The Mpemba effect is the curious phenomenon that hot water can freeze faster than cold water. Consider that! A container of hot water, identical in all other aspects except temperature to another container of water, can freeze before the cooler water. Surprised? Confused? You are not alone! The question of whether this is real or not has been discussed for over 2300 years.

The Mpemba effect is named after Erasto Mpemba. As a high school student in Tanzania, he re-invigorated the scientific discussion about this effect, along with a professor that worked with him. Mpemba was making ice cream by boiling milk and adding sugar, then freezing it. He noticed that the boiling hot mixture that he put into a freezer formed into ice cream faster than a cooler similar milk and sugar mixture that a fellow student had added to the freezer. (I think it is important that this fascinating effect was elevated in scientific circles through making ice cream faster than your friends.) At first Mpemba was more mocked than believed for his assertion, but through dogged determination Mpemba got a professor interested, and a series of university experiments supported Mpemba’s observation. Mpemba and the professor, Dr. Osborne, put out a paper together in 1969 entitled simply “Cool?” This increased scientific interest in this odd cooling effect.

Mpemba was one in a long line of people looking into this effect. It had been noticed by none other than Aristotle in the 300s BC. Aristotle wrote that “The fact that water has previously been warmed contributes to its freezing quickly…” and implied that this was well known (if not well understood) by saying that “Many people, when they want to cool water quickly begin by putting it in the sun”. Aristotle was unable to empirically prove, or even put forth a reasonable mechanism for this effect.

In the 1200s, Roger Bacon, an English philosopher and scientist, experimented with this then-called ‘hot begets cold quicker’ phenomenon without resolution.  In the 1400s (now 1700 years after Aristotle), the Italian physicist, doctor, and mathematician Giovanni Marliani is said to have the first empirical proof of this effect, although the reasons why were not understood. Francis Bacon wrote about hot water freezing quicker than cool water in 1620 in his “Novum Organum Vol VIII”, as did Rene’ Descartes in his 1637 “Les Meteores”. Over three hundred years later when Mpemba revived interest in this effect it was still not understood.

After Mpemba reinvigorated discussion about this effect there was of course considerable doubt in the scientific community. And why not? The very idea of it seems to contradict or at least skirt basic rules of thermodynamics. The fact that the mechanism could not be explained, and that the Mpemba effect only worked (if it indeed did work) in certain circumstances and seemingly none of the experiments were able to isolate all other factors or be reproducible also cast doubt.

Forty some years after the publication of “Cool?”, there was still controversy. In 2012 Britain’s Royal Society of Chemistry had a worldwide competition to find the best explanation of the Mpemba effect. It had 22,000 submissions (including one from the author). The winning submission (not this author’s) focused on four possible reasons for the effect. While none are proven to be the single reason for the effect, some think that a combination of these four plus an additional fifth possible reason known as “Super cooling” might account for the effect. Others deny still that the effect exists and point to some experiments showing this.

Here are the explanations put forth for the Mpemba effect:

1. Evaporation – hotter water evaporates more. Evaporation itself is a cooling effect, besides which hot water would have less mass to cool due to evaporation.
2. Dissolved gasses – hot water contains less dissolved gasses (and boiling water removes almost all gasses). Less gasses could change the ability of the water to conduct heat, could reduce thermal insulation in the water, or could change the freezing point of the water.
3. Convection – hot water will have greater temperature gradients and as water density is a product of temperature the cooling effect of convection currents from temperature gradients will affect the total cooling of the water.
4. Surroundings – the theory that hot water somehow affects the surrounding of its container in such a way that facilitates quicker cooling. For example, a container of hot water might melt ice that the container is on, increasing physical contact between the container and the ice, which would assist cooling.
5. Supercooling – the idea that cooler water would possibly need to supercool below 0 degrees C in order to actually freeze. This idea also incorporates the idea that a nucleation site might be needed in the cooler water in order to form ice crystals, while something called temperature shear in non-thermally homogenous cooling hot water would cause freezing closer to 0 degrees C for hot water.

This is all just fascinating to me! The Mpemba effect to date has neither been totally disproved or proved. As they say, “We can put a human on the moon, but we still don’t know if cold water freezes before hot water.” Also, a great learning subtext to this story is that a young student put forth a proposition that seems preposterous, but that may be true. It is a lesson in keeping an open mind and not dismissing different ideas out of hand.

What do you think? Could the Mpemba effect be true? Would it defy accepted laws of thermodynamics? Could it be the key to other learning? Or maybe more importantly, would it change how you make ice cubes at home? (We tried to test this several times in our freezer, but our experimental plan failed every time due to “Look – squirrel!”. However, my wife and I do agree that boiled water makes much prettier ice cubes).

Thanks!

Richard MacAlpine

Richard is a Lab Supervisor at Metro Wastewater Reclamation District in Denver. His opinions and any facts presented in this blog are his responsibility solely and do not reflect at all on his employer. Richard is also the RMWEA Lab Practices (LPC) Committee Chair and is on the RMWQAA Board.

Resources:

https://math.ucr.edu/home/baez/physics/General/hot_water.html More detail on the five main mechanisms considered to cause the Mpemba effect

https://edu.rsc.org/download?ac=13093 Copy of Erasto Mpemba and Dr. Osborne’s 1969 paper “Cool?” in Physics Education, 1969

https://www.nytimes.com/2008/03/18/health/18real.html A quick article arguing that while the Mpemba effect is real, its obverse (cold water boiling faster than hot water) is not

https://www.nature.com/articles/srep37665#:~:text=The%20Mpemba%20effect%20is%20the,to%20the%20writings%20of%20Aristotle. Nature article arguing against the Mpemba effect

http://www.eoht.info/page/Aristotle-Mpemba%20effect  Brief history of the Mpemba effect, general info, and how the Mpemba effect may apply to the Cold War (the social Mpemba effect).

https://www.rsc.org/news-events/articles/2012/06-june/rsc-offers-1000-for-explanation-of-an-unsolved-legendary-phenomenon/ Royal Society of Chemistry Mpemba effect competition from 2012 (which again, the author did not win), and other info

https://en.wikipedia.org/wiki/Giovanni_Marliani#cite_note-Hot_water_freezes_faster_than_cold_water-1  Marliani’s supposed empirical proof of the Mpemba effect, and his inability to ascribe a mechanism to it

http://www.eoht.info/page/Roger%20Bacon Info on Roger Bacon, who was called the first scientist, was the originator of the scientific method, and one in a line of people who did not solve the Mpemba effect