It is one of the most famous paradoxes of science, the fact that hot water freezes faster than cold water. Why is this so? The reality is that the scientific community is still struggling to explain it.
The fact that hot water freezes faster than cold water has a name, this is the Mpemba effect, or “Mpemba paradox”, named after Erasto Mpemba, a Tanzanian student who discovered this effect in 1963. That year, the student noticed during cooking classes that his hot milk – put in the freezer – was turning into ice cream more quickly than the same milk cold.
This phenomenon seems to challenge the laws of thermodynamics, scientists are still struggling to find its origin. The simplest hypothesis evoked concerns the mass of water. Indeed, hot water generates steam and therefore, its mass becomes lower than for cold water and is therefore likely to ice faster. Another track has also been explored with supercooling, which allows the water to remain in the liquid state, even below 0 ° C. The fact is that hot water – less sensitive to this phenomenon – would freeze less quickly than cold water.
In 2012 the Royal Society of Chemistry, in the UK, wanted to have an answer and launched a great competition for researchers, so that they can seriously study the issue and bring experiences and concrete conclusions. A Croatian doctoral student won the competition, arguing that the convection effect is the most plausible explanation of the Mpemba effect.
The convection motions are all the internal movements (horizontal or vertical) generated in a fluid mass. Here, if a container of hot water is placed in the cold, convection movements will happen, and it is the latter that will transport the heat out of the container, thus allowing the water to see its temperature drop significantly.