Posted by ann on Sunday, October 17, 2010 at 4:26am.
In melting point determination, why shouldn't a substance be re-melted? (i.e. why should we not re-melt a substance that has previously been melted in a melting point tube?)
I think the original melting process may drive out some of the impurities in the sample and those condense in the upper part of the tube; therefore, taking the m.p. a second time means we are using a "different" sample. (But I've done that. ;-)]
When determining the melting point of a substance, it is important to avoid re-melting the substance because the process of melting and cooling can sometimes cause the formation of a new crystal structure. This new crystal structure might have a different melting point than the original substance.
To explain why this happens, we need to understand the concept of crystal lattice. A crystal lattice is the repeating arrangement of particles (atoms, ions, or molecules) in a crystalline solid. The specific arrangement of particles determines the properties of the solid, including its melting point.
When a substance is melted, the crystal lattice breaks down, and the particles move freely as a liquid. As the substance cools, it forms a new crystal lattice when it solidifies again. The formation of this new lattice can vary depending on the cooling rate and conditions.
If we re-melt a substance that has previously been melted, we disrupt the existing crystal lattice and allow the particles to rearrange themselves again. This can result in the formation of a new crystal lattice, which may have a different arrangement than the original one. As a consequence, the melting point of the substance can change.
To accurately determine the melting point of a substance, we want to ensure that we are measuring the temperature at which the original crystal lattice melts. By avoiding re-melting the substance, we can prevent the formation of a new lattice and obtain a more accurate measurement of its melting point.
Therefore, it is crucial to handle the substance carefully during the melting point determination process and not subject it to multiple melting cycles.