How does adding salt to ice make it colder?

Answer 1

It doesn't make the ice colder. Salt lowers the freezing point of water. Normally water freezes at 32 degrees Farenheit/0 celsius. The addition of salt will lower the temperature at which water freezes so that the salt water solution is still a liquid below 32 degrees Farenheit/0 celsius. This effect will work for several degrees below the normal freezing point of water. So, this is why you put salt on your driveway in winter and the ice will melt, because the freezing point has been lowered.

Answer 2

Why can adding salt to ice water make the ice melt slower?

My son is doing a 5th grade science fair project using ice cubes and salt water. We hypothesized that ice cubes would melt faster in salinated water but they melt slower. Is that because the salt lowers the temp of the water thereby making the water colder so the ice takes longer to melt?

Yes. Adding salt to the ice/water mix causes a temperature drop that slows the melting rate and increases the freezing rate [3]. The net result is that the ice melts more and more slowly after the initial addition of salt.
Why does salt melt ice? In pure water, at 0°C, ice melts just as fast as water freezes. You won't see any of the ice melt as long as the freezing rate and melting rates are exactly equal [1].

Adding salt (or any foreign substance) to the water upsets the delicate balance between freezing and melting. Fewer water molecules reach the surface of the ice in a given time, so water freezes more slowly. The melting rate isn't changed by the salt, so melting "wins" [2].

Does adding salt to ice and water cause a temperature drop? Yes. This is how old-fashioned ice cream makers lowered the temperature of the ice cream below water's ordinary freezing point. A mixture of rock salt, ice, and water packed in the bucket around the ice cream mix can bring the temperature down as low as -21°C.

Why does the temperature drop? Energy is required to snap the hydrogen bonds that hold the ice together. The melting ice draws that energy from the surrounding solution as heat.

If you'd like to extend your project, here are some suggestions:

Watch how the temperature of the ice water falls after the initial addition of salt. What does a plot of temperature vs. time look like? (Stir constantly and completely!)
Try adding different amounts of salt. What does a plot of temperature vs. salt concentration look like? Can you keep lowering the temperature this way indefinitely, or is there a point where adding salt produces no additional cooling? What happens when you add salt beyond that point?
Suppose you forced the saline ice water and the pure ice water to have exactly the same temperature. Would ice in the saline solution melt faster than ice in the pure water, then?
Does adding salt to water without any ice result in a temperature change?

Answer 3

This Site Might Help You.

RE:
How does adding salt to ice make it colder?

Answer 4

Simply put:
Ice melts faster in contact with salt.
Faster melting absorbs more heat.
The mix gets colder.

Answer 5

Salt lowers the freezing point.

Answer 6

Colligative properties, specifically molal depression of melting point.

http://www.chemguide.co.uk/physical/phaseeqia/salteutect1.gif
http://www.ec.gc.ca/nopp/roadsalt/reports/ParkingLot/EN/phasediagram.jpg
ice-salt-water phase diagram

Answer 7

It lowers the freezing temperture.