Warming and Cooling Behavior of a Pure Substance
Elements and compounds are pure substances. Although there are more than 100 elements known to man, there are over 6,000,000 compounds that have been studied.
A pure substance is:
An element is:
A compound is:
Each year several hundred thousand new compounds are reported. Each substance has a characteristic set of properties that essentially identifies it as that substance. One of these properties is called, melting/freezing point.
The melting point of a solid is:
The freezing point of a solid is:
In this lab a solid will be heated to a temperature that is slightly above is melting point. In order to determine its phase change point you will cool the solid and record its temperature. Conversely, in the second part of the experiment, temperature/time data will also be collected as heat is slowly added to the solid substance by a water bath.
Cooling Behavior
Make a cooling bath by filling a 600mL beaker about 4/5 full of cool tap water (~30-35°C).
Obtain a test tube containing the melted pure substance from the teacher. Place a designated thermometer into the test tube. Take an initial (time zero) temperature measurement and return to your lab bench. DO NOT WASTE TIME the test tube begins to cool immediately and will soon approach the point when it turns into a solid.
Clamp the test tube in position above the cool water bath; being sure to take
a temperature measurement every 30 seconds while setting up the bath.
When ready, immerse the test tube in the cool water bath. The water level outside the test tube should be above the level of the liquid inside.
Continue to record the temperature of the substance every 30 seconds. GENTELY stir the liquid with thermometer. DO NOT POLE A WHOLE IN THE BOTTOM OF THE TEST TUBE. Be sure your observations include the time solidification begins and when the entire tube is a solid.
Warming Behavior
Remove the test tube from the cool water bath.
Make a warming water bath by placing your cool water bath on a hot plate and heat the water to 70-75°C.
When, ready, take an initial (time zero) temperature measurement and immediately lower the test tube into the warming water bath. The water level outside the test tube should be above the level of the liquid inside.
Continue to record the temperature of the substance every 30 seconds. Be sure your observations include the time melting begins and time when no solid remains in the tube. ONCE THE SOLID BEGINS TO MELT, GENTELY stir the liquid with thermometer. DO NOT POLE A WHOLE IN THE BOTTOM OF THE TEST TUBE.
Remove the thermometer from the melted substance and quickly wipe it off with a paper towel before the substance solidifies on it. Be careful not to get the substance on your hands.
Observations: (Be sure to include the times are which you saw changes being and when the changes were complete.)
Data: Make a data table here to record your times and temperatures.
Processing the Data:
(Assign “time” as your independent variable to be graphed horizontal axis; assign temperature as dependent variable to be graphed on the vertical axis)
Use your data points to make 2 separate graphs; one cooling, one warming. Enter and plot your data points into excel. DO NOT use the excel program to “connect the dots” making your graph but instead draw a smooth curve by hand that represents the “best fit” to your data points.
From your cooling graph, determine the melting point temperature for the substance.
From your warming graph, determine the freezing point temperature for the substance.
Were the melting and freezing point temperatures similar?
Explain why you think they should be similar or different.
Explain how errors you observed during the experiment could result in melting and freezing point temperatures that are incorrectly similar or different.
Use the data table below to identify the unknown substance used.
Substance / Melting Point (°C)2-cyanophenol / 94
p-anisidine / 59
stearic acid / 70
p-nitrotoluene / 54
napthalene / 83
3-hydroxyacetophenone / 89
phosphoric acid / 43
My substance used is ______.
If your data does give you evidence to identity the substance used “without a doubt,” in order to receive credit for your lab you must explain how errors you observed affected your results allowing you to pick the best possible substance used.