Why does salt reduce the freezing point of water?

  • Thread starter lekh2003
  • Start date
  • #1
lekh2003
Gold Member
588
339
I've been looking into how adding sodium chloride or calcium chloride to snow reduces the freezing point, and why this occurs.

The really simple reason I found on this website: https://www.thoughtco.com/how-salt-melts-ice-3976057 , which simply stated that the salt ions get in the way of the atoms or molecules coming closer together and creating a solid.

The more complex reason I found was related to entropy, which I did not understand. I understand entropy, but the explanation on this website is not making much sense: https://van.physics.illinois.edu/qa/listing.php?id=1580 . I tried making some sense of it, but it all seems very convoluted. Why is it absolutely required that we need to create a net entropy gain? I am referring to this explanation:
Say you have a cup of pure water and a cup of somewhat salty water. As you lower the temperature some of the pure water starts to form ice crystals. The reason is that although the frozen water molecules, lined up into a crystal, have fewer ways to move around (lower "entropy") than the liquid molecules, they release heat when they freeze and that raises the entropy of the surroundings even more. So the net entropy goes up as the water freezes, as it always does on the way to any equilibrium state.

What about in the salty water? There's one extra term in the entropy change. The salt doesn't fit into the ice crystals. So as they form, the remaining salt is left with less room to roam around in, and thus less entropy. So you have to get the salt water even colder before you get a net entropy gain from freezing it.
Please help me understand what the relation to entropy is.
 

Answers and Replies

  • #2
Mister T
Science Advisor
Gold Member
2,735
949
Why is it absolutely required that we need to create a net entropy gain?

It's not a requirement, strictly speaking. It's a statistical tendency: Given the opportunity to increase entropy, Nature will take it.
 
  • #3
lekh2003
Gold Member
588
339
It's not a requirement, strictly speaking. It's a statistical tendency: Given the opportunity to increase entropy, Nature will take it.
Exactly, I'm aware of that, but I feel like the explanation for it is assuming that the aim of the system, be it pure or salty water is to gain entropy. This is why I'm slightly annoyed at it. Is this something I should trust. Can someone provide a more sound reasoning when related this concept to entropy?
 
  • #4
1,538
131
What about in the salty water? There's one extra term in the entropy change. The salt doesn't fit into the ice crystals. So as they form, the remaining salt is left with less room to roam around in, and thus less entropy. So you have to get the salt water even colder before you get a net entropy gain from freezing it.

I would say that because it takes a lot of energy to remove salt from water, there is less heat energy released when salty water becomes non-salty ice, compared to when non-salty water becomes non-salty ice, and thus less entropy is generated. So you have to get the salt water even colder before you get a net entropy gain from freezing it.

I don't disagree with the claim that the remaining salt is left with less room to roam around in, and thus less entropy. I just think that's less important.
 
  • #6
Exactly, I'm aware of that, but I feel like the explanation for it is assuming that the aim of the system, be it pure or salty water is to gain entropy. This is why I'm slightly annoyed at it. Is this something I should trust. Can someone provide a more sound reasoning when related this concept to entropy?
I’d see it thus - with salt, at the same temperature there are more configurations of water + salt molecules (more disordered, more entropy) where the salt “gets in the way” and prevents the formation of ice. So the water needs to cool down even further to have the same probability of h2o molecules lining up to form ice as under the no salt scenario. So the melting point of ice is lowered.
What this also means is that it’s not just salt, even adding sugar should have the same impact as you just need things to “get in the way”. Conversely, no additive can raise the melting point of ice, only lower it. Short of chemical bonds forming and breaking, that is. Are the last two true?
 

Related Threads on Why does salt reduce the freezing point of water?

  • Last Post
Replies
2
Views
3K
Replies
7
Views
17K
  • Last Post
Replies
5
Views
9K
  • Last Post
Replies
3
Views
7K
  • Last Post
Replies
8
Views
7K
Replies
1
Views
3K
  • Last Post
Replies
6
Views
2K
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
1
Views
2K
Top