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A reasonable start to analysing the problem is to consider the process in two stages; first, cooling the water to an average temperature of 0 °C (or enthalpy equivalent thereof), and second, freezing the water to form solid ice. In so doing any effects associated with the supercooling of water are entirely contained within the second stage. We restrict our definition of the Mpemba effect to the first stage of the process, i.e. the process of cooling a sample of warm water to 0 °C in less time than it takes to cool a sample of water, which is notionally identical except that it is initially at a lower temperature, to 0 °C.
The total entropy of an isolated system always increases over time, or remains constant in ideal cases where the system is in a steady state or undergoing a reversible process.
There is no fully satisfactory theoretical proof for the Second Law, although there are some connections to Quantum Mechanics, Probability and Relativity.
As the video explains the lack of a clear definition of the mpemba effect is part of the problem, but my definition was the same as the citation in the OP from the 2016 paper which is "freezing the water to form solid ice" so just a bit of surface ice wouldn't count, though in my experiments that distinction wouldn't have made any difference because the surface ice always appeared on the cooler water first unless I had sources of experimental error like the vent blowing on the top shelf.
originally posted by: Phage
a reply to: swanne
Really? So, just a bit of surface ice could be the criterion?
Arb? What did you use for "freeze" in your experiments?
Yes my experiment failed to display it but some experimenters observed it with water, this researcher 28 times in 28 trials, and he thinks it's related to supercooling:
originally posted by: swanne
a reply to: Phage
He used water. Which has no fat. And he reported that the experiment failed to display the effect. This is why I think saturation of fat is the possible culprit in the ice cream thing.
An explanation for why hot water will sometime freeze more rapidly than cold water is offered. Two specimens of water from the same source will often have different spontaneous freezing temperatures; that is, the temperature at which freezing begins. When both specimens supercool and the spontaneous freezing temperature of the hot water is higher than that of the cold water, then the hot water will usually freeze first, if all other conditions are equal and remain so during cooling. The probability that the hot water will freeze first if it has the higher spontaneous freezing temperature will be larger for a larger difference in spontaneous freezing temperature. Heating the water may lower, raise or not change the spontaneous freezing temperature. The keys to observing hot water freezing before cold water are supercooling the water and having a significant difference in the spontaneous freezing temperature of the two water specimens. We observed hot water freezing before cold water 28 times in 28 attempts under the conditions described here.
According to just the laws of thermodynamics, it is backwards, because they say the cold water should freeze first. Some researchers think there is more going on here such as supercooling, and if that's the case, then the experiments don't violate the laws of thermodynamics as some might suggest.
originally posted by: loveguy
It seems backward.
Hot molecules (less viscous) move about quicker than cold molecules (more viscous) do? The less movement the quicker the freeze?...
Still seems backward to me.
There are ways to mitigate the transfer of heat but if heat transfer is all that is taking place there should be no conditions for which the hot water will freeze first, because it doesn't have any way to transfer enough heat to the colder water to make it hotter than itself, right?
originally posted by: TheAlleghenyGentleman
You can't put both hot and cold samples in a small freezer together at the same time as the warmth from the hot sample will change the conditions for the cold sample.