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Q & A: Why does salt cool ice?

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Most recent answer: 02/20/2008
Q:
Explain why salt makes ice cooler.
- vicky (age 14)
Australia
A:

Let's say you start with some ice and some salt, each at 0°C. Combine them. The freezing temperature of salt water is below 0°C, as low as -21 °C for very salty water. So some of the ice will start to melt, forming salty water. However, it takes energy to pull water molecules out of an ice crystal, where they fit very well, into the liquid. The name for that effect is the "latent heat of melting". That energy has to come from somewhere. So long as the ice and salt are more or less thermally isolated, say in a styrofoam bucket, very little energy will flow in from outside as heat.  The energy comes from all the little thermal jiggling in the ice and salt themselves. So as the ice melts, that leaves less thermal jiggling. In other words, it gets cooled down.

Mike W.


(published on 02/20/2008)

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