This phenomenon, which seems completely illogical, was already observed by Aristotle himself. He noted that some inhabitants of present-day Turkey sprinkled the stakes of their palisades with hot water to safeguard them. Apparently, they did so because that way they froze faster. Francis Bacon and René Descartes also took note of this physics curiosity. Yet, it wasn’t until the 1970s that it received a name, the Mpemba effect.
It was named after a young Tanzanian, Erasto B. Mpemba, who noticed that hot ice cream mix freezes faster. But until 2013 scientists had not been able to find a satisfactory explanation for this quirk of nature. Now we know it has to do with the way in which energy is stored in the hydrogen bonds between water molecules.
How does water cool?
So, what is water to begin with? Water molecules have one oxygen atom and two hydrogen atoms, all linked by covalent bonds. Which is basically sharing of electron pairs between atoms. Hydrogen atoms are attracted to oxygen atoms in nearby water molecules, a force called hydrogen bonding. But, at the same time, water molecules as a whole are repelled from each other.
The hotter the water is, the more distance there is between its molecules, due to the repelling force between them. This forces the hydrogen bonds to stretch, so that energy is stored. That energy is released as water cools, allowing molecules to get closer to each other. Releasing energy means cooling.
Hot water has more hydrogen bonds stretching than cold water, so it stores more energy. Accordingly, it has more energy to release when exposed to freezing temperatures. That’s why it freezes faster than cold water.
This phenomenon is much more evident when containers of hot and cold water are placed in a sub-zero environment (-30º C). In this case, hot water is close to the boiling point and tends to evaporate more easily than warm / cold water.
The phenomenon of evaporation requires heat to occur. Since the environment is below zero, most of this heat will be taken from neighboring water molecules. Some molecules will evaporate and some will cool. This is the principle by which water cooling towers work. In the case of cold / warm water, there is much less evaporation and it is negligible in mathematical terms.
Simply put, less mass to cool, greater heat transfer surface, and higher evaporation speed make hot water (apparently) easier to freeze. Why apparently? Mainly because part of it does not freeze. It evaporates, and with the passage of time will end up freezing.
A simple experiment
But don’t take our word for it. You can just follow this simple experiment we found in Quora to see the Mpemba effect in action.
- Pour ½ cup of cold tap water in a small Pyrex container and ½ cup of boiling water in an identical container.
- Put both of the containers in the freezer at the same time. Place them near the back of the freezer, where the icy air comes out but not in the direct blast. Both containers should have plenty of air space on all sides. Also, place the cold water behind the boiling water. This way you ensure that the heat from the boiling water will not interfere with the freezing of the cold water.
- When all is done, start a timer and observe the freezing process.
What should we expect?
If all goes well, you should be able to observe two things:
First, part of the mass in the hot container will evaporate and will not be considered as a solid mass in the experiment. That is, there will be less ice (or snow) than the mass of water that was initially in the water container. Therefore, having evaporated part of the water, now there will be less mass to freeze. It will freeze faster because it has given up part of its heat to evaporate part of the water, favoring the freezing process.
The second consequence is caused by the increase in volume of the evaporated mass. Water increases a thousand times its volume when it goes from a liquid state to a vapor state. In the process, molecules that don’t evaporate push away from each other and become more dispersed. It increases the heat transfer surface and the speed of evaporation. Both of these things increase the speed with which the heat contained in these molecules is transferred to the environment, rapidly cooling and freezing.