Here’s a link to the article: Hot water freezes faster than cold – and now we know why.
Here’s my initial theory on the matter:
Water is one of the only molecules whose solid takes up more volume than it’s liquid state…which means that the molecules in the solid form of H2O are further spaced apart in the crystalline structure than near freezing water. As water temp rises, the distance between molecules increases, until we get a gas…which requires energy. Due to the solid taking up more volume than the liquid, near freezing water would therefore have molecules closer together than necessary for the crystalline bonds required for ice. Near freezing water molecules would therefore require a little more energy to expand for the bonds to form than water at a temp with the correct molecular distance to create the bonds. Therefore, there would be a water temp at something above freezing where the molecules were spaced most appropriately for the quickest freezing, so to speak, with the least transfer of thermal energy necessary to do so. Which is why it doesn’t appear to be consistent. Water at a precise temperature above freezing can therefore freeze quicker than both water at a lower and higher temp than that equilibrium point, but warm water ‘in general’ does not freeze faster than cold water. Since a similar phenomenon does not happen with water turning to steam (i.e. the liquid does not take up more space than the gas), cold water would not boil faster than warm water.
There would also probably be a range of temperatures just above the equilibrium temp where the water could reach the equilibrium temp quicker than colder water could release thermal energy, and gain enough kinetic to cross over the threshold for crystallization, and freeze….further adding to the ambiguity and uncertainty in most data sets during experimentation. This phenomenon would also require that the water be placed immediately in a sub freezing environment and not merely having the surrounding temperatures drop below freezing on both samples. Some surface crystallization would have to immediately be able to take place, otherwise warm water would become cold water before Any crystallization could ever take place…thus defeating the premise by not isolating that variable.
Or maybe think of it something like this?: a regular filament bulb basically has a linear energy requirement to glow due to the resistance in the wire, much like warm water at the perfect temp; where cold water freezing has a small peak energy threshold it must overcome to crystalize before it then loses whatever thermal energy might be left in below freezing temperatures, much like how a ballast is necessary to light a halogen bulb crossing a peak energy threshold requirement, but then that quick boost of energy to cross that threshold is no longer necessary to keep the bulb lit.
To me their article seems to describe a symptom of the phenomenon or the means by which the phenomenon ultimately stabilizes…not necessarily the reason for the phenomenon or any explanation as to the ambiguity in the data set. Of course any insight into the matter is welcome, I only gave this a cursory thought. ???
But, hey, it’s not rocket science… : ) [yes, that’s intended as a joke]
ADDENDUM: One other observation. Crystals forming at the surface of warmer water already taking up a larger volume might also form a larger outer frozen boundary than crystals in colder, lower volume water. Cold water crystallizing from the outside in would then put more pressure on the liquid trapped in the center not yet crystallized. This would require more energy for the unfrozen water to expand the exterior crystalline structure already formed in order for the inner water to crystallize. If warm water is able to basically ‘flash freeze’ at the surface, it’s volume may not compress the inner water nearly as much inhibiting the expansion necessary to create further crystals, thus reducing the transfer from thermal to kinetic energy required for the final crystallization threshold. So I would want to determine if ice from cold water has a higher propensity for cracking at the outer surface when it freezes, than would water that was at that equilibrium temperature where the molecules are already properly spaced for crystallization.