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Everyone knows that you use antifreeze to make sure the radiator in your car doesn't freeze, but have you ever wondered how it works? The answer is something called Freezing Point Depression. The depression of a freezing point means to literally lower the point at which something freezes. This property, known as a colligative property, means that you can add antifreeze, or Ethylene Glycol to water, you can lower its freezing point from 0 degrees Celsius to a much lower temperature, and therefore prevent your radiator from turning into a chunk of ice. Here follows an experiment that demonstrates the natue of such a substance.






Freezing Point Depression

Abstract:

The purpose of this experiment is to determine an unknown percentage of Ethylene Glycol in Water from a twenty-milliliter sample by measuring the freezing points of Ethylene Glycol and Water in twenty-milliliter samples of known percentage.

Introduction:

In order to test the affects of Ethylene Glycol on the freezing point of water, we will be mixing known percentages of Ethylene Glycol with water to produce twenty-milliliter samples that can then be used determine the percentage of Ethylene Glycol in the Unknown by comparing its freezing point with a visual graph.

Procedure:
  • 1. Calibration of the Thermometer
    • Using a sample of pure water, calibrate your thermometer using a standard thermometer. For purposes of this calibration, temperatures of 15°, 5°, 0°, and -5° Celsius. This step is only necessary if you are using electronic measuring equipment.

  • 2. Preparing the Cold Water Bath
    • Take two Styrofoam cups and place one inside of the other.
    • Place the cups inside of a copper basin (to catch runoff water and ice)
    • Fill the cup with water and ice.

  • 3. Measuring the Freezing Point of Water
    • Using a 25 mL burette, fill a test tube with 20 mL of water.
    • Place the thermometer in the test tube and then place the test tube into the ice-water bath. (The whole time you are recording temperature, you should be stirring both the water in the ice-water bath using a stirring rod and the water/solution in the test tube using a piece of copper wire)
    • Once the temperature of the water begins to approach 0°C add salt to the ice-water bath. The salt will allow the solution of salt-water and ice to fall below 0°C and freeze the contents of the tube.
    • Wait for the water inside of the tube to freeze. When this occurs, a period of steady temperature occurs as the fluid in the tube freezes. This is your freezing point.
    • Once the temperature of the water inside the test tube begins to fall below the freezing point, it is frozen solid. Pull the test tube out of the ice-water bath and use your hands to melt the ice and return the contents in the test tube to room temperature (or around 15°C)
    • Repeat the previous steps in section 3 until you have 3 measurements. Record your results.

  • 4. Measuring the Freezing Point Depression of Ethylene Glycol
    • Using a 25 mL burette, fill a test tube with 19 mL of water.
    • Using a 1 mL pipette, add 1 mL of Ethylene Glycol to the water in the test tube.
    • Place the thermometer in the test tube and then place the test tube into the ice-water bath. (The whole time you are recording temperature, you should be stirring both the water in the ice-water bath using a stirring rod and the water/solution in the test tube using a piece of copper wire)
    • Once the temperature of the solution begins to approach 0°C add salt to the ice-water bath. The salt will allow the solution of salt-water and ice to fall below 0°C and freeze the contents of the tube.
    • Wait for the solution inside the tube to freeze. When this occurs, a period of steady temperature should appear as the fluid in the tube freezes. This is your freezing point.
    • Once the temperature of the solution inside the test tube begins to fall below the new freezing point, it is frozen solid. Pull the test tube out of the ice-water bath and use your hands to melt the ice and return the contents in the test tube to room temperature (or around 15°C)
    • Repeat these steps until you have 3 measurements. Record your results.
    • Repeat these again, until three measurements have been taken for each 5% Ethylene Glycol, 10% Ethylene Glycol, 15% Ethylene Glycol, and 20% Ethylene Glycol. Alter these steps as follows:
  • 5. Measuring the Freezing Point Depression of the Unknown
    • Using a 25 mL burette, fill a test tube with 20 mL of the Unknown
    • Place the thermometer in the test tube and then place the test tube into the ice-water bath. (The whole time you are recording temperature, you should be stirring both the water in the ice-water bath using a stirring rod and the water/solution in the test tube using a piece of copper wire)
    • Once the temperature of the Unknown begins to approach 0°C add salt to the ice-water bath. The salt will allow the solution of salt-water and ice to fall below 0°C and freeze the contents of the tube.
    • Wait for the Unknown inside the tube to freeze. When this occurs a period of steady temperature should appear as the fluid in the tube freezes. This is your freezing point.
    • Once the temperature of the Unknown inside the test tube begins to fall below the new freezing point, it is frozen solid. Pull the test tube out of the ice-water bath and use your hands to melt the ice and return the contents in the test tube to room temperature (or around 15°C)
    • Repeat these steps until you have three measurements. Record your results.

  • 6. Preparing the Data for Analysis
    • Take all of the data and place it in a table using some spreadsheet software with graphing utilities.
    • Record all three measurements for each % concentration of Ethylene Glycol and the Unknown. Average the results.
    • Using the averages of all the data (excluding the Unknown), create a linear graph of % Ethylene Glycol vs. Freezing Points in °C
    • Using this newly made graph, locate the average freezing point of the Unknown. Once this is done, locate the corresponding Y value for the temperature; this will be the percentage of Ethylene Glycol present in your unknown sample.

  • 7. Testing Your Results
    • Using the procedures you used previously, and the information gained from the chart, prepare a 20 mL sample with a percentage equal to the theoretical value of the Unknown. Once you discover the freezing point depression of this test sample, it's freezing point should be identical of the Unknown.

Conclusion:

Once the data was prepared, we saw that the freezing point of the unknown sample fell between 10% and 15% Ethylene Glycol. We guessed that the unknown must lie between these two concentrations. Once the data was used to make a graph we found that the temperature of the Unknown placed it at roughly at 13.0% Ethylene Glycol. This matched our assumptions, and believed that values to be within acceptable range of error. Any errors made during this experiment might be attributed to the inability to freeze the solutions without super cooling. Also, there were doubts in the accuracy of the calibration of the thermometer after it had been allowed to sit for two days. Overall, our results were fairly accurate, with measurements in temperature having an average standard deviation of ± 0.051336 °C, and our accuracy being ± 0.35%







Data

Concentration by %              First Exp.       Second Exp.       Third Exp.       Average
Tf of pure water, °C              1.51             1.49             1.54            1.51
Tf of 5.00 % antifreeze, °C      -0.13            -0.14            -0.12           -0.13
Tf of 10.00 % antifreeze, °C     -2.08            -1.99            -2.07           -2.05
Tf of 15.00 % antifreeze, °C     -4.89            -4.81            -4.84           -4.85
Tf of 20.00 % antifreeze, °C     -7.92            -7.81            -7.69           -7.81
Tf of Unknown % antifreeze, °C   -3.56            -3.59            -3.69           -3.61

Standard Dev. 
Tf of pure water, °C               0.025166115 
Tf of 5.00 % antifreeze, °C        0.010000000 
Tf of 10.00 % antifreeze, °C       0.049328829 
Tf of 15.00 % antifreeze, °C       0.040414519 
Tf of 20.00 % antifreeze, °C       0.115036226 
Tf of Unknown % antifreeze, °C     0.068068593 

Average Standard Deviation         0.051335714 

Unknown     % of Ethylene Glycol     0.125 
Theoretical % of Ethylene Glycol     0.130 



Overall Accuracy was  ± 0.35 % 

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