Why Did the Melting Point Decrease After Re-Solidification?

[twitsedgirl]

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.

Homework Equations

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?

 

[Borek]

Have you ever seen what happens to sucrose when it is heated to melting?

 

[chemisttree]

Have you heard of polymorphism?