CHEM 2123 Austin Community College Week 6 Determination of Melting Points Experiment

Austin Community CollegeCHEM 2123 – Organic Chemistry I Lab
Online Learning Module
Determination of Melting Points
To learn to use a melting point apparatus and to use mixture melting points to determine the
identity of a substance.
Students will learn to:
• Pack melting point tubes
• Use a melting point apparatus
• Determine melting point ranges
• Correctly assess identity of an unknown based on mixture melting point process
Please read the following material in “Laboratory Techniques in Organic Chemistry” (4th ed.) by
Mohrig, et al:

Chapter 14 – Melting Points and Melting Ranges
Austin Community College
CHEM 2123 – Organic Chemistry I Lab
Online Learning Module
The melting point is the temperature at which the solid and liquid phases of a substance
exist in equilibrium. For example, and ice cube at exactly 0°C will not melt. Similarly, water at
exactly 0°C will not freeze. Both physical states are stable at the melting point. It is only when
the temperature is lowered or raised that freezing or melting will occur.
Melting point is a physical property of a pure substance. By comparison to known
literature values, it can be used as one piece of data to identify substances. Melting point can
also be used to judge sample purity. If a sample is extremely pure and the melting point is taken
using high quality equipment, it is possible to observe the sample melting all at once. In this
case, a single number is listed for the melting point.
In introductory organic labs, it is more often the case that melting occurs over a range of
temperatures. Impurities typically cause the sample to melt over a range that is lower than the
melting point of the pure substance. Students are expected to record melting ranges. The first
number of the range is the temperature at which the sample begins to melt and the first droplet
of liquid is observed and is continually changing (not to be confused with sweating). The final
number of the range is the temperature observed when the sample is totally liquefied.
Mixture melting points will be used to confirm the identity of your unknown obtained
from previous experiments. By this process, an unknown is mixed with a known sample. If the
known and unknown are the same compound, the melting range will be very similar to that of
the pure unknown. If the known and unknown are different substances, the melting range will
be broader and very likely lower than the melting range of the pure unknown.
When measuring melting points, it is important to adjust the melting point apparatus so
that the temperature is ramping up at a rate of no more than 1-2 °C per minute. This is especially
important with analog devices. If the apparatus is heated too quickly, the thermometer lags
behind, resulting in a falsely low temperature reading.
Austin Community College
CHEM 2123 – Organic Chemistry I Lab
Online Learning Module
mp (°C)
109 – 111
113 -115
benzoic acid
121 – 123
125 – 128
malonic acid
132 – 135
140 – 144
salicylic acid
158 – 161
169 – 171
D-tartaric acid
172 – 174
Austin Community College
CHEM 2123 – Organic Chemistry I Lab
Online Learning Module
Watch the video to get a detailed procedure for determining your unknown. Your unknown is
different than the video. Your unknown is either 4-hydroxyacetanilide or D-tartaric acid. You
will still include the fast melting point series, the slow melting point series, and the mixed
melting point series using your unknown and 4-hydroxyacetanilide in one case and your
unknown and D-tartaric acid in the second case.
Fast melting point series: This first series will be used to determine the approximate
melting point of the pure unknown (the unknown without being mixed with anything).
Slow melting point series: This will be used to determine a more accurate melting range
of the pure unknown (the unknown without being mixed with anything).
Mixed melting point series: This will be used to determine the identity of the unknown.
This is based on the idea that if a pure compound is mixed with a compound that it is not, then
the observed melting point will be lower and occur over a broad range as compared to the pure
compound. On the other hand, if a pure compound is mixed with a compound that has the same
identity, then the observed melting point would be at the expected value and consist of a narrow
Fast Melting Point Series (oC)
Slow Melting Point Series (oC)
165 – 175
170 – 173
Slow Melting Point Series (oC)
Unknown + 4-hydroxyacetanilide
141 – 163
Unknown + D-tartaric acid
171 – 173
In the results section, determine the identity of the unknown.
Austin Community College
CHEM 2123 – Organic Chemistry I Lab
Online Learning Module
1. Draw structures for the 2 possible unknown components. Indicate their functional
groups. Are they polar or nonpolar and why? Are they expected to be water soluble or not
and why?
2. Explain what a melting point is using a definition – cite your source. Relate this to the
experiment – how did we use mp to identify your unknown.
3. Explain how a mp is assigned and why it is always assigned as a range.
4. Explain what a mp range indicates about purity. Also explain what the presence of
impurities do to a mp and how; you will need to look up the how and cite your source.
5. Explain what a mixture mp is using a definition – cite your source. Relate this to the
experiment. Explain how mixture mp can be used to identify an unknown.
6. Using your mp from your data, explain how you arrived at what the unknown component
was for your sample.
7. Explain what a fast melting point series is and why it is used.
8. Explain what a slow melting point series is and why it is used.
9. Write a section in your theory and conclusions explaining how you would decide what
your unknown was if the slow mp series for your unknown was 110 – 114oC using the
information from the table of properties on page 3. You will need to explain what 2
possibilities your unknown could be and why you chose those 2 possibilities. You will then
have to explain in detail how you would ultimately decide what the unknown is using mixture
melting points.
10. etc
Enter the sources that you used and be sure to include either superscripts or the
author’s name in ( ) in the body of the paper to show where you used the source.
The following is an example of an investigative lab report. Most of the
experiments in Organic Chemistry I will follow this lab report format.
Everything shown in bold below is what should actually be written in your
Both your name and date should appear on separate lines on the right hand side of
the page.
Your Name
Title of the Experiment Centered in the Page
Experiment 2 Extraction and Evaporation. Separating the Components of
I. Introduction
The introduction should be in your own words and explain the purpose behind the
experiment. This will only require at most 10 sentences. The purpose can be found
in the reading section from the departmental handout.
II. Safety
This section should have the NFPA rating for each chemical presented in the
demonstration video. This will include the Health, Flammability, Reactivity, and
other hazard ratings. You can find these ratings by doing an internet search of the
III. Experimental Procedure
While watching the demonstration video for the experiment, write the procedure
stepwise, numbering your steps, in your own words. There should be enough detail
such that anyone who reads your report could do the experiment without the use of
a lab textbook.
IV. Observations and Results
This section will include things such as boiling range, melting point, color, time for
reaction, spectra and etc. This section will include results related to your
observations such as melting point and purity relationship, percent yield, percent
recovery etc.
V. Theory and Conclusions
This section should indicate how your results related to the purpose of the
experiment and what can be interpreted from your results. This section should
indicate how your results prove the theory behind the reaction or procedure. Most
of the information on the theory section can be found in your lecture book or in the
prereading section of the lab experiment as well as the reading sections from the
textbook, and research using the ACC library particularly AccessScience, KirkOthmer Encyclopedia of Chemical Technology, Van Nostrand’s Scientific
Encylopedia, and Gale ebooks. This section will be graded the hardest and
rambling results in grade deduction. You should be able to make your point in
VI. Citations
Any format is acceptable, I tend to use MLA.

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