3 ms·

What I've never understood about this phenomenon, and what this article fails to clear up is: the phrase "freezes faster" is ambiguous. It could be interpreted

What I've never understood about this phenomenon, and what this article fails to clear up is: the phrase "freezes faster" is ambiguous. It could be interpreted as hot water "drops in temperature at a faster rate than cold water", or "reaches 0°C before cold water, if both placed in a freezer at the same time." I would almost certainly think only the former interpretation makes sense. Because hot water becomes cold water on its way to becoming ice. So if I have two glasses of water, one at 20°C and one at 30°C, and I put them both in a freezer, the hot one may cool down faster, but how does it overtake the cooler one? Because once the 30° glass cools down to 20°, it should now be cooling at the same rate as the 20° one did when it was first put in the freezer... Right?

zem 13y ago
no, hot water can indeed freeze (i.e. turn into ice) faster than cold water, in the same environment. http://math.ucr.edu/home/baez/physics/General/hot_water.html http://math.ucr.edu/home/baez/physics/General/hot_water.html explains it well. he addresses your point directiy: [The "impossibility proof"] implicitly assumes that the water is characterized solely by a single number — its average temperature. But if other factors besides the average temperature are important, then when the initially warmer water has cooled to an average temperature of 30°C, it may look very different than the initially cooler water (at a uniform 30°C) did at the start. Why? Because the water may have changed when it cooled down from a uniform 70°C to an average 30°C. It could have less mass, less dissolved gas, or convection currents producing a non-uniform temperature distribution. Or it could have changed the environment around the container in the refrigerator. All four of these changes are conceivably important, and each will be considered separately below. So the impossibility proof given above doesn't work.

zem 13y ago

no, hot water can indeed freeze (i.e. turn into ice) faster than cold water, in the same environment. http://math.ucr.edu/home/baez/physics/General/hot_water.html http://math.ucr.edu/home/baez/physics/General/hot_water.html explains it well. he addresses your point directiy: [The "impossibility proof"] implicitly assumes that the water is characterized solely by a single number — its average temperature. But if other factors besides the average temperature are important, then when the initially warmer water has cooled to an average temperature of 30°C, it may look very different than the initially cooler water (at a uniform 30°C) did at the start. Why? Because the water may have changed when it cooled down from a uniform 70°C to an average 30°C. It could have less mass, less dissolved gas, or convection currents producing a non-uniform temperature distribution. Or it could have changed the environment around the container in the refrigerator. All four of these changes are conceivably important, and each will be considered separately below. So the impossibility proof given above doesn't work.