S '05
MELTING POINT DETERMINATION
PURPOSE
Identify an unknown compound by its melting point.
MATERIALS
•Mel-Temp melting point apparatus
•capillary tube
•unknown sample
•Table of Melting Point Data
•laboratory notebook
THEORY
The melting point (mp) of a substance is one of the physical properties that chemists use
to identify a substance. The melting point is the temperature at which a substance
changes from a solid to a liquid state. A pure crystalline organic compound usually has a
sharp and characteristic melting point range of 0.5 to 1 °C. The melting point range is
determined by recording the temp at which melting first begins and the temp at which
melting is complete. You may recall that impurities depress the melting (freezing) point
of a substance. They also increase the range of melting. When a sample melts at a lower
than expected temperature over an extended range, this is a sign that the sample was not
pure. Consequently, the melting point of a compound is also a criterion for purity and
well as for identification.
A technique known as a "mixed melting point" may be used as additional evidence in
identifying a given compound. First, a melting point is taken of the unknown and a
tentative identification is made using literature data. Then the unknown sample is mixed
with some authentic sample of the suspected compound and the melting point is taken of
the mixture. If the mixture shows no depression in the melting point, the two compounds
almost certainly were the same and the identification of the unknown is confirmed. If the
mixture shows a depression of melting point, the two compounds were not identical.
Often solid substances undergo some unusual behavior prior to actual melting.
Compounds may decolorize, decompose, soften, or shrink. It is normal for compounds to
appear to shrink immediately before melting. Actual melting begins when the first drop
of liquid becomes visible. When you research your compounds, be sure to note any
decomposition temperature listed, as some solids decompose into other products before
they melt.
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