Measuring a Melting Point
A Mel-Temp is commonly used to measure the
melting points of compounds. A sample is loaded into a capillary
tube and placed into the Mel-Temp. While the tube is being viewed
through an eyepiece, the Mel-Temp gradually heats the sample.
By carefully observing the temperature range at which the sample
turns from a solid into a liquid, the melting point is determined.
Preparing a Sample
First, the sample should be crushed to a fine powder.
Video: Crushing the sample (3.20M )
Next, the sample is loaded into a capillary tube.
Video: Filling a capillary tube (1.98M )
Tap the capillary tube on a hard surface until the sample
packs into the bottom.
If the solid will not go to the bottom of the capillary tube
when the tube is tapped on a hard surface, drop the tube through
a long, narrow cylinder.
Video: Dropping down cylinder
( 1.71M )
About 1 to 2 mm of sample should be added to the capillary tube.
Use a new capillary tube for every sample.
Why is this sample not appropriate for use in a melting point experiment?
Measuring the Melting Point
Place a thermometer with a relatively high temperature range
(greater than 200 degrees) into a relatively cool Mel- Temp.
Also place the capillary tube into the machine.
Why must the Mel-Temp be cool before the experiment?
The Mel-Temp can hold up to three samples at once.
Video: More than one sample (
Turn the heating stage on to a medium
setting. This setting should result in a heating rate of about 15
degrees per minute.
Video: What does 15 degrees per
minute look like? ( 2.75M )
Carefully observe the sample through the eyepiece. About ten
degrees before the temperature approaches the expected melting
point, reduce the rate of heating to one degree per minute. Why?
Be sure to decrease the rate of heating well before melting actually
takes place. Why?
If you are unsure what the melting point of your sample should be, perform an initial melting point experiment in which you melt the sample rapidly to determine the approximate melting point. Then perform another experiment using the technique described above to measure the melting point of the sample more accurately.
Video: What does one degree per
minute look like? ( 2.92M )
Instead of recording one temperature as the melting point,
measure the melting point range. To measure the range, record
the temperature at which the solid first begins to turn to liquid
and the temperature at which the solid has melted completely.
The melting range may span several degrees.
Video: Recording a melting range
( 8.34 M )
Video ( 8.55M )
What melting range should you record for the sample shown in the video? Answer
Discard used capillary tubes into a broken glass container
unless directed otherwise.
The melting range, if measured properly, can give you useful
information about a sample.
You carefully measure the melting range of a sample of salicylic
acid that you prepared in the lab to be 140.7 to 149.3 °C. The known value of the melting point is 159 °C.
What does the melting range you measured tell you about your sample?
What is wrong with this student's work? Answer
Using a Thomas Hoover Melting Point Apparatus
The Thomas Hoover Capillary Melting Point Apparatus is used
in a manner similar to the Mel-Temp. However, while the Mel-Temp
uses a heating stage to melt a sample, the Hoover uses an oil
To measure a melting point using a Hoover apparatus, prepare the sample
in a capillary tube as described previously. If the sample has not been packed well into the capillary tube, place the tube into the vibrator compartment near the back of the machine and turn the vibrator on briefly. The button must be held continuously
for the vibrator to work.
Video: Using the vibrator to pack the sample ( 3.20 M )
Place the sample into one of the sample compartments.
Up to five samples may be placed into the apparatus at once.
Begin stirring the oil bath. The faster the rate of heating,
the faster the oil bath should be stirred.
Heat the sample and record the melting point in a
similar manner as if using a Mel-Temp.
Remember to heat the sample relatively rapidly at first, but
then to slow the rate of heating well before the expected melting
point is reached. Also remember to record the melting range,
not just a single temperature.
When finished with the Thomas Hoover Melting Point Apparatus,
turn off the stirrer and the power, and discard the capillary tube
in a broken glass container.
Copyright © 1995-1996 NT Curriculum Project, UW-Madison