Why does melting point decrease for impure solid?

Click For Summary

Discussion Overview

The discussion revolves around the phenomenon of melting point depression in impure solids, specifically comparing crude acetaminophen to pure acetaminophen. Participants explore the underlying reasons for the observed differences in melting points, considering thermodynamic principles and intermolecular interactions.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant notes that crude acetaminophen is expected to have a lower melting point than pure acetaminophen and questions the underlying reasons.
  • Another participant suggests that the substance mixed with crude acetaminophen may have a lower melting point, proposing this as a possible explanation.
  • A different participant introduces the concept of entropy, explaining that mixtures have higher entropy than pure substances, which may lower the driving force for melting and thus require higher temperatures for the phase transition.
  • Another viewpoint discusses intermolecular forces, stating that the addition of another compound affects the packing of molecules in the crystal lattice, leading to weaker interactions and requiring less energy to break them apart.

Areas of Agreement / Disagreement

Participants present multiple competing views on the reasons for melting point depression, including entropy considerations and intermolecular forces. There is no consensus on a single explanation.

Contextual Notes

The discussion does not resolve the specific mechanisms involved in melting point depression and relies on assumptions about the behavior of mixtures and crystal structures.

erjkism
Messages
54
Reaction score
0
Okay, i am doing a lab involving crude acetaminophen and pure acetaminophen. The crude acetaminophen is supposed to have a lower melting point than the pure acetaminophen...

how does that work and why? does it have to do with thermodynamics or what??
 
Physics news on Phys.org
The substance its mixed with may have a lower melting point; that's my guess anyway.
 
Mixtures have higher entropy than pure substances. Because the transition from solid to liquid is mostly driven by the increase in entropy (ΔS = Sliquid - Ssolid), increasing Ssolid lowers the overall ΔS, lowering the driving force for melting. Since melting is less favorable, you need higher temperatures to accomplish it.
 
ahhh... i see

thanks man
 
You may also think in terms of intermolecular forces. In crystal molecules are packed in such a way that their interactions are strongest. When you add some other compound it stretches the crystal lattice, molecules are not in optimal positions and their interactions are weaker, thus less energy is required to break them apart.
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
6K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 1 ·
Replies
1
Views
542
  • · Replies 7 ·
Replies
7
Views
5K
  • · Replies 15 ·
Replies
15
Views
4K
Replies
0
Views
2K
  • · Replies 1 ·
Replies
1
Views
5K
  • · Replies 9 ·
Replies
9
Views
4K
  • · Replies 3 ·
Replies
3
Views
23K
  • · Replies 19 ·
Replies
19
Views
13K