|MadSci Network: Physics|
The key principle involved here is freezing point depression. When salt is mixed with water, the salt causes the freezing point of the water to decrease. This means that instead of turning to ice at 32 F, the ice will freeze at a slightly lower temperature. This is exactly the reason why salt is poured on city streets during a winter snowstorm. When mixed with the salt, the snow requires a lower temperature to remain frozen. The salt, therefore, helps the snow to melt and prevents countless accidents.
In the case of a string with salt on it, exactly the same principle is being applied. The salt that comes in contact with the ice melts the surface, forming a thin layer of water. This layer of water, however, freezes very quickly causing the string to stick to the ice.
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