Can Salt Freeze Ice?

[Jinx]

The Olympic sportcasters just said the lower portions of the women’s speed skiing run is slushing. As such, it has been treated with salt to help it freeze up. Huh? I wager it’s like why NaCl salt is placed on ice cubes surrounding the churn when making homemade ice cream, but I’m not clear on why this is, either. You want to draw the heat out of the churn, but again…does salt really help accelerate the process?

I must be picturing something wrong. I’m sure the SD can set me straight!

[ZenBeam]

They said the same thing when they were talking about them having problems with the half pipe getting warm. They also brought out fire hoses to spray it with. I would think the salt and spraying with water would both make it worse, so I’m interested in what the theory is as well.

[Great_Antibob]

Based on this article, it looks like the salt is used to partially melt the slush, which then freezes in a smooth layer. You can use a snow machine to add more snow on top of the ice or not as the situation may require.

[Telemark]

http://en.wikipedia.org/wiki/Alpine_skiing#Salting

Chemicals can be applied to the snow surface in order to harden the course. The most common chemicals used are sodium chloride, calcium chloride, urea, ammonium nitrate, and potassium nitrate. This “salting” is done mostly when the snow is wet and slushy. When a race course is salted, the salt crystals break up into ions. These ions lower the freezing point of the snow, which hardens the surface. This provides a dense layer of snow with a consistent surface throughout the course, making the race more fair for athletes skiing later in the race.
Two important factors that determine whether salt will work are crystal structure and moisture. Crystals that are too sharp will prevent the deep penetration of the salt into the snow. Crystals that are too round lack flat surfaces that can bond tightly to adjacent crystals. The snow also needs to have a moisture content of around 60%. Salting a race course works best when the weather is sunny and warm. Salting may also work when it is raining, since the rain adds moisture to the snow. In general, salt will not harden the snow if it is cloudy and just above freezing.
Salting is typically only done in slalom and giant slalom events. It is important to salt the apex of the turn especially well, because that is the part of the turn when the skier exerts the most pressure on the snow. Speed events (downhill and super giant slalom) have a larger surface area, making salting expensive; the high speeds of these events also make salting potentially dangerous.

[Carl_Pham]

You add salt to the ice-water mixture around an ice-cream machine to prevent the water from freezing, because the inner tub (containing the ice cream) usually rotates with respect to the outer. If the water were to freeze, this wouldn’t be possible. You want water in there in the first place because it gives good thermal contact between the cream mixture and the cooling jacket.

[JWT_Kottekoe]

@Carl_Pham

You add salt to the ice-water mixture around an ice-cream machine to prevent the water from freezing, because the inner tub (containing the ice cream) usually rotates with respect to the outer. If the water were to freeze, this wouldn’t be possible. You want water in there in the first place because it gives good thermal contact between the cream mixture and the cooling jacket.

To make ice-cream you need to produce a temperature that is below the freezing point of the creamy mixture. Salt water freezes at a lower temperature than water. By adding salt to an ice-water mixture, you lower its temperature. This is an inexpensive way to obtain the required temperature, as long as you have a source of ice. It also stabilizes the temperature, which is now controlled by the ratio of water to salt. As heat flows into the ice/brine, more ice melts, but the temperature stays roughly constant (it rises a bit because the brine becomes more dilute).

[Colophon]

Indeed - in fact the zero point on the Fahrenheit scale was originally set as the lowest temperature attainable by a mixture of salt and water. Well, kind of.

[leadfoot]

@Colophon

Indeed - in fact the zero point on the Fahrenheit scale was originally set as the lowest temperature attainable by a mixture of salt and water. Well, kind of.

Not according to Cecil…

http://www.straightdope.com/columns/read/3146/did-cecil-err-in-explaining-the-significance-of-zero-fahrenheit

[Omar_Little]

I’ve always understood that adding salt to ice will allow the melted ice to sustain temperatures below freezing or at freezing and still be in a liquid format. That is why you add salt to ice surrounding ice cream freezers.

[Bill_Door]

But when you add salt to ice it melts. As it melts it’s going to take up the heat of fusion, somewhere around 6 kJ/mol. If the melted water is removed it takes that heat with it, leaving the rest of the ice colder, the same way water turning into vapor cools a pot of hot water.

[AlmostPerfect]

I think the term everybody would like to hear is freezing point depression.

I don’t know why it is useful in the aforementioned scenarios, but that is the reason that the freezing point goes down.

[UTejas]

@Bill_Door

But when you add salt to ice it melts. As it melts it’s going to take up the heat of fusion, somewhere around 6 kJ/mol. If the melted water is removed it takes that heat with it, leaving the rest of the ice colder, the same way water turning into vapor cools a pot of hot water.

This is the key point. It’s a bit like a Zamboni on a skating rink. The slush is warm. Dissolving NaCl is endothermic, meaning it takes up heat from its surroundings as the reaction proceeds. A salty layer of water will be present at the surface, but, because it has drawn heat out of the slush, the underlying snow/water can freeze. The surface water can then be pushed off, leaving behind a nice and compact layer of frozen ice.