DETERMINATION OF MELTING POINTS
Introduction:
The melting point of a compound is the temperature at which it changes from a solid to a
liquid. This is a physical property often used to identify compounds or to check the purity
of the compound.
It is difficult, though, to find a melting point. Usually, chemists can only obtain a melting
range of a 2 - 3oC accuracy. This is usually sufficient for most uses of the melting point.
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
digital thermometer
mortar and pestle (optional)
capillary tubes
solid organic compounds
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 break very easily, handle them with caution.
Procedure:
1. Obtain a capillary melting point tube and a known 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. Start
with a setting of two to two and a half; the temperature should slowly rise. 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. Record the melting
range, which begins when the sample first starts to melt and ends when the sample is
completely melted.
5.
Obtain an unknown sample and determine its melting range. Identify the unknown
by comparing the data on the knowns the class has obtained.
6.
Time permitting, pulverize a mixture of two known substances used for practice with
a mortar and pestle, and determine the melting point of the mixture.
Data Table:
Known Compounds:
compound
compound
melting
range
-
o
melting
range
-
o
melting
range
-
o
C
C
Unknown:
compound
Unknown # ___________
C
identity of
compound
(OPTIONAL) Melting points of mixture _______: _________C and _________C
Components of mixture _______ are ________________________and
________________________
Questions:
1. Define the "melting point" of a substance.
2. What is the purpose of determining melting points?
3. Why is this method not used for finding the melting points of inorganic compounds?
4. Why could the rate of heating influence the melting point?
DETERMINATION OF MELTING POINTS
TEACHER NOTES
Standards Met:
3.1.10.E – Describe patterns of change in nature, physical and man made systems.
 Describe the effects of error inmeasurements.
 Describe changes to matter caused by heat, cold, light or chemicals using a rate
function.
3.4.12.B – Apply and analyze energy sources and conversions and their relationship to
heat and temperature.
 Determine the heat involved in illustrative chemical reactions.
3.4.12.A – Apply concepts about the structure and properties of matters.
 Explain how the forces that bind solids, liquids and gases affect their properties.
3.7.12.A – Apply advanced tools, material and techniques to answer complex questions.
 Demonstrate the safe use of complex tools and machines within their
specifications.
 Evaluate and use technological resources to solve complex multistep problems.
3.7.10.B – Apply appropriate instruments and apparatus to examin a variety of objects
and processes.
 Describe and use appropriate instruments to gather and analyze data.
Lab Time: 35-40 minutes
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. Why is this method not used for finding the melting points of inorganic compounds?
Inorganic compounds have extremely high melting points.
4. 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.
Compound
palmitic acid
stearic acid
p-nitrobenzaldehyde
vanillin
benzoin
ethyl-p-aminobenzoate
Melting point
63 oC
68 / 69.5 oC
106 / 107 oC
81 / 83 oC
137 oC
88 / 90 oC
Considerations:
The opportunity for students to use the Mel - temp apparatus is very valuable. Most
college chemistry labs do not even have this convenience. It is good to show students the
ease of a Mel - temp so they can appreciate their use later when they are stranded using a
Theile tube.
Last updated 11-02.