Boiling water becomes air

By Markus C. Schulte von Drach

Some people there try to demonstrate how insanely cold it is in the USA by throwing boiling water into the air. Before it even reaches the bottom, the liquid turns into a cloud and hailstones that audibly patter on the earth. A fake? No.

However, the pictures show less the extreme cold than the astonishing physical behavior of water. The fascinating thing is that if cold water is used instead of the hot liquid, the effect is not there.

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Physicists have not yet been able to conclusively explain why this is so. There are, however, some factors that are likely to play a role that have been discussed for a long-known paradox: hot water freezes faster than cold water.

The Greek philosopher Aristotle pondered this fact in the fourth century BC. The English monk and philosopher Roger Bacon then dealt with it in the 13th century and his compatriot Francis Bacon, scientist and philosopher of the 17th century, failed in the search for explanations, as did his French colleague and contemporary René Descartes. After that, the matter was somehow forgotten.

It was not until 1963 that researchers began to focus on the effect again, after 13-year-old Erasto Mpemba messed up a school project in Tanzania. When trying to make ice cream from milk, he impatiently placed the still hot starting liquid in the refrigerator. His cream froze faster than samples from his schoolmates, who waited obediently for their liquids to reach room temperature and only then put them in the refrigerator. Since then, the effect that Mpemba developed together with the British physicist Denis Osborne, at the University College Daressalaam, Tanzania, in 1969 in the Journal Physics Education described, referred to as the Mpemba effect or paradox.

In a narrower sense, the name refers to the following experiment: Two identical open containers are filled with the same amount of water. One liquid is warmer than the other. Then both are cooled down at the same time under the same pressure and the same ambient temperature (for example in the refrigerator or outdoors in winter). The temperature of the warmer water then drops more quickly to around three degrees, maintains this temperature for a while and then freezes significantly earlier than the previously colder liquid.

Since then there have been a number of suggestions to explain the phenomenon.

  • It can therefore be assumed that there is a connection with the faster evaporation of hot water. This reduces the volume of the liquid that ultimately has to freeze. The process of evaporation itself also has a cooling effect.
  • Gas bubbles, of which there are fewer in warmer water, presumably play a role.
  • Salts dissolved in the water lower the freezing point. In hot water, however, these tend to fail and their influence is reduced as a result.
  • In addition, hot water is naturally warmer in the middle than on the faster cooling edges. These temperature differences cause currents within the water, which also contribute to cooling. This "convection" is stronger in warm water than in cold water.
  • "Supercooling" is also one of the suggested explanations. This is the phenomenon that under certain conditions liquids remain liquid well below their actual freezing point and only solidify when a crystallization nucleus enters - that can be a dust particle.

But all of these explanations were so unsatisfactory that in 2012 the British Royal Society of Chemistry initiated a competition: Whoever offered the "best and most creative explanation of the phenomenon" was to receive £ 1,000. The winner was announced in January 2013: Of the 22,000 entries, the declaration by Croat Nikola Bregovic convinced the jury the most. The chemist from the University of Zagreb had carried out a number of his own experiments and came to the conclusion that he, too, could not find a final solution.

Rather, James D. Brownridge from the State University of New York rightly pointed out the importance of hypothermia a few years ago. However, Bregovic noted, the role of convection needs to be emphasized more.

Researchers led by Xi Zhang from Nanyang Technological University in Singapore recently presented another explanation. They attribute the Mpemba paradox to the unusual processes that take place at the level of the bonds between water and oxygen when hot water is cooled. (If you want to know more about the complicated explanations, you will find the original literature behind the links in this text.)

It is a different matter whether water is placed in a bowl in the refrigerator or thrown into the air. However, some factors that are discussed for the Mpemba phenomenon should of course also play a role here. For example, the surface of the hot water mass changes abruptly and significantly when it is poured into the air from a bowl, which should significantly intensify the evaporation process and thus the cooling process.

The fact that hot water is currently pounding on the ground as hailstones in the USA is not a surprising consequence of the polar cold there. If it were this alone, some Americans would not have got scalded trying to repeat the experiment.

The observation, however, proves what Nikola Bregovic also found after his experiments: "Once again, this small, simple molecule surprises and fascinates us with its magic."

P.S. Even without knowing why it works, manufacturers of ice cream use the Mpemba effect. And every motorist knows that warm water should not be used to wash the car when it is below zero.

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