Why salt makes ice cream freeze: ice melting below 0 °C pulls out the heat
30seconds
Ice is already at 0 °C, so how can a handful of rock salt make it colder? Because salt makes ice want to melt below 0 °C, and melting costs energy: about 334 joules per gram, taken from the bucket and the cream.
The effect has a floor. A salt-and-water brine bottoms out at the eutectic, 23.3% salt by mass and −21.1 °C. Drag along the brine curve, then churn a tub to see why plain ice can chill cream but only a salty bath can freeze it.
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