student practice with the heat equation the previous day.

For this lab, we have improved on our open calorimeter from the water lab, and are using the calorimeter set from PASCO.  The PASCO calorimetry lab kit we purchased contains 6 foam calorimeters with lids, a metals set containing aluminum, copper and tungsten and a thermometer.  Each metal has a hook at the top to attach a string to, or hold using crucible tongs.  Metals not included in this kit would be fine to use, so long as they were pure, and following the link to PASCO above leads to a set of five metal samples to use for this lab.

While the kit is not essential, we purchased it using grant funding as we are updating all of our probeware in an effort to get more hands on in our implementation of NGSS.  Ideally, we would be using temperature probes instead of regular thermometers, but the time to train the students isn't present at this point, and it has been too long since using them first semester to break them back out.

This is an improvement as it is a closed system for the lab, which should result in more accurate results.  Given more time to conduct the lab, I would have students conduct three trials with each metal sample and average out their specific heats.

To help speed up the start of the lab, I try to get the hot plates on ahead of the start of class so students do not have to wait as long for boiling water.  I also recommend using distilled or filtered water, as the hard water at my school left a lot of mineral stains on the metals and the beakers after four periods of lab.

Each lab table begins with the following materials:

  • Calorimeter
  • Thermometer
  • Stirring rod
  • Two Unknown Metal Samples
  • Electronic balance
  • Hot plate
  • 600mL Beaker
  • Graduated Cylinder
  • Crucible tongs

 This lesson connects to Science and Engineering Practice 3, conducting investigationsScience and Engineering Practice 4, analyzing and interpreting data, and Science and Engineering Practice 5, using mathematical and computational thinking.  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.

Opener -- Lab Safety

5 minutes

When students enter, I return their Calorimetry problems from the previous day, and distribute the Identifying Metals via Calorimetry lab sheet.  I want students to have their problem examples to help them with the mathematical analysis of the lab.

I explain to students that the primary safety concerns today are in spilling or splashing the hot water we are using, and that they will need to wear goggles and aprons due to those hazards. I stress gently adding the metal samples to both the glass beakers and the foam calorimeters to not damage either.

I hold up each part of the lab equipment for them to observe, particularly the new calorimeters with lids.  I show them the setup on the middle lab table if they are running low on boiling water to heat the metals.  I remind them to use the hot hand to bring me their beaker and that I will refill it from the 2000mL flask.

I also point out that they should refill their calorimeter water between each trial, and take the initial temperature again each time.

I give students 2 minutes to read the procedure, and then ask if they have questions.  If they do not, they are then invited back to the lab space.

Metals Investigation

25 minutes

When students come to the lab, they immediately mass out their metal samples while the water is heating.  At this point in the year, the lab goes pretty smoothly except for step 4, when they have to measure out 250mL of water using a 100mL graduated cylinder.  Many students need to be reminded that it means two full 100mL and a 50mL measurement to get that amount of water.

One step I should have added is to insert the thermometer through the lid of the calorimeter.  Many students had delays in closing the calorimeter lid due to this step missing.

Once students have collected all their data, I encourage them to clean up and move to the front of the class to complete their analysis.  Some students have already split the tasks and worked on calculations for sample 1 while their partner did the procedures for sample 2.

Analysis and Conclusions

15 minutes

Once students are ready to work on their analysis, they move to the front of the room and get their calculators out.  Based on the previous day's equation practice, I split the analysis into solving for the heat gained by the water and the heat lost by the metal to help students navigate the fact that they need to use the heat equation twice.

This student had a nicely organized data table, and was able to remember that the negative heat of the water is the heat lost by the metal.  From this data, she was able to use the table of specific heats provided and determine the identity of her first sample.

This student forgot to change the sign on the heat of the water, when plugging into the equation of the heat lost by the metal.

However, it did not deter him from finding the identity of the metal sample, made more difficult by the inclusion of tin in the specific heat table.

This student had to use the prior knowledge that copper had a brown/orange color to explain his results.  Many students left this sample as "copper or tin" on the paper, and required some push to use ALL of their data in determining the identity.

I made the Extension question extra credit for the students who finished with enough time.  This is a modification of the famous example of Archimedes using density to determine if a crown were pure gold, using specific heat instead.

All students correctly identified at least one of their metal samples.  Those who did not tended to have had procedural errors, such as leaving the lid off the calorimeter, having the thermometer bulb in contact with the hot metal, or not immediately transferring the hot metal into the calorimeter.  If I had an extra day to debrief, these errors would have made for a rich discussion.  However, with final exams looming, students turned in their papers when they completed them.

Closing -- Test review

5 minutes

When students turn in their papers, they are to pick up the Calorimetry Test Review.  Although we shortened the unit, I want students to still attempt the unit exam to assess what they have learned.  The following day in class students will finish the review in class and I will post the key online.  On the next day students will take the exam.  This will accomplish two things for me as an instructor:

  1. All students will be taking an exam during the time my handful of seniors are taking the final exam, ensuring a quiet testing environment for them.
  2. Allow me to target my final exam review guide to the parts of the final unit that students appeared to be weakest at.

To be fair to students, given that we shortened the unit by over a week, I will apply a curve to the exam prior to entering it into the grade book.  After looking at my results from all four classes, I ended up applying a 17% curve to equate my top score with a 95%.