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Effects of salt on freezing point of water

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Mr. Blank
4/9/02
Effects of Salts on the Freezing Point of Water
In this experiment, you will study the effect that several solutes have on the Freezing Point of water. When a solute is dissolved in a liquid, the temperature at which that liquid freezes decreases, because the molecules of the solute become attached to the water molecules, making it more difficult for the water to form its crystaline shape and form into ice. This process is called Freezing Point Depression. In this experiment, you will be determining which of five salts most affects the Freezing Point of water.

Each salt will have a concentration of one mole. A mole is the atomic weight of the compound multiplied by one gram, in 1000 ml of water. Since you will be using only 100 ml of water each time, then you must divide the atomic weight by 10 before multiplying it by one gram. You will be using a computer-interfaced Temperature Probe to monitor how much each salt decreases the freezing temperature of water, and a graph of your results will be plotted using the computer.

*PURPOSE: To learn which type of salt lowers the Freezing Point of water the greatest amount.

*PROBLEM: Which type of salt lowers the Freezing Point of water to the lowest point?

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*HYPOTHESIS: The KCl will lower the waters Freezing Point the most, because Potassium has the lowest ionization energy out of the five metals that are bonded with Chlorine, and perhaps it will bond more easily with the water molecules, keeping it from freezing.

*MATERIALS:
*Power Macintosh or Windows PC*10-ml graduated cylinder
*Vernier computer interface*ice
*LoggerPro*salt
*Vernier Temperature Probe*stirring rod
*ring stand*100 ml of 1M calcium chloride
*utility clamp*100 ml of 1M magnesium chloride
*one test tube*100 ml of 1M sodium chloride
*600-ml beaker*100 ml of 1M potassium chloride
*water*100 ml of 1M lithium chloride
*triple-beam balance*weighing dish
*PROCEDURES:
1. Put about 400 ml of water and only 5 to 6 ice cubes into a 600-ml beaker.

2. Obtain from your teacher 100 ml of 1M solution of each of the following solutions:
*CaCl*KCl
*MgCl*LiCl
*NaCl

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3. Using a graduated cylinder, measure and pour 5 ml of the CaCl solution into a test tube and use a utility clamp to fasten the test tube to
a ring stand. The test tube should be
clamped above the ice water bath.
Place the Temperature Probe into
the solution inside the test tube.

4. Prepare the computer for data collection by opening the Experiment 3 folder from Physical Science with Computers. Open the experiment file for the Stainless Steel Temperature Probe. The vertical axis will have temperature scaled from 20 to 30 degrees C. The horizontal axis will have time scaled from 0 to 15 minutes. Change these so that the temperature is scaled from -30 to 30 degrees, and the time is scaled from 0 to 25 minutes.

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

6. Soon after lowering the test tube, add 50 g of salt to the beaker and stir with a stirring rod. Continue to stir the ice-water bath during the experiment.

7. Slightly, but continuously, move the probe during the first 12 minutes of the experiment. Be careful to keep the probe in, and not above, the ice as it forms. When 12 minutes have gone by, stop moving the probe and allow it to freeze into the ice. Add a few ice cubes to the ice-water bath, as the original ice cubes get smaller.

8. When 25 minutes have gone by, data collection will stop. On the displayed graph, analyze the flat part of the curve to determine the freezing temperature of the solution:

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*Move the mouse pointer to the beginning of the graphs flat part. Press the mouse button and hold it down as you drag across the flat part to select it.

*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 in your data table.

9. Repeat steps 3, 5, and 7-8 with the remaining 4 solutions, refreshing the ice-water bath for each.

10. Print each of your 5 graphs and label them:
*Choose Make Annotation from the analyze menu, and type CaCl Freezing Cuve, etc. in the edit box. Then drag each box to a position on or near its respective curve.

11.Clean up your work area as directed by your teacher.


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Data Table
Chemical SolutionAverage Temperature from 17.5 to 22.5 Minutes in Experiment
CaCl1.4 degrees C
MgCl-0.3 degrees C
NaCl0.8 degrees C
KCl0.9 degrees C
LiCl1.2 degrees C
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