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I understand that adding/sprinkling, say NaCl, on a highway depresses the freezing point by making any moisture on the road harder to freeze as the NaCl molecules get in the way of phase transition. So now we need lower temperatures to freeze the moisture on the road.

However, why can I cool my drink "better" by placing it in a salt + ice bath instead of just an ice bath? It would seem that a salt + ice bath only keeps the water from freezing or in other words, the ice from melting for longer duration of time. Where does the lowering of temperature come into this picture?

I am not able to connect these two phenomenon although people tell me they are the same.

Links to what I have already seen:

http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml

http://revision3.com/forum/showthread.php?t=34351

I need someone to rephrase something and I cant put my finger on it.

Edit:

I was thinking more about it and here is what I can further add to my question:

  1. Sprinkling salt on the highway PREVENTS water to ice transition by intereference due to NaCl molecules. It is actually preventing the liquid-solid transition and it would seem to people that NaCl is melting the ice.

  2. When I place my drink in a ice + salt (NaCl) mix, the NaCl molecules dissociate into Na+ and Cl- ions into the water and this is an endothermic reaction where the NaCl molecules remove heat from the water molecules decreasing the temperature of water molecules and converting them to ice. I am assuming that the NaCl molecules strip the ice of heat as well further decreasing their temperature.

Any comments?

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3 Answers 3

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Adding salt to water makes it freeze at a lower temperature. This fact is being used in two different ways in the two scenarios you mention. Dissolving sodium chloride in water is slighly endothermic, but this effect is small and to the best of my knowledge isn't important in the drink cooling process.

Putting salt on the highway is quite straightforward: we don't want ice to form, so we put salt in the water to prevent that. This doesn't just change the amount of time it takes ice to form, it actually completely prevents ice from forming, unless the temperature gets so low that the water can freeze even with salt in it.

Cooling your drink is a bit more complicated, because in this case the rate at which things happen is important. You don't want your drink to be less than $0^\circ C$ because it would freeze; instead you want to cool it down to a few degrees Celsius nice and quickly.

The rate at which it cools depends on two things: the temperature of its surroundings (the colder the better) and the heat conductivity between it and them. You could try to cool it by putting it in a bowl of ice at $0^\circ C$, but the problem is that the ice is solid and will only touch the bottle at a few points. This results in a poor thermal conductivity, so the drink will only cool slowly.

To get around this, you could try mixing the ice with some water. Now the bottle is touching the liquid over a large surface area, and the liquid itself has a higher thermal conductivity than solid ice due to mixing, so heat will be transferred much more quickly. But the problem is that the water won't be at zero degrees any more, at least not at first (I'm assuming the water comes from a tap, so it's not chilled initially). You have to wait for quite a bit of the ice to melt before the water's temperature will drop. Also, once you put your warm drink into the water it will heat the water up as the drink cools down, so again you have to wait for the ice to melt in order for the water to cool again.

The solution to this is to make the ice melt faster. You can do this by adding salt. This lowers the freezing point, making the water less "happy" about being in the liquid state, so it melts more quickly. This means firstly that the tap water you've added to the ice will cool to close to $0^\circ C$ much more quickly, and secondly that once you've put your drink in there the water will stay cold as the ice continues to melt.

It's also possible that, with the salt added, the water can go to below $0^\circ C$, but this will only happen if the ice is quite a bit colder than $0^\circ C$. This could be the case, but my intuition is that the rate of cooling due to the ice melting faster is more important here than the final temperature. You could easily test this by putting a thermometer in the salty ice water and seeing if it goes much below freezing.

There's also the fact that dissolving the salt is endothermic, as you mentioned. To test whether this is important, you could try adding salt to some chilled water without any ice, and see if the temperature drops a lot. My feeling is that it will only drop by a tiny amount that will be hard to measure with a normal kitchen thermometer, but you can always try the experiment.

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Just so that I got this right: About the drink cooling scenario -- When I add salt to a ice + water bath, the salt molecules interfere with the transition of water-to-ice, at the ice surface. So because of salt added in this situation, more of the water remains water. But since this water is in contact with the ice, it gets cooled. Wouldn't I be better served if I didn't use salt then? I am thoroughly confused now. –  drN Dec 25 '12 at 2:22
    
Perhaps I should also take this answer into account. didn't find it previously. –  drN Dec 25 '12 at 2:24
    
The water is not being cooled (much) by being in contact with the ice, it's actually the ice melting that does the cooling, because it takes energy to break the molecules apart from one another. Eventually all the ice will melt (unless the ambient temperature is below freezing - but then you probably wouldn't be trying to cool a drink!) - you want this to happen quickly in order to make the water stay cold when you add the warm bottle of beer. It's not about how much of the water remains solid, it's about how fast the ice melts. –  Nathaniel Dec 25 '12 at 3:45
    
Note that this is different from salting a highway. In that case the ambient temperature is below freezing, so you're trying to stop the water freezing, rather than slow the melting. In the highway case it is about how much water remains solid, and not about how fast the phase change happens. –  Nathaniel Dec 25 '12 at 3:47
    
I think I'm just being very dense! I'll sleep on it and come christmas day, all should be clear! –  drN Dec 25 '12 at 5:50

As you noted, adding salt to the ice causes the freezing point of the ice to drop. When ice (or any other substance) is in the process of melting, it always does so at the constant freezing point temperature, which you just lowered by adding salt. The new, lower freezing point temperature keeps the ice cooler and therefore cools your drink faster.

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When add salt to ice, you're just decreasing its freezing/melting point. (So at a low temperature, like -5 celcius the ice would melt) "In the real world, on a real sidewalk, sodium chloride can melt ice only down to about -9°C (15°F)." [1]

It doesn't help a drink to cool down because what really matters then is the rate of heat transfer from drink to ice/ice+NaCl (which doesn't change significantly when you add salt to the ice)

If you put ice+salt and ice in a freezer for a long time, the final temperature of the ice+salt would be lower than the final temperature of the ice. Thus, when using ice+salt to cool the drink, the rate of heat transfer from the ice+salt to the drink would be faster than the heat transfer from the ice to the drink because the temperature difference between the ice+salt and the drink is greater than the temperature difference between the ice and the drink.

Wikihow explains how to cool drinks quickly, "Add table salt to the ice. A small handful should do. The salt will disassociate into its constituent sodium and chloride ions. The water molecules, being polar, will orient themselves accordingly. This is work, and work requires energy which comes from thermal energy in the water, thus reducing the ambient temperature further."

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So when I add salt to ice, the rate of heat transfer from ice to water is decreased hence keep the ice as ice for longer? –  drN Dec 25 '12 at 0:20
1  
"Increase" the freezing point? I think you are wrong there. –  drN Dec 25 '12 at 0:21
    
I noticed that you changed "increase the freezing point" to "increase the melting point" in an effort to rectify the error pointed out by drN. However, the melting point and freezing point are the same temperature, so you should have modified your answer to read "decreasing the freezing/melting point." –  Draksis Dec 25 '12 at 1:26

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