Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Impurities and Melting Point
Name: Widya
Status: student
Grade: 9-12
Country: Indonesia
Date: Summer 2010



Question:
Can impurities in a substance increase the melting point of that substance? Or it just decreases it?


Replies:
Widja,

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)
Canisius College


hi Widya,

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) metals.

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 temperature.

Regards,
Bob Wilson


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.

Vince Calder



Click here to return to the Material Science Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory