how is it that salt is used to de-ice road & also used to freeze icecream?

Answer 1

Rock salt forces the ice surrounding the can of ice cream mix to melt. The "brine solution" or liquid that forms in the wooden bucket absorbs heat from the mix and gradually lowers the temperature of the mix until it begins to freeze. If there were no salt added to the ice, it would melt at 32 degrees Fahrenheit and eventually the ice water and mix would come to equilibrium at 32 degrees. The ice cream mix, however, does not begin to freeze until its temperature falls below 27 degrees. Therefore, in order to freeze the mix, we need a salt concentration, or a ratio of 5 cups of ice to 1 cups of salt. At this concentration, our brine temperature should remain constant at 8 to 12 degrees F. This will give the rapid cooling and freezing that is essential to making smooth creamy ice cream.

More detailed information provided by David Winer, Bethesda, Maryland For ice to melt into liquid water it must absorb a lot of heat energy. In an ice cream maker this heat needed to melt the ice comes from the mix, and so the mix chills. If the ice were not combined with salt, the mix’s cooling would be slow and would stop when the mix reached the temperature of melting freshwater ice-- at 32 degrees. But salt makes ice melt faster, and drops the temperature to that of freezing salt water. The heat used to melt the ice is drawn much faster from the surroundings, which in this case consist of the melted ice AND the ice cream mix. Since the surroundings lose large quantities of heat to the ice, they cool rapidly and continue to cool until they reach the freezing temperature of melting saltwater ice. This temperature is below the cream mixture's freezing point.

Answer 2

To state it simply, , in both cases the salt is doing the same thing: the salt is causing the ice to melt, via chemical reaction.

On the road: wherever salt is sprinkled, it melts the ice. Melted ice surface is no longer slippery, ie road is now safe to drive on. Same with sidewalk, steps, etc..

For icecream: icecream is put in container with a lid. The container full of icecream is put in a larger container. The larger container around the icecream, is filled with salt and ice. The salt melts the ice, creating a constant steady cold which freezes the icecream inside the inner container. Icecream would yet freeze if surrounded by only ice and not salt - but melting the ice by adding the salt, increases the cold and decreases the amount of time required to freeze the icecream.

There is a more scientifically proper way of stating it, I'm sure ! Maybe someone else can explain the chemical reaction that results in what I've described.

Answer 3

Salt lowers the freezing/melting point of water, so in both cases the idea is to take advantage of the lower melting point.

Ice forms when the temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C). On a roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the ice and lowers its freezing point.

If you ever watch salt melting ice, you can see the dissolving process happen -- the ice immediately around the grain of salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then the salt really won't have any effect -- the solid salt cannot get into the structure of the solid water to start the dissolving process. In that case, spreading sand over the top of the ice to provide traction is a better option.

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze. Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture.

Answer 4

Salt REALLY, REALLY wants to react with water to dissolve into it and disassociate into sodium+ and Chloride- ions. When this happens, heat is absorbed from the surrounding environment as the salt melts the ice.

When one uses salt on ice to churn ice cream, the dissolving salt is causing the melting ice to absorb heat. It absorbs it from the chamber in which the ice cream ingredients are being stirred, thereby cooling those ingredients gradually and causing micro-crystaline freezing, which gives the smooth texture of ice cream.

Answer 5

Go to makeicecream.com, it tells you why rock salt is used and how it works.