Cause of melting point depression

In summary, the discrepancy in the melting point of an unknown compound could be due to impurities within the crystal lattice. However, the fact that the sample was not cooled quickly and was allowed to sit at room temperature may have allowed the crystal lattice to recombine without any impurities. Another possible explanation could be the presence of polymorphism, where the compound can exist in different crystalline forms with different melting points. Additionally, the heating process itself could have caused changes in the compound, such as in the case of sucrose.
  • #1
twitsedgirl
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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.

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?
 
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  • #2
Have you ever seen what happens to sucrose when it is heated to melting?
 
  • #3
Have you heard of polymorphism?
 

1. What is the cause of melting point depression?

Melting point depression is caused by the addition of a solute, such as salt, to a solvent. This disrupts the crystal lattice structure of the solvent, making it more difficult for the molecules to arrange themselves in an orderly fashion to melt.

2. How does the amount of solute affect melting point depression?

The amount of solute added directly correlates with the degree of melting point depression. The more solute added, the greater the disruption to the crystal lattice structure and the lower the melting point of the solvent.

3. Why is the melting point of a solution lower than that of the pure solvent?

The presence of solute particles in a solution decreases the intermolecular forces between the solvent molecules, making it easier for them to overcome these forces and melt at a lower temperature.

4. Can any solute cause melting point depression?

Yes, any solute that is able to dissolve in a solvent and disrupt its crystal lattice structure can cause melting point depression. However, the degree of depression may vary depending on the chemical properties of the solute and solvent.

5. How is melting point depression used in practical applications?

Melting point depression is commonly used in industries such as pharmaceuticals and food production to lower the melting point of substances and make them easier to work with. It is also used in antifreeze solutions to prevent the freezing of liquids in cold temperatures.

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