Dave 02/03/01
 | Vocabulary |
equilibrium freezing point depression hydrogen bond melting point solute |
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?
Notes
- Some of the ice actually is melting, but new ice forms as fast as old ice melts.
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See these previous questions for more:
"Why does salt melt ice?" (includes a Flash simulation of freezing point depression)
"How can freezing point depression be explained in terms of free energies?" - The melting ice also adds more water to the solution. The higher water concentration gives the freezing rate an additional boost.