Which water is heavier, hot or cold

Why does warm water freeze faster than cold water?

At a glance

  • It's hard to believe: warm water freezes faster than cold water
  • This can be demonstrated with a simple experiment

It sounds illogical at first, but it is a fact: under certain conditions, hot water freezes faster than cold water. Scientists know the phenomenon as the Mpemba effect. A precise explanation of the process is still pending, but there are a number of possible interpretations. An overview.

In areas where the thermometer falls deep into the double-digit minus range in winter, an experiment that is as fascinating as it is beautiful to look at can be carried out: If you throw hot water from a glass into the air in the open air, it freezes before it falls to the ground falls. The amazing thing about it: The trick doesn't work nearly as well with cold water. Behind this so-called Mpemba effect, which can be admired in numerous videos on YouTube, is the paradoxical fact that warmer water turns into ice faster than colder water under certain conditions.

Researchers have puzzled over the effect since ancient times

The effect has been known at least since the ancient Greeks - the philosopher and scientist Aristotle mentioned it as early as the fourth century BC. And other researchers documented the phenomenon again and again afterwards. The Mpemba effect did not get its name until 1963, when the then 13-year-old student Erasto Mpemba from the East African Tanzania discovered that hot milk in the refrigerator frozen faster than cold milk when making ice cream.

The race to freezing point between two bowls filled with liquids at different temperatures does not work unconditionally: You need two exactly the same vessels with the same amount of liquid. The temperature difference between the two quantities should be as large as possible, whereby the temperature of the colder liquid must not be too close to the freezing point, otherwise it would have an unassailable lead. Air pressure, temperature and other external conditions must be identical, and the experiment must take place in a so-called open thermodynamic system - that is, the liquids must be able to exchange energy and matter with their surroundings.

Different explanatory models

If you put the vessels in the freezer compartment, the liquid in both initially cools down to a temperature of around three degrees Celsius. The warmer one takes a little longer for this, but copes with the cooling significantly faster. Both liquids now remain at around three degrees Celsius for a while. Then the originally hotter one begins to cool down further and is the first to reach freezing point.

So far there is no generally accepted explanation of the Mpemba effect. In 2012, the Royal Society, the British academy of science, promised those who could conclusively unravel the phenomenon that they would receive £ 1,000 in prize money. Although he did not find a complete solution either, the physical chemist Nikola Bregović from the University of Zagreb received the reward the following year - he considers convection to be a major cause of the phenomenon in his approach.

Convection of the riddle?

Convection occurs due to the temperature differences in a liquid: a bowl of hot water, for example, first cools down on the walls of the vessel and on the surface. The resulting difference in density to the warmer water in the middle of the vessel sets a circulation movement in motion, which also has an effect on the temperature.

The greater the heat difference between hot water and the cooler environment, the stronger the convection and thus the heat dissipation of the liquid. At the same time, the flow of water increases with increasing heat, which, according to Bregović, in turn increases convection - and, according to the researcher, beyond the point at which both liquids reach the same temperature. As a result, the originally warmer sample can continue to cool faster than the previously colder one.

Evaporation also serves as an explanation

Other scientists explain the Mpemba effect with the process of evaporation, which takes place more strongly in hot water than in cold water. With the same amount of water and identical ambient conditions, according to the theory, a larger proportion of the originally hotter water has evaporated when the same temperature is reached. This now smaller amount stores less thermal energy than the previously colder one (and now larger) and therefore freezes faster. Still other explanations suggest the salt content in the water as the cause of the Mpemba effect.

Whatever is behind the fascinating phenomenon, there has so far been no practical use for the Mpemba effect. However, if you want, you can use it in winter with little effort to become a hobby scientist yourself and get to know the apparently unspectacular element of water anew. At the end of his award-winning work, chemist Bregović puts it this way: "Once again, this small and simple molecule amazes and fascinates us with its magic."

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