Why Did the Melting Point Decrease After Re-Solidification?

AI Thread Summary
The melting point of a compound can decrease after re-solidification due to the presence of impurities or changes in the crystal structure. Melting point depression often occurs when impurities disrupt the crystal lattice, leading to a lower melting point. If the sample was allowed to cool slowly, it may have formed a different crystalline structure or incorporated impurities from the environment. The discussion also references the behavior of sucrose upon heating and introduces the concept of polymorphism, which can affect melting points. Understanding these factors is crucial for explaining the observed discrepancy in melting points.
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Homework Statement



An unknown compound gave a melting point of 230C. When the molten liquid solidified the melting point was determined to be 131C. Give a possible explanation for this discrepancy.

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The Attempt at a Solution



I understand that melting point depression arises from impurities within the lattice of the crystallized sample. But, I cannot seem to find why re-melting a "used" MP sample would lead to a MP depression. Most likely the sample was not cooled quickly, but slowly sitting at room temperature, which should allow the crystal lattice to recombine without any impurities within the capillary tube. Any ideas?
 
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Have you ever seen what happens to sucrose when it is heated to melting?
 
Have you heard of polymorphism?
 
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