Impurities and Melting Point
Date: Summer 2010
Can impurities in a substance increase the melting point
of that substance? Or it just decreases it?
Based on your question, I think you are already familiar with the
colligative property of "freezing point depression" - which states
that a solute will lower the freezing point of a solvent. Thus, as
long as the additional material remain at the level of "impurity"
(and not be at a very high concentration where we would confuse the
additive as the actual solvent), then impurities only lower the freezing point.
Greg (Roberto Gregorius)
The melting point of a pure substance is always higher than one with
impurities. The most common example of this is a metallic alloy, where
the addition of a second metal is made to a "pure" metal. In this case,
the second metal that is added is also pure, but can be thought of as
an impurity with respect to the first metal. The result is that the mixture
(or alloy) has a lower melting point than either of the two original (pure)
Another result of adding impurities to a pure substance is that the
melting point tends to become less well defined. Again, looking at
metal alloys as an example, pure metals have a sharp melting point,
but many alloys become mushy as the temperature approaches their
melting points, and only become true liquids after a further increase in
One should "never say never, never" -- someone can find some
conditions where the conditions a leave the unexpected -- "expected".. The
publication "The Entropy of Iodine Monochloride...Heats of fusion and
Vaporization" J. of Physical Chemistry, 69, 2443, (1965) lays out the
issues of the fusion of solids in detail. Melting a solid is not so
"obvious" as it appears at first sight.
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Update: June 2012