RSC Offers $1,600 for theory.

[taking Pat's idea of bound gasses]

 

Ice made from heated water is clearer than ice made from cold water. This is also a known effect, partly due to the reduced aeration of the water caused by boiling and partly by the way inpurities settle-out in the freezing water. It would be a good guess (and without accurate measurement of the aeration and impurity levels and appropriate controlled experiment it is only a guess) that the speed of freezing could be related to the amount of oxygen in the water at the moment of freezing. This could explain why it doesn't work for all temperatures and conditions. Also, pure water is an insulator, ice is also an insulator, poor conductors of electricity are also poor conductors of heat, how the water freezes determines how quickly an insulating layer of ice is formed that could slow down the freezing process, therefore another guess is that colder water freezes around the outside first, creating an insulating barrier slowing the freezing of the water inside the cube - you can see this effect in ice produced in bar ice-machines they put in your Cuba Libre (they are hollow) - it could be that hotter water (the molecules being more active) freezes more evenly and does not create this insulating barrier of ice.

 

 

/edit: the last part also raises questions of conduction vs. convection in removing the heat from the water and that leads to questions over fluid dynamics which, if memory serves, are non-linear. (and would cost a bit more than £1000 to solve).