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Impurities effect on melting point
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Lauren_6408
#1 Posted : Monday, June 21, 2021 7:08:11 AM
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Hi,
I was wondering why impurities in a compound would only ever decrease the melting point and never increase it. I understand why the melting range is broadened and why the melting range would change but not the direction of the change. In my head I'm currently thinking that it's possible an impurity may have a higher melting point than the pure compound therefore increasing the melting point. If someone could explain this that would be awesome :)

Thanks
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INSTR_Katrina_128
#2 Posted : Tuesday, June 22, 2021 6:10:50 PM
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Awesome question, Lauren!

Let's use water as an example. Water usually freezes/melts at 0°C. The molecules form a crystalline structure at this temperature composed of beautifully-arrangedd hexagonal rings (crystal lattice).

https://www.iceandclimat...IMGP0265_cut_500x220.jpg

Salt (an impurity that we like to add in the winter!) likes to GET IN THE WAY of this perfect crystal lattice structure and disrupt its formation. Instead, the water needs to force the salt crystals OUT, but this requires the water molecules to get even colder.

https://static.scientifi...190131-112389-ya1j16.jpg

I like to think that the salt is like a fake daredevil who's like "I can handle it, guys", but then when the water starts to get colder, it nopes right on out of there. 🙄

Adding an impurity will never increase the freezing/melting point, only DECREASE it.

Let me know if that helps. 😀
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