Determining the Freezing Point of Water

G Period Revised ONLY

Introduction

Background topics should include:

·  The 3 basic phases of matter and describe the molecule arrangement and behaviors and properties of each phase.

·  A description of all 6 phase changes – melting, freezing or solidification, evaporation/boiling, condensation, sublimation, and deposition

·  A discussion of what happens during phase changes in terms of energy (potential and kinetic), intermolecular bonds being broken and created, and what that means for temperature changes and cooling and heating curves. (Hint – plateau/flat part why?)

·  It would be great to discuss the difference between temperature and heat in terms of energy.

·  We will discuss some of this in class in the coming days before the report is due, but students are expected to use outside sources as needed to back up their statements and of course to cite in text (APA style) for all of their background information. Information must be paraphrased well, not copied NOR quoted directly. Ch 3 of textbook is useful.

Objectives:

·  To create a cooling curve for water.

·  To use the cooling curve of water to determine the melting and freezing point of water.

·  To learn the relationship between energy and molecules during phase changes

Hypotheses:

·  Use your Ipad to do research to find answers: What theoretically will happen to the temperature of water while it is freezing at its freezing point? Why?

Materials and Methods

Part I –Cooling Curve/Freezing Point

1. Fill a 400-mL beaker 1/2 full with ice, then add up to 300 mL mark with water.

2. Measure out 5 mL of water and pour into a test tube. Use a utility clamp to fasten the test tube to a ring stand. The test tube should be clamped above the water bath.

3. Place the Vernier temperature probe into the water inside the test tube.

4. Prepare the computer interface for data collection as directed by instructor.

5. Add 5 teaspoons of salt to the beaker and stir with a plastic stirring rod.

6. Click to begin data collection. Then lower the test tube into the salt/ice-water bath.

7. ***Continue to stir the ice-water bath throughout Part I.

8. Slightly, but continuously, move the probe during the first 10 minutes of Part I. Be careful to keep the probe in, and not above, the ice as it forms. When 10 minutes (600 seconds) have gone by, stop moving the probe and allow it to freeze into the ice. Add more ice cubes to the ice-water bath as the original ice cubes get smaller.

9. When 15 minutes (900 s) have passed, data collection should stop. Keep the test tube submerged in the ice-water bath. Now, start to copy the data from your computer interface into Table 1 below in the results section. (If time allows, Mrs. Jimenez will show you how to use the wi-fi to send your data to your email and download/use the Vernier app to retrieve it.)

Lab report reminder about methods – be sure that they are in past tense, organized into many paragraphs, specific with respect to details, and using passive voice to AVOID any personal pronouns like I, you, we, us, or a random subject like “The student…” See sample lab report for what this should sound like.

Results

Table 1 – Cooling Curve

Time (s) Temperature (°C)

0
30
60
90
900 s

You can stop your data at 900s even though we may have collected slightly longer than that.

For your lab reports, you must create a line graph by hand of temperature on the y axis vs. time on the x axis. This will be a large graph with many data points so be sure to hold the graph paper landscape. Also you should have a y axis that goes BELOW your x axis because there will be negative temperatures. This is ok!! Your graph will have one line that plots the cooling/freezing data from Table 1. Be sure to follow all graphing rules. If appended, be sure to note proper format on how to indicate this.

*** Once graph is completed, analyze your graph to determine the freezing point of your water. The freezing point is the temperature on the graph which can be determined to be the flattest part of your data (your plateau.). If the line is not quite flat, it’s ok. We are looking for the “flattest” part and then estimating that temperature. Experimental error may prevent a perfect plateau as theorized. Note the freezing point you determine in Table #3.

Table #3 – Summary of Results

Phase Change Point Temperature (°C)

Freezing point

For your lab reports, all data tables must be recreated (typed). There are 2 of these.

You must do 1 calculation for percent error for this report. Calculate the percent error for freezing point, keeping in mind that the theoretical freezing point of water is 0.0°C. As always be sure to use the proper 3 step method with equation, sub-in WITH units, and final answer with units.

Discussion

Follow the usual guidelines for what should be included in the discussion section. To help you out, here are some inferences that definitely need to be made… What was happening to temperature while the water was freezing? WHY? What was happening to the water molecules as their temperature got lower? WHY? Use theory you will learn this week next and also use your text and the internet! Talk about results and processes in terms of kinetic energy of molecules and making or breaking attractions between molecules. Be SURE to reference specific results (tables or figures) or parts of results as EVIDENCE. Inferences mean nothing in the discussion without results being used as evidence.

How was the graph useful in determining the freezing point of the water? Why? What does the graph look like at these points? What’s going on with the molecules and energy?

As always, be sure to discuss MANY specific possible error sources, comment on the accuracy of your results (referring to % error calc) and discuss HOW SEVERAL specific error sources could have affected the results, state if objectives were met and how, and note if the hypothesis was proven and why/why not. I don’t want to read “human error,” only specific errors that may have been made and an explanation of exactly HOW the error would have affected the results.

Be sure to clearly organize your discussion section into paragraphs even if you need to make an outline first. A good discussion is several well developed paragraphs not a couple sentences as some of you turned in for your first reports.

References

Don’t forget References section and use my quick reference APA guide and/or the link on our class website.

*****Use the original guidelines!! Use the hints and reminders to do a check of your report before handing it in!

Turn it in using Turnitin.com by class time on the day the report is due. Due date for your class is Thur 10/30 Day 3 . A hard copy is also due at the beginning of class. Late 1 minute to late 1 day = - 15 points. Any day after that lab reports are accepted but can receive a maximum of 50% credit. Last day accepted to count for this quarter is Friday November 6.