Can Salt Freeze Ice?

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!

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.

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.

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

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).

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

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.

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.

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.

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.