DETERMINATION OF MELTING POINTS
LAB PP 1
From Gannon University SIM and Auburn University SIM
INTRODUCTION
The melting point of a pure compound is an intensive property, like density and boiling
point. Intensive properties are independent of the amount of substance present. The
melting point of a compound is the temperature at which it changes from a solid to a
liquid. Experimentally, melting point is actually recorded as the range of temperatures in
which the first crystal starts to melt until the temperature at which the last crystal just
disappears.
Reasons for determining melting points:
1.
The melting point indicates the level of purity of a sample. An impure
compound melts over a wider range of temperatures, usually greater than 2
degrees.
2.
The melting point helps to identify unknown samples, narrowing the number
of possibilities, because a pure solid melts reproducibly over a narrow range
of temperatures.
3.
The melting point helps to characterize new compounds.
In this lab, the identity of an unknown organic compound will be determined by
comparing its experimental melting point to those of a variety of known compounds.
PURPOSE
The purpose of this experiment is to determine the melting points of various organic
compounds and to use these to identify unknowns.
EQUIPMENT/MATERIALS
Mel-Temp apparatus
Thermometer
Capillary tubes
Mortar and pestle (optional)
Dropping tube
Westminster College SIM
Organic compounds, known
Organic compound, unknown
Hairdryer with cool setting
Weighing boat
PP1-1
Determination of Melting Points
SAFETY
•
•
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Always wear safety goggles in the lab.
The parts on the top of the Mel-Temp are HOT while it is turned on. Do not touch
these parts or place your eye on the eyepiece. You will get burned!
Capillary tubes break very easily, handle them with caution.
Wash hands after performing experiment.
PROCEDURE
1.
Obtain a capillary melting point tube and a known compound. The known
compound may need to be ground into a fine powder with a mortar and pestle.
2.
Place a small amount of the finely ground known compound in a weighing boat.
Push the open end of the capillary tube into the compound to load sample into tube.
Load only 1-2 mm of sample into the tube. Larger samples will heat unevenly.
3.
Hold the closed end of the capillary tube over a dropping tube; the dropping tube
should be held perpendicular to the table and on top of the table. Drop the capillary
tube into the dropping tube; the capillary tube will bounce on the table packing the
powder into the bottom. Remove the capillary tube from the dropping tube.
4.
Place the capillary melting point tube in the Mel-Temp apparatus chamber, with the
closed end pointed down.
5.
Turn power switch ON.
6.
Set the power level to obtain the desired heating rate. Start with a setting of 40, and
adjust if needed to control the rate of temperature increase. The sample should be
observed continuously, so that the melting point of the sample is not missed. Heat
slowly to acquire the most accurate results.
7.
Record the melting range, which begins when the sample first starts to melt and
ends when the sample is completely melted. The known sample should melt within
the range found in Table 1, on the following page.
8.
Turn off the Melt-Temp to allow it to cool to about 50ºC before trying your
unknown. Use a hairdryer set on the cool setting to lower the temperature of the
Mel-Temp faster.
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PP1-2
Determination of Melting Points
9.
Prepare a sample of your unknown in the same way that you prepared the known
sample and find its melting range.
Known Compound
palmitic acid
stearic acid
Vanillin
oxalic acid(dihydrate)
benzoic acid
acetylsalicylic acid
Melting point(ºC)
63-64
69-70
81-83
101-102
122
138-140
Table 1 Melting Points of Known Compounds
10.
Identify your unknown compound by comparing the experimental melting range of
the unknown compound to the melting points of the known compounds in Table 1.
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PP1-3
Determination of Melting Points
DATA SHEET
Name ________________________
Name ________________________
Period _______ Class ___________
Date ___________
DETERMINATION OF MELTING POINTS
DATA TABLES
Known Compound
Unknown Number
Melting Range (ºC)
Melting Range (ºC)
Compound Identity
QUESTIONS
1.
Define the "melting point" of a substance.
2.
What is the purpose of determining melting points?
3.
Why could the rate of heating influence the melting point?
4.
Why is this method not used for finding the melting points of inorganic
compounds?
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PP1-4