Freezing Point Determination

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Objective

SWBAT use probeware to determine the freezing point of water experimentally.

Big Idea

While a substance undergoes a phase change, its temperature is constant but the energy and behavior of particles change.

Background

This lesson is to help students focus on energy during state changes.  Our district has chosen to focus on phases of matter in our unit on Intermolecular Forces. 

The original lesson plan (which comes from Vernier, the maker of the probeware we use in the laboratory) is do both the freezing and melting point of water in the same class.  The full lab requires a minimum of 35 minutes of working time.  As we only have 42 minutes on our shortened day, I have chosen to modify this heavily, and only have students complete the freezing of liquid water. 

The previous day was spent in pre-lab preparation, planning our lab steps and visualizing each step of the lab.

This lesson is connected to the NGSS as follows:

  • Science and Engineering Practice 3- Planning and Carrying Out Investigations   While students did not plan this lab, they will have to carry it out correctly.
  • Science and Engineering Practice 4- Analyzing and Interpreting Data   Students will have to analyze the temperature vs time graph and align it with their observations to determine what happened at the molecular level.
  • Science and Engineering Practice 7- Engaging in Argument from Evidence  Students will use their observations and graphs to write a conclusion regarding how the water molecules behaved during the different stages of the lab.
  • Cross Cutting Concept 5- Energy and Matter   Students will be asked to describe the flow of energy within the experimental system.

Materials needed for this lab are:

  • Computer with LoggerPro software and LabPro
  • Vernier temperature probe
  • Ring Stand
  • Test Tube Clamp or Three pronged Clamp
  • Large Test Tube
  • 400 mL beaker
  • 10 mL graduated cylinder
  • Stirring Rod
  • Spoon
  • Distilled and tap water
  • Ice
  • Salt

Introduction- Lab Safety

3 minutes

When students enter the room, I ask them to get out their Freezing Water Lab papers and filmstrips.  Freeze Melt Student Data is a student filmstrip for a student assigned to the chemicals, and a Filmstrip for the student running the computer.

We review their filmstrip procedure from the day before- that one lab partner will handle the computer and the other will run the experimental setup.  Since this is our first lab using probeware, the student jobs have been explicitly split to one using the computers, and the other handling the chemicals and other lab equipment.

Alternately, this lab could be done without the probes, but students would need to set up or be provided a data table with the times to record the temperature.  At the least, it would be beneficial to perform this lab with electronic thermometers to help with rapid measurements and the extra significant figure of the tenths place.

The significant advantage to using the probes is that they collect and graph the data together, saving a tremendous amount of class time.  The gain in procedural efficiency allows students to focus on the questions and observations present within the investigation.

Along with the excitement of the lab comes the critical importance of lab safety. I do a quick review of lab safety. Better to be proactive, than reactive! Due to the safe nature of this lab, I remind them that they must wear aprons, but that goggles are not required today.

At this point, I dismiss the students to their lab stations to begin working.

Student Lab- Vernier Probeware

25 minutes

Students come back to their lab tables and get aprons on and log into the computers. As students begin I do a quick sweep of the classroom. That means I'm on the lookout for obstacles or issues – mostly technical – as students begin their work. This includes the safety precautions. 

They complete the lab as follows:

1.   Fill a 400 mL beaker 1/3 full with ice, and then add 100 mL of tap water.

2.   Measure 5 mL of distilled water into the graduated cylinder and pour it into the test tube.

3.   Use a utility clamp to fasten the test tube to a ring stand. The test tube should be clamped above the water bath. Place a Temperature Probe into the water inside the test tube.

4.   Prepare the computer for data collection by going to File --> Open and selecting the folder Chemistry with Vernier and the file: “02 Freeze Melt Water”.

5.   When everything is ready, click to begin data collection. Then lower the test tube into the ice-water bath.

6.   Soon after lowering the test tube, add 5 spoons of salt to the beaker and stir with a stirring rod. Continue to stir the ice-water bath during Part I. Important: Stir enough to dissolve the salt.

7.   Slightly, but continuously, move the probe by twisting it during the first 10 minutes of Part I. Be careful to keep the probe in, and not above, the ice as it forms.

8.   When 10 minutes have gone by, stop moving the probe and allow it to freeze into the ice. Add more ice cubes to the beaker as the original ice cubes get smaller. Record your observations in the space provided.

9.   When 15 minutes have passed, data collection will stop. If it doesn't, click "Stop" before proceeding.

10.   On the displayed graph, analyze the flat part of the curve to determine the freezing temperature of water:

  1. Move the mouse pointer to the beginning of the graph’s flat part. Click and drag across the flat part to select it.
  2. Click on the Statistics button. The mean temperature value for the selected data is listed in the statistics box on the graph. Record this value as the freezing temperature in your data table.
  3. To remove the statistics box, click on the upper-left corner of the box.

11. Print a graph of temperature vs. time graph. In the footer, type your name(s) and the number of copies of the graph you want.

12.  Clean up the lab.

Projecting Calm is about classroom demeanor. We are working with outdated equipment, and it can feel like a frustrating experience for students. I use a deliberately calm demeanor to help keep students engaged with the lab rather than focused on their frustration. While students are working, I circulate the room checking that they are proceeding correctly, troubleshooting, and answering questions. Most common pitfalls:

  • Not opening the correct file for the lab.
  • Loose connections on the Vernier LoggerPros.
  • Bad USB connectors on my computers keeps from finding the probe.
  • Not stirring salt/ice bath enough.
  • Not recording observations of the water in the test tube.

When students finish and clean-up, I send them to the printer to get their graphs, and have them work on the analysis at the front of the room.

Data Analysis and Conclusion Writing

14 minutes

Students tend to compartmentalize their learning. Even within one subject! I’ve found students disconnect the classroom from the labs. So during a lab I make it a point to keep bringing students back to their classroom learning.

Students analyze their graph to determine the freezing point of the water. Students were asked to make observations of the water in the test tube during the lab. Many noted that the water was freezing around the probe, so I ask them what the graph shows. Conversely, many notice the graph had flattened out and ask why that is happening, so I direct them to observe the probe getting frozen into place.

This student focuses on the temperature when making the observations, and not the actual visual observations of the results.

Alternately, I can refer students back to the ExploreLearning Exploration from two days prior, where they also investigated this phenomena.

Below is a sample of decent student data. It still varies a bit off of zero due to a lack of stirring and not twisting the probe in place to keep it from freezing initially.

This data is from the student who answered the questions above. The graph is nearly perfectly flat, indicating the students did a good job of dissolving the salt in the ice water and continuously moved the probe about.

The more difficult portion of the analysis is to think of the kinetic energy. Students have not had physics, so we previewed that kinetic energy as the energy of the molecules moving around prior to this experiment. Many students may need refreshing on the concept, so I am handling that from one group to the next as the questions arise.

If students are struggling with this concept, I ask them to sketch, next to their observations, what they think the water molecules are doing in each part of the lab. The way to show molecule motion, I explain, is by using lines behind the molecules to indicate the speed of the molecules, with longer lines meaning greater speed. This usually helps students understand the changes in kinetic energy.

When students are writing their conclusions, I remind them that it must include evidence from the lab. They state their idea like a thesis statement in English, and then provide the evidence for that statement.

This student example completely misses tying the conclusion to the data. However, it does indicate that he made solid observations of what was happening during the freezing process, a piece that was missing above.

When students finish, they turn in their lab sheets at the end of the period.