LAB 2: MELTING POINTS
PRE-LAB 2: Complete the pre-lab assignment BEFORE entering lab. Your instructor
will give you instructions about how it should be handed in.
1) Write a safety section for this experiment.
2) In your own words define melting point. How can you tell a substance is pure by
taking the melting point of a sample?
3) Look up and record the melting points in the experimental procedure section (Part 1
and 2) for Lab 2.
4) Explain your predicted melting points for Part 2.
PURPOSE: You will learn why the melting point of a solid is important in organic
chemistry and you will practice taking melting points.
BACKGROUND INFORMATION:
The melting point of a solid substance is the temperature range where the
substance goes from the solid to the liquid state. It is an important physical property of a
substance because it is accurate, reproducible, and easily done. The melting point range
for a pure sample is usually small if the measurement is made carefully. Usually the
range is 1 to 2 degrees centigrade (or Celsius). The lower temperature of the melting
point range is the temperature at which the solid first begins to liquefy. The upper
temperature is the temperature at which the entire sample is in the liquid state. For the
range to be small the sample size should be small and the heating should be done slowly.
Usually, the temperature should increase about one degree per minute near the melting
point. Unlike the boiling point of a substance (see the distillation experiment), the
melting point does not vary much with changes in the atmospheric pressure.
The melting point can be used as a criterion for the identity of a compound and a
qualitative indication of its purity. The observed melting point of a substance can be
compared to the melting point of a substance given in the literature. If the two
temperatures are different, the compounds are not the same. If the two temperatures are
the same, the compounds may be the same but not necessarily. Many different
compounds have the same melting point. One could do a mixed melting point (described
below) to further help identify an unknown. Pure samples generally have a very sharp
melting point range while impure samples have a broad (roughly 3 to 15 ºC) melting
point range. Further, impure samples melt lower than expected.
A mixture melting point is done when one wishes to fully show whether two
compounds with the same melting point are indeed the same. This can help identify an
unknown. A good example would be the two compounds urea and cinnamic acid (shown
on the next page) both of which have a melting point close to 132-133 ºC. Let’s say you
have an unknown that melts at 132-133 ºC. If the unknown is urea and you mix it with
some pure urea from another source, the melting point of the mixture would be 132-133
as the entire sample is urea. However, if the unknown were cinammic acid and you
mixed it with urea the melting point of the mixed sample would now be lower and
broader than 132-133º as the mixture was impure. So, if you suspect the identity of an
unknown, you can test this by doing a mixed melting point by mixing the unknown with
a sample of the pure compound and measuring the melting point of the mixture.
Melting point composition diagram for a mixture of the solids X and Y
Liquid X + Y
mpt. Y
mpt. X
Liquid +
solid Y
T
em
p
Co
Liquid +
solid X
ET
(Eutectic point)
Solid X + Y
Mole % X
100
75
60
0
Mole % Y
0
25
40
100
Since the melting point merely records a change in state (solid to liquid), it should
be possible to solidify the liquid and retake the melting point again as a second check of
the measurement. This is never done. If the measurement is to be repeated, a fresh
sample of the solid is taken and a new melting point capillary tube is used. Melting point
capillary tubes are only used once and then discarded in the broken glass container. The
reason a second melting point is not taken on a given sample is that the solidification of
the liquid rarely gives pure crystals (as some decomposition may have occurred) or may
give a different crystal structure which often has a different melting point.
Sometimes, a solid decomposes as it melts. This is true of certain classes of
compounds or compounds with high melting points. In such cases, the melting point will
have the letter d after it to indicate decomposition. Melting points with decomposition
are not as accurate as a true melting point. Often you will observe the decomposition by
a strong gas evolution or the charring of the sample when melting.
An error that can cause your melting points to be wrong is that the thermometer or
temperature measuring device may be badly calibrated. For very careful work, the
temperature measuring device is calibrated before measurements are made. This is done
by taking the melting points of very pure standard substances which melt at different
temperatures so the accuracy of the device can be noted at various temperatures. A
correction value is then added or subtracted from your later measurements as needed. In
this course, temperature corrections will not be done. Please keep in mind that our actual
observations may vary from the literature values by a degree or two for this reason.
EXPERIMENTAL PROCEDURE:
Melting point measurements are easy to do. Good eyesight and patience are
needed. Measurements can be made in any one of a number of commercial melting point
apparatuses. Some use oil baths (silicon oil is the best to use as it is thermally stable and
can be heated to 350 ºC safely). We use Mel-Temp devices in our laboratory. This
device uses a heated metal block in place of the oil bath. An advantage of the heated
block is that it cools quickly and hence speeds up the process when many melting points
need to be done. Temperature measurement in our equipment uses a thermocouple with a
digital readout device which works very nicely.
PART 1 - TAKING A MELTING POINT:
Take the melting point of salicylic acid, caffeine, urea, and cinnamic acid. The purpose
of recording melting point of the salicylic acid and caffeine in this experiment is to test
the calibration of the apparatus.
Caffeine has a melting point of ______________ ºC.
Salicylic acid has a melting point of ______________ ºC.
Urea has a melting point of ______________ ºC.
Cinnamic acid has a melting point of ______________ ºC.
To make a melting point determination you need to:
1. Put the sample in a melting point capillary tube. Do this by taking a melting point
capillary tube (be sure to use the capillaries that are sealed at one end - do not use the
capillaries that are open at both ends) and placing a small amount of solid in the tube.
Tap the solid down so it is tightly packed in the tube. The solid should be about 1
millimeter or 1/8 inch in the bottom of the tube.
2. Place the tube in the melting point apparatus and turn the device on.
3. Observe the sample as the temperature rises and record the melting point range of the
sample noting both the temperature at which the first drop of liquid appears (be careful
to differentiate sample softening from melting) and the temperature at which the entire
sample is liquid. This means that your melting point should always be reported as a
temperature range.
4. Make a Table in your Notebook to record in your notebook that includes the sample
identification, the chemical structure (Lewis structure, condensed structure, or bondline struncture), observed melting point range and literature melting point range.
5. Discard the capillary tube in the broken glass container.
PART 2 - TAKING A MIXED MELTING POINT:
You will be assigned an unknown compound by the instructor. You must indentify your
unknown as urea or cinnamic acid using only melting point. To do this you should use
the mixed melting point method.
BEFORE starting the experiment estimate the melting point of the following mixtures:
Urea + Urea = ______________ ºC
Cinnamic acid + Urea = ______________ ºC
Cinnamic acid + Cinnamic acid = ______________ ºC
To make a mixed melting point determination you need to:
Take a small amount of your unknown and mix it with a small amount of urea. USE
ONLY WHAT YOU NEED – keep in mind how little sample is needed for a melting
point. Try to make your mixtures ~50:50. Good mixing is important here. Take the
melting points of the mixture and record it in your notebook.
At this point you probably know what your unknown is, however repeat the mixed
melting point procedure using cinnamic acid just to be sure. Record the mixed melting
point in your notebook.
Discard the capillary tube in the broken glass container.
Discard any left over solid waste in the appropriate container in the hood.
IMPORTANT INFORMATION ABOUT THE REPORT:
1) Report your results. Briefly, compare the melting point ranges you observed with the
literature values. Explain any differences you noted and sources for the differences.
2) How does the melting point of a mixture of two different compounds compare to the
melting points of the individual compounds? Briefly, explain reason for the difference.
3) How accurate were your determinations of the melting points? Were your
measurements uniformly high or low or were they random? Suggest reasons for any
observed trends, if any.
4) How can melting point be useful to an organic chemistry student?
5) Write an introduction for this experiment.
END OF EXPERIMENT.
© 2011 Carberry and Carreon