Determination of Melting Points
PA State Standards:
1.2.11.A
Read and understand the central content of
informational texts and documents in all academic
areas.
3.4.10.A
Explain concepts about the structure and
properties of matter.
3.7.10.B
Apply appropriate instruments and apparatus
to examine a variety of objects and processes.
Introduction:
The melting point of a compound is the temperature at
which it changes from a solid to a liquid. Melting point
is a physical property often used to identify compounds
or to check the purity of the compound. It is difficult to
find a melting point to the nearest degree. Usually,
chemists can only obtain a melting range of 2 - 3oC.
This level of precision is usually sufficient for most uses
of the melting point. The following table contains the
data for the compounds used in this experiment.
Compound
palmitic acid
stearic acid
naphthalene
vanillin
ethyl-p-aminobenzoate
ortho-phenanthroline
Melting point (oC)
63
68-69.5
77
81-83
88-90
93-94
Mel-Temp Apparatus
Guiding Question:
Please answer the following question before beginning the lab.
What changes might the presence of an impurity cause in the melting point of a
compound?
Equipment / Materials:
Mel-Temp apparatus
digital thermometer
capillary tubes
dropping tubes
Determinatin of Melting Points
solid organic compounds
solid organic compounds with impurities
unknown organic compounds
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Science in Motion
Juniata College
Safety:
 Always wear safety glasses 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 and dropping tubes break very easily, handle them with caution.
Procedure:
1. Obtain a capillary melting point tube and a known organic compound.
2.
Place a small amount of the compound on a clean surface. Push the open end of the
tube into the compound. Some of the sample will now be in the top of the tube.
3.
Hold the closed end of the capillary tube over a dropping tube; the dropping tube
should be held perpendicular to the table and a couple of inches above the table
surface. Drop the capillary tube into the dropping tube; the capillary tube will
bounce on the table, packing the powder into the bottom.
4.
Place the capillary melting point tube in the Mel-Temp apparatus chamber. If
instructed to test a sample with and impurity and/or an unknown sample, prepare
those and insert them into the Mel-Temp. Be sure to indicate the location of each
tube in the data table. (The additional samples may be tested individually if time
permits.)
5.
Start with a setting of two to two and a half; the temperature should slowly rise. The
samples should be observed continuously, so that the melting points of the samples
are not missed. Heat slowly to acquire the most accurate results. Record the
melting range for each sample, which begins when the sample first starts to melt and
ends once the sample has completely melted.
Data:
Sample
Determinatin of Melting Points
Beginning Melting Temp (C)
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Ending Melting Temp (C)
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Science in Motion
Juniata College
Questions:
1. Define the "melting point" of a substance.
2. What are the purposes of determining melting points?
3. List the unknown number tested and its identity. Indicate the reason for the
determined identity.
4. Why is this method not used for finding the melting points of inorganic compounds?
5. Why could the rate of heating influence the melting point?
6. Based on the experimental data, what changes does the presence of an impurity cause
in the melting point of a compound?
Credit:
The graphic of the Mel-Temp Apparatus was obtained from
http://jan.ucc.nau.edu/~jkn/235L1-MeltingPoint.htm.
Determinatin of Melting Points
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Science in Motion
Juniata College
Determination of Melting Points
Teacher Notes
Time for Completion:
This lab can be completed in a single 40 – 45 minute period. More time will be necessary
if the samples are not run simultaneously.
Target Grade Level:
This lab is appropriate for physical science or chemistry classes.
Objectives:
1. Students will determine the identity of an unknown compound based on its melting
point.
2. Students will determine the effects of an impurity on the melting point of a compound.
Major Concepts:
This lab is best taught in a unit on physical properties. Other chemicals may be used, but
the melting point must not exceed the limits of the thermometer. SIM digital
thermometers should be turned off and cooled when they reach 145 oC. If other
thermometers are used, be sure to check their upper limit.
Preparation:
To save time, samples with impurities may be prepared in advance in a mortar and pestle.
However, students my do this step themselves if desired.
Answers to Questions:
1. Define the "melting point" of a substance.
The temperature at which a solid changes to a liquid.
2. What is the purpose of determining melting points?
A pure compound will have a definite melting point; therefore, the identity of a
substance can be determined, or the purity of a substance can be determined.
3.
List the unknown number tested and its identity. Indicate the reason for the
determined identity.
Answers will vary depending on the chemicals tested. The reason for the
identification should be that the experimental melting poin collected for that
unknown most closely matched the identified chemical on the reference chart.
4. Why is this method not used for finding the melting points of inorganic compounds?
Inorganic compounds have extremely high melting points.
5. Why could the rate of heating influence the melting point?
If the rate of heating was too high, the substance may melt before the thermometer
has a chance to register the actual temperature. The melting point will appear to
be lower than the actual value.
Determinatin of Melting Points
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Science in Motion
Juniata College
6. Based on the experimental data, what changes does the presence of an impurity cause
in the melting point of a compound?
The data should indicate that the impurity decreased the melting point of the
compound, and that the impure compound melted over a wider range.
Extensions:
If students are unsure of the identity of their unknown compound, they may test one or
two pure compounds to help them decide, if time permits. Melting point can be used to
check the purity of chemicals synthesized by students, providing the chemical melts at a
low temperature. For example, melting point is a good follow-up activity for the
Microscale Synthesis of Acetylsalicylic Acid lab, which can be provided by SIM.
Determinatin of Melting Points
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Pre-Test for Determination of Melting Points (Key)
1. What is a purpose for determining the melting point of a compound?
A.
B.
C.
D.
Decomposition
Exasperation
Identification
Separation
2. Pure vanillin is a compound in vanilla beans. What is the effect of an impurity on the
melting point and range of this compound?
A. It would decrease the melting point and broaden the range.
B. It would decrease the melting point and narrow the range.
C. It would increase the melting point and broaden the range.
D. It would increase the melting point and narrow the range.
3. Why are melting points usually performed on organic compounds and not inorganic
compounds?
A.
B.
C.
D.
Organic compounds are less expensive.
Organic compounds are less hazardous.
Organic compounds are more fragrant.
Organic compounds have lower melting points.
4. Melting points are determined with the MelTemp or similar apparatus. Where is the
sample placed?
A.
B.
C.
D.
Between two salt plates.
In a 1-cm cuvette or test tube.
In a glass capillary tube.
In a syringe.
5. Two chemicals have the same melting point. Which of the following statements best
interprets these results?
A.
B.
C.
D.
The two chemicals are the same.
The two chemicals are not the same.
The two chemicals may be the same.
None of the above.
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