Why does salt reduce the freezing point of water?

In summary: Shorter answer: Salts lower the freezing point because they disorderedly occupy water molecules and impede their ability to form ice crystals.
  • #1
lekh2003
Gold Member
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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.
 
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  • #2
lekh2003 said:
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
Mister T said:
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
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
lekh2003 said:
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 to Why does salt reduce the freezing point of water?

1. How does salt lower the freezing point of water?

When salt is added to water, it breaks down into positively charged sodium ions and negatively charged chloride ions. These ions interfere with the formation of ice crystals, making it more difficult for water molecules to bond together and freeze. This results in a lower freezing point for the solution.

2. Why does salt only lower the freezing point of water, and not other liquids?

Salt lowers the freezing point of water because of its unique chemical properties. When dissolved in water, salt forms ions that disrupt the hydrogen bonds between water molecules, which are responsible for the formation of ice crystals. This does not occur in other liquids that do not have hydrogen bonds.

3. How much salt is needed to lower the freezing point of water?

The amount of salt needed to lower the freezing point of water depends on the concentration of the salt solution. Generally, the more salt that is added, the lower the freezing point will be. However, there is a limit to how much salt can be dissolved in water, and adding too much salt can actually cause the freezing point to increase.

4. Does the type of salt used affect the freezing point of water?

The type of salt used does not significantly affect the freezing point of water. However, different salts have different solubility levels, which can affect the concentration of the salt solution and therefore the freezing point. For example, table salt (sodium chloride) is more soluble in water than rock salt (calcium chloride), so a solution of table salt will have a lower freezing point than a solution of rock salt.

5. Can other substances be used to lower the freezing point of water?

Yes, there are other substances besides salt that can lower the freezing point of water. These include sugar, alcohol, and antifreeze. However, salt is the most commonly used substance because it is inexpensive and readily available. It is also non-toxic and does not harm the environment when used in small amounts.

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