Ice and salt

Main Question or Discussion Point

The guy in the video says something like "the salts going to allow the ice to stay at its freezing point without melting."
I've always thought that adding salt to ice/water mixture lowers the freezing point, and therefore melts the ice. Isnt that the reason why they pour salt on the streets in the winter?

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cepheid
Staff Emeritus
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I've always thought that adding salt to ice/water mixture lowers the freezing point, and therefore melts the ice. Isnt that the reason why they pour salt on the streets in the winter?
Yes.

Maybe that's what the guy in the video meant, or maybe he meant what he said, and he is just wrong.

I concur, I think he meant "the salts going to allow the water to stay at its freezing point without freezing".

Ice, by it's nature, will 'remain at it's freezing point without melting'.

symbolipoint
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Solute lowers the freezing point of the solvent. For ice bath in water, the ICE used is NOT at the freezing point of water ( 0 degree C); the ice is at LESS THAN the freezing point of water. This ice will lower the temperature of the surrounding water. Naturally, the ice will gain energy (heat) from the water as the water becomes colder and eventually, the ice may be mostly at its freezing point and will melt.

When I make ice cream , and i put the salt on the ice around the edge , Why does putting salt on the ice make the ice cream colder than i ever could with just plain ice. Why does putting salt on ice melt it, why does it lower the freezing point?

Well water has a freezing point of ~0oC.

I'm not entirely sure of the why part, but the salt lowers the freezing point - allowing you to make 'colder ice cream'.

I would think it's because of the water-salt solution having a lower freezing point. (I believe road grit is only effective down to ~-15oC

The why part is because the salt ions dont want to form a crystal with the water molecules that easily.

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ok so it melts it because the salt ions prevent it from forming crystals, so the salt breaks bonds, Why does it cause the ice cream to get colder, When the salt is put on the ice does it temperature change?

It doesn't cause it to get colder, it allows it to.

Your freezer causes it to get colder.

The salt allows the liquid to lose more heat energy before becoming a solid (a bit simplistic but I feel it covers the required ground).

russ_watters
Mentor
Actually I would say "causes". If you take an ice/water mixture at 0C and add salt the temp will drop.

K^2
ok so it melts it because the salt ions prevent it from forming crystals, so the salt breaks bonds, Why does it cause the ice cream to get colder, When the salt is put on the ice does it temperature change?
Because when you have ice in the liquid, you have molecules at the surface randomly departing the ice crystal and randomly returning to it. Even if there is no melting or freezing, the balance is dynamic. When molecules leave the ice crystal, they loose kinetic energy equal to binding energy. When a molecule attaches to crystal, binding energy is released as vibration energy in the crystal.

When you add slat, or any other impurity, you reduce the probability of the molecule of water attaching to the crystal. So the balance shifts towards melting. Since that takes up energy, the net temperature drops.

Specifically in case of NaCl, which is ordinary cooking salt, the process of salt dissolving in water also happens to be endothermic. So there is cooling due to that as well. Any impurity will help reduce the temperature, but sodium chloride is probably your best household agent for that.

If your goal is to melt as much ice as possible, however, you want the temperature to stay up. In that case, you want a salt that dissolves exothermically. Potassium chloride, KCl, is a good candidate for that. That's the one they usually use on the roads to keep them ice-free.

Actually I would say "causes". If you take an ice/water mixture at 0C and add salt the temp will drop.
Really? Didn't know that.

Might even give that one a go. Sounds like a nice bit of fun for my free time!

K^2
That's exactly how they made ice cream before refrigerators.

That's exactly how they made ice cream before refrigerators.
Definitely going to give that a go - fantastic end result to that experiment!

symbolipoint
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Actually I would say "causes". If you take an ice/water mixture at 0C and add salt the temp will drop.
The temperature of the water with salt in it can drop because the ice used is at a temperature of LESS than 0 degree C. Also, the freezing point depression effect.

russ_watters
Mentor
The temperature of the water with salt in it can drop because the ice used is at a temperature of LESS than 0 degree C. Also, the freezing point depression effect.
*Unless you pre-mix salt into water and chill it close to 0C before pouring it into the ice, the effect of the sub-cooling of the ice will not take the water below 0C: the melting of the ice is what cools the water below 0C.

IMO, this is an important distinction because after you mix your water and ice, you can let it sit for hours in an insulated container at a reasonably steady temp of below 0C (it will slowly rise as ice melts and changes the concentration of salt). On the other hand, any heat transfer from the ice to the water prior to melting is transient, affecting the temperature of the solution for mere minutes. Heck, maybe more to the point, you can use [salt] water to cool the ice below the temperature it came out of the freezer!

I've got a test set up and will share the results...

*Lemme expand on that a little. Certainly you can take a lot of ice at, say, -15C and a little salt water at +5C and mix them together and end up with all ice or a mixture of ice and water at <0C. I don't think this is the typical scenario, though, nor do I think it is the most instructive/interesting. I think the most interesting/instructive scenario is where you start with some ice at, say, -5C, some salt water at +5C and end up with ice water at -10C. It is counterintuitve, but true, that you can end up with a ~50/50 mixture that is colder than the starting temp of either part.

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symbolipoint
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russ_waters tell us this:
*Lemme expand on that a little. Certainly you can take a lot of ice at, say, -15C and a little salt water at +5C and mix them together and end up with all ice or a mixture of ice and water at <0C. I don't think this is the typical scenario, though, nor do I think it is the most instructive/interesting.
That is how it is done in practice. The intent is to rely on ice which is much much below 0 degrees C, and to have it immersed in salty water (not all of the salt needs to be dissolved). The salt dissolved in the surrounding water will allow the water to reach termeratures of much less than 0 degrees C. Negative 10 C can easily be reached.

russ_watters
Mentor
That is how it is done in practice. The intent is to rely on ice which is much much below 0 degrees C, and to have it immersed in salty water (not all of the salt needs to be dissolved). The salt dissolved in the surrounding water will allow the water to reach termeratures of much less than 0 degrees C. Negative 10 C can easily be reached.
I think the ratio of water and ice is typically 50/50 or even more than 50% ice - it is less common to have "a lot of ice and a little water". My point was that you rely on the melting of the ice, not the temperature of the ice. Perhaps I'm not making myself clear.

In any case, I ran a test:

I filled an insulated travel coffee mug completely with ice cubes and put it in my refrigerator (I wanted to start with "warm" ice and my freezer is very cold), covered, with a temperature probe in it. I pre-mixed 400ml of water with 1/2 cup of salt and put it in my refrigerator. When the ice reached -2.5C (this took a couple of hours) I took the mug out, poured the water (starting temp +3.8C) in and mixed. After a few minutes, the temperature dropped to -11.6C.

My point is, the starting temperature of the ice is not really that important here. You can even have a final temperature below the starting temperature of the ice.

davenn
Gold Member
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In any case, I ran a test:
I filled an insulated travel coffee mug completely with ice cubes and put it in my refrigerator (I wanted to start with "warm" ice and my freezer is very cold), covered, with a temperature probe in it. I pre-mixed 400ml of water with 1/2 cup of salt and put it in my refrigerator. When the ice reached -2.5C (this took a couple of hours) I took the mug out, poured the water (starting temp +3.8C) in and mixed. After a few minutes, the temperature dropped to -11.6C.
My point is, the starting temperature of the ice is not really that important here. You can even have a final temperature below the starting temperature of the ice.
that is really "cool" pun intended ;) something I am going to have to try for myself
as said it sounds so counter-intuitive, now to obtain a reasonable thermometer :)

Dave

GT1
I filled an insulated travel coffee mug completely with ice cubes and put it in my refrigerator (I wanted to start with "warm" ice and my freezer is very cold), covered, with a temperature probe in it. I pre-mixed 400ml of water with 1/2 cup of salt and put it in my refrigerator. When the ice reached -2.5C (this took a couple of hours) I took the mug out, poured the water (starting temp +3.8C) in and mixed. After a few minutes, the temperature dropped to -11.6C.

My point is, the starting temperature of the ice is not really that important here. You can even have a final temperature below the starting temperature of the ice.
Can you explain in more detailed what cause it?