Chemistry
Change of Physical State Lab
Introduction
In this lab, you will investigate the freezing behavior of an organic compound
called lauric acid,
CH3(CH2)10COOH. In any pure substance, changes of physical state occur
at constant, discrete temperatures that are uniquely characteristic of the
substance. You will use a Vernier LabQuest to measure the temperature of
lauric acid as it freezes over time and graph these data to determine its
freezing point.
Purpose
Based on the introduction above, determine the purpose of this lab and record
in your lab notebook.
Equipment
beaker (400 mL)
large test tube w/wire stirrer
ring stand
utility clamp
Vernier LabQuest
Vernier temperature probe
Materials
lauric acid [CH3(CH2)10COOH] (in test tube)
tap water
Safety Considerations
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Lauric acid is mildly irritating – avoid contact with the skin and eyes.
Safety goggles must be worn at all times.
Sometimes chemicals from previous labs still remain in glassware and on other lab equipment;
wash all lab equipment before and after performing this lab.
Wash your hands thoroughly after completing this lab.
Procedure
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Turn on the Vernier LabQuest and connect the temperature probe. Tap on the settings box on
the right of the sensor screen and set the collection time to 40 minutes and the frequency to 10
times per minute. Save the settings by tapping the “Done” button.
Fill a 400 mL beaker with about 250 mL of cold tap water. Place the beaker on the lab counter
next to a ring stand.
Attach a utility clamp to the ring stand and place it above the beaker.
Obtain a large test tube with melted lauric acid and a wire stirrer from the hot water bath in the
back of the room. Place the large test tube in the beaker and hold it in place using the clamp.
Make sure the water level is above the level of the lauric acid in the test tube; add water to the
beaker if necessary.
Place the temperature probe in the test tube, making sure it does not touch the glass, and begin
recording data by pressing the play button.
Gently stir the lauric acid with the wire stirrer with constant motion. Continue stirring the lauric
acid throughout the experiment.
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Make a note in your lab notebook of the time at which the lauric acid started to freeze and when it
was totally frozen.
Continue stirring the lauric acid and recording temperature readings with the LabQuest until the
temperature of the material has fallen below 35°C. When most of the lauric acid has solidified,
you will no longer be able to stir the contents.
Return the lauric acid test tube to the hot water bath in the back of the room. Dump your tap
water in the sink. Clean and return all lab equipment. Disconnect and clean the temperature
probe with soap and water using a test tube brush.
Using the LabQuest, copy the graph of your data into the Data section of your lab write-up. Label
each axis and title your graph. Label these three sections of your curve: “liquid lauric acid”,
“freezing lauric acid”, “solid lauric acid”.
Using the LabQuest stylus, highlight as much of the flat section of the curve as possible. Record
the average of these data points (shown on the right) as the melting point of lauric acid in the Data
section of your lab write-up.
Turn off the LabQuest (discard the data) and return it to the supplies table.
Calculations
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The accepted freezing point of lauric acid is 44.0°C. Using temperatures in Kelvins, calculate
your percent error for this lab.
Draw the temperature vs. time graph from the Vernier LabQuest.
Questions
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How could you determine the melting point of lauric acid?
Does the temperature of a substance vary while it is freezing? Explain why or why not.
Would an increase in the amount of lauric acid used affect your curve? Explain why or why not.
Why is it important to keep stirring the lauric acid during the experiment?
Is solidification an endothermic or exothermic process? Explain.
In this lab, you recorded temperature readings every 6 seconds. Between temperature and time,
which is the manipulated variable and which is the responding variable? Explain.
Errors
List four possible errors you may have committed in this lab that may have somehow affected your results.