Why when we mix salt with a cup of ice,the temperature of the ice will drop?

Question

Why when we mix salt with a cup of ice,the temperature of the ice will drop?

Answer 1

If you carefully measured the temperature of a sample cup of ice and then added salt to it, it wouldn't change the temperature very much. Now, I will explain why salt is used in ice cream makers.  When the inner bucket filled with cream is rotated inside a larger bucket of ice, the ice will melt (or some water is already added).  This means that the liquid that is actual contact with the inner bucket is not any colder than the freezing point of water.  If you add salt to this mixture, the freezing point of the saltwater will be lower, or colder, which does make it easier to freeze the cream.  But how on earth can "it get colder just by adding water"?  Actually, what a great many, many people forget about or don't know about is that when you get ice from a freezer, the temperature of the ice itself is actually usually quite a bit COLDER than the freezing point of water.  Thus, by adding salt to the water, you are ensuring that a colder liquid would be contact with the inner bucket of cream.  If you dont' believe me, go buy a modern refrigerator where they have independent temperature controls for the fresh food and freezer compartments---the freezer compartment can be set as low as 30 below the freezing point of water.

Addendum:  I have read pisgahchemist's expanded answer, and deleted my wording about raising the temperature of the ice slightly by the salt added--it actually does drop. I agree that is what happens.  However, it does not cause the mix to drop as much as 30 degrees, so the correct answer will depend on what the Asker wants to know--as a chemist interested in the reaction between ice and salt, or as an ice cream maker.

Answer 2

Yes, the Ice will LOSE 'Sensible Heat' (Specific Heat of Cooling') To the salt causing a decrease in the Ice temperature. This heat is absorbed in dissolving the salt to liquid in the water it produces. (Latent Heat of Fusion of the Salt) as it melts.

Answer 3

The simplistic answer is that the freezing point of a solution of water and salt will be lower than zero C which is the melting point of ice.

In reality the temperature of the ice won't change immediately.  Any water on the ice will be in thermal equilibrium with the ice and it will dissolve the salt to make a concentrated solution with a lower freezing point.  The solution and the ice will be in contact but no longer in equilibrium.  Since the natural tendency is to increase in entropy, the ice will melt to form water, which becomes part of the solution with the lower freezing point.  The reason the ice melts is that energy is required to melt the ice - break the intermolecular hydrogen bonds.  As heat goes into the ice it cools the water, lowering its temperature.  Water from the ice melting at zero C loses heat as it reaches thermal equilibrium with the water which goes into melting ice.  It's a positive feedback loop that continues until all of the ice has melted, and the solution has a temperature below zero C.