How does salt in icewater cause foods to freeze more quickly?

The rock salt melts faster. It is placed in the container on the outside of the ice cream bucket, not in the ice cream. As you turn the handle, the ice on the outside of the bucket melts and the ice cream inside the bucket get harder and harder.

It lowers the freezing point as well.

Salt in water also causes its freezing point to drop. That’s why they put salt on side-walks and roads.

Edit: Ninja’d

Right… so, how does lowering the freezing point cause whatever solution you’re surrounding to freeze faster?

@andrew

It allows you to lower the temperature of the water below 0 C.

@andrew It freezes faster because as the temperature of the ice goes down, it sucks the heat out of the ice cream.

Ice is not at 0 degrees Celsius, it is below that. By adding the salt, the freezing point of water is lowered further so that the entire liquid mixture itself can become lower than 0 degrees.

Water cannot drop below its freezing point until all of it is frozen. By lowering the freezing point, you let lose more heat before it gets stuck at one temperature. This is called the heat of solidification.

The same holds true for boiling: the water will not exceed 100 degrees Celsius until the entire amount has changed to gas. This is called the Heat of Vaporization.

Adding any substance to water (or any liquid, for that matter) that creates a solution, will both lower the freezing point and increase the boiling point. This is because the salt throws off the equilibrium of the reaction, and also affects water’s ability to form a crystalline solid structure.

Hrm. And how does the ice lower the freezing point?

The ions from salt in aqueous solution interfere with the water molecules’ ability to interlock into a solid.

is awaiting @Harp’s answer

Liquid water is far more efficient at conducting the heat away from the sides of the container than is solid ice. The salt allows for a soupier (yet still very cold) ice mixture, which pulls the heat out of the ice cream mix faster.

If you want to talk about the ice cream, there are two types of heat transfer in play; conduction and convection. Condution refers to the heat transfer from the ice cream into the water because they are adjacent to each other. (i.e. the energy from the milk/sugar etc. leaves and goes into the water/ice bath surrounding it. and the temperature of the ice cream drops.) This occurs at a FASTER rate when the difference in temperature between the two is greater. Therefore, you want to add the salt, so that you can make this water/ice bath colder and the difference in tempurature greater. The second part of the equation is convection. This refers to the heat transfer that takes place because a fluid is in motion as it passes over the heat sink (our ice/water bath). It’s pretty much common sense, but the ice cream machine stirs the milk and sugar so that the heat is dissapated to the water/ice bath quicker than if it were to simply sit still.

@andrew
The salt lowers the freezing point.

And to @Harp‘s and @Bcarlyle’s points (the best on here), I’d figure that one of the aspects of the efficiency of using supercool liquid is that it’s also able to stay in continuous contact with the ice cream container over a much, much greater surface area than chunks of ice.

It’s able to effectively cool – below freezing – the entire surface of the ice cream container, not just the random points that jagged chunks of ice would contact.

And I’d doubt that even using chunks of dry ice would work as efficiently as salty, icy water.