Identification of Unknown Solids by Melting Point
Determination
Note: Print and bring to lab with you the document “Using the MelTemp Apparatus.” This is
the next link on the CHM230LL web page after this one.
Introduction
Most crystalline organic solids melt at moderate temperatures and their melting points are
charisteristic, conveniently measured physical properties that can be used to help determine a
compound's identity and its purity. Pure crystalline organic solids melt sharply (over a narrow
temperature range) when heated to their melting points. If a solid is not pure, however, its melting
temperature is reduced and the melting will occur over a wider range of temperature. The reported
melting point is generally the midpoint of the melting range.
Melting points can be used to determine whether two samples are the same material, even if
they both melt at the same temperature. If two samples are the same compound and if portions are
mixed together the mixture should melt at the same temperature as the individual components. If
they are different compounds, however, the mixture’s melting point will be lower and the melting
will occur over a broad temperature range.
In this experiment you will mix two sharp-melting compounds that have the same melting
point and determining the melting point of the mixture. The presence of a second compound, even if
it has the same melting point, acts as a contaminant and lowers the melting point of the mixture. In
addition, the melting point range, the temperature range between the first indication of melting and
the point at which the entire solid has been liquefied, will be much wider than either component
alone.
In this laboratory you will use these characteristics of solids to determine the identity of an
unknown compound. An initial determination of a compound's melting point will allow you to
narrow the possible identity of the compound to a few choices. Once the possibilities have been
identified, you will test your compound mixed with authentic samples of those possibilities. If you
observe a mixed melting point with one of the candidates that exhibits a normal melting point and
narrow melting range, you can take that evidence as confirmation that your unknown and the sample
are identical.
Before you attempt to identify your unknown, however, you will perform a short experiment
that will demonstrate the effects of a mixed melting point. This experiment will enable you to see the
magnitude of the melting point depression and the widening of the melting point range when two
different compounds are mixed and melted.
Melting point determinations are performed most accurately when you increase the
temperature in your system slowly. This will allow heat to flow rapidly enough through the system
so that the sample and the thermometer will always be at nearly the same temperature. It is also
important to finely grind the sample and pack it tightly in the sample tube. Air pockets and larger
crystals will affect how well the heat moves through the sample. Improper sample preparation can
lead to inaccurate melting point determinations and wider melting point ranges.
Procedure − Part 1- Mixed Melting points
Print out and read http://www.mesacc.edu/~minger/Organic Lab/235LL/Using the Mel Temp
Apparatus.pdf
Place approximately 10 mg of urea in a small test tube and pulverize the crystals with a clean
stirring rod. In a second small test tube, pulverize 10 mg trans-cinnamic acid in the same fashion.
CHM 230, Identification of Unknown Solids by Melting Point Determination, Page 1
Finally, in a third test tube, mix 10 mg urea and 10 mg trans-cinnamic acid and pulverize them
together with a glass stirring rod.
Prepare two melting point capillaries from the material in each test tube. The melting point
capillaries are very thin (ca. 2 mm in diameter) glass tubes, about 90 mm in length. One end is open
and the other end is sealed. The tube is loaded by pressing the open end of the capillary tube down
into the pulverized powder to force a sample of the powder into the tube. If you can see the sample
in the tube you have enough! Using too much material will make your melting point less accurate.
The sample can be moved to the bottom, sealed end of the tube by tapping the tube, open side up,
vigorously on the bench top. Alternatively, you may drop the capillary tube, open end up, down
through a piece of larger PVC tubing one or two feet long. It may also help to tap the capillary tube
gently on the side with a spatula or a stirring rod. You will get the best results if you have a small,
tightly-packed sample of finely-divided powder. After you have loaded two samples each from all
three test tubes, label them or store them in labeled beakers so that you don’t mix up your samples
and place them in the Mel-Temp apparatus. (see the melting point web page). If you take care to
keep track of the identity of two samples, you can run them simultaneously in the apparatus.
Remember that you should see the pure solids melt around 132 ˚C. The mixture should melt at a
noticeably lower temperature. Be prepared to observe and record the results as they occur.
Use the report sheet found at the end of this experiment to record the melting points and
melting point ranges of each sample. Although the beginning of the melting point range can
sometimes be difficult to determine, a "sweating" of the crystals or any distinct change in the
appearance of the crystals usually marks the beginning of the melt. The end of the melting point
range is identified as the temperature at which all of the crystals have melted. You should report the
midpoint of the melting range as the melting point of the sample
Procedure − Part 2- Identification of an Unknown Solid
Obtain your sample of unknown solid from the chemistry stockroom. As you did in the
previous section, place a few milligrams of your unknown solid in the bottom of a test tube and, after
pulverizing the solid with a glass rod, load two capillary tubes with the solid unknown. Use the first
tube to find the approximate melting point of your unknown by adjusting the heater control dial to a
relatively high setting. There is a graph with the title Knob Settings for MelTemp which shows the
approximate knob settings which give a 10 °C/minute and 1 °C/minute heating for a given
temperature. For your first try, use a rapid heating temperature (read the blue line for knob setting.)
At a high setting, the temperature will increase rapidly and you will be able to quickly determine an
estimate of the melting point. After determining the approximate melting point, use your second
capillary sample to accurately determine the melting point and the melting range of your unknown.
To do this, you should lower the setting on the heating control dial as you approach the melting
point. The reduced setting should create a heating rate of no more than 1 - 2 ˚C/min. Under these
conditions, you should be able to accurately determine your melting point.
After you have determined the melting point of your unknown, compare your results to the
melting points listed in Table 1. After identifying the most likely possibility for your unknown from
the table (your unknown will be found in Table 1), run an experiment similar to that run in
Procedure 1 above with your unknown and an authentic sample of the suspect compound checked
out from the stockroom. If you find that your unknown, the authentic sample of the suspect
compound, and a mixture of your unknown and the authentic sample all melt at the same
temperature, you can feel confident that your unknown and the authentic sample are identical. If, on
the other hand, the mixture sample melts at a significantly lower temperature than either of the other
two samples, you can be confident that your unknown is not identical to the authentic sample and
you will need to return to Table 1 to identify another candidate for testing. Repeat this procedure
until you have correctly identified your unknown.
CHM 230, Identification of Unknown Solids by Melting Point Determination, Page 2
Table 1; Melting Points of Various Solid Compounds
Compound
Citraconic Acid
Pyrocatechol
Resorcinol
Acetanilide
Succinic Anhydride
d,l-Mandelic Acid
Benzoic Acid
Urea
trans-Cinnamic Acid
Maleic Acid
Malonic Acid
Benzoin
Anthranilic Acid
Cholesterol
Adipic Acid
Citric Acid
Salicylic Acid
M. P. ˚C
93-94
104-106
109-110
113-114
118-120
119-121
121-122
132-133
132-133
134-136
135-137
136-137
145-147
148-152
152-153
153-155
156-159
CHM 230, Identification of Unknown Solids by Melting Point Determination, Page 3
Report
Name ____________________________________
Identification of a Solid by Melting Point
Part 1 − Mixed melting point of Urea and trans-Cinnamic Acid
Melting Point
Material
Start
End
Midpoint
Urea
trans-Cinnamic Acid
Mixture
Part 2 − Identification of a Solid by Mixed Melting Point
Unknown # ________________
Melting Point
Material
Start
End
Midpoint
Unknown
First Known (Which compound? ____________________ )
50/50 mixture
Second Known (Which compound? __________________ )
50/50 mixture
Third Known (Which compound? ___________________ )
50/50 mixture
The unknown is ____________________________ (name of chemical).
Summarize the reasons for your choice.
CHM 230, Identification of Unknown Solids by Melting Point Determination, Page 4
Postlaboratory Questions
1. Why is it important to grind the sample finely and pack it tightly before performing a melting
point determination?
2. What might be the effect of using too large a sample in your determination?
3. If compound A and compound B both melt at 150 ˚C, and if a 50-50 mixture of these compounds
has a melting point of 134 ˚C (midpoint of the melting range), what would you predict about the
melting point of a 75-25 mixture of A and B? How about a 25-75 mixture?
CHM 230, Identification of Unknown Solids by Melting Point Determination, Page 5