Since we have started discussing solid liquid and liquid gas transformations (and their opposites), here’s an interesting empirical observation that is yet to be explained clearly and convincingly: Mpemba effect.
The Mpemba effect, named after Tanzanian student Erasto Mpemba, is the assertion that warmer water can sometimes freeze faster than colder water.
There appears to be a lot of uncertainty about the conditions under which this effect can be observed in a reproducible way. And while many explanations have been suggested, the issue has not been settled.
Last year, the Royal Society of Chemistry ran a competition for “the best and most creative explanation of the phenomenon by which hot water sometimes freezes faster than cold water, known today as The Mpemba Effect“. It offered a prize of £ 1000. And the winner was Nikola Bregović, a research assistant in Chemistry at the University of Zagreb, Croatia. In his summary, he says,
The fact that this effect is not fully resolved to this day, was an indication to me that fundamental problems lie underneath it […]. Once again this small, simple molecule amazes and intrigues us with [its] magic.