Friday, May 12, 2017

physical chemistry - Why does freezing point decrease on adding impurities?


It is a well known fact that freezing point decreases on adding impurities. I feel that it should increase on adding impurities.



Reason: I have watched this video on supercooled water. It says that when ice forms, it needs some nucleation points. If water is pure and left undisturbed, then there would be no nucleation points and water won't freeze even below $0^o$C.


So, my reasoning is that when impurities are added, they provide additional nucleation points where water crystals can gather. This would cause water to freeze faster.


But it doesn't happen. Why?




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