Just binging YouTube video’s just led me to this one:
The faster cooling of hot water as opposed to cooler water is called the Mpemba efffect. Named after Erasto Mpemba, who was making ice cream in a school project. The students were supposed to boil together sugar and cream, and let the mixture cool before chilling it in the freezer. However, worried that he might not have a spot in the freezer, Mpemba immediately placed the boiled mixture into the freezer, rather than letting it cool. After an hour and 30 minutes later, when he checked on the freezer, he noticed that his concoction had frozen while his classmates had not.
While there have been many theories, these are 5 proposed in the past to reason as to why hot water freezes faster than cold water, although right now, there is not a definite answer.
- Frost melting
- Dissolved Gases
- Supercooling
- Evaporation
- Convection
The theory of frost melting comes from the fact that in the freezer, the frost around the hot beaker will melt, and this will act as a rapid-cooling effect from the layer of liquid water surrounding the beaker. However, this was also found disproved from other experiments which insulated the base of the beaker, or basically removed the surrounding water, and still claim to find the Mpemba effect occurring.
The second suggestion was that there was a higher concentration of dissolved gases in colder water. And so as warm water cools down, gases are constantly being dissolved into it. But since this is an exothermic process, it should actually slow cooling, and is therefore counteractive, and proves nothing.
The third states that while water may freeze at 0° Celsius, is when water freezes, however, realistically its temperature may be significantly lower than this before freezing occurs. The reason behind this is because in order for water to freeze, it requires a nucleation site like an impurity or air bubble. So that things like hot water, which will gradually gain more gases will freeze faster as they have more nucleation sites. Although, the results from multiple tests are widely inconsistent, as supercooling relies on microscopic impurities such as a speck of dust, it provides an ambiguous answer.
The fourth suggestion is that the hot beaker will evaporate water, and therefore have less water to cool or freeze after the temperature has dropped and freeze faster. Although, realistically the total mass lost from the hot water is usually less than 3%, which fails to explain the drastically different cooling rates. And the Mpemba effect is still supposedly found in sealed beakers, which would reveal that changes in mass is not the reason that hot water freezes faster than cold water.
The fifth proposition was regarding convection: That water cools primarily from the sides and top, and so the water in the middle would remain a higher temperature than around it. This would cause the cool water to sink, and the hot water to rise, exposing it to the sides and causing it to cool quickly; these are called convection currents. Since warmer water has greater temperature differences, it would also have greater convection currents, so even when it reaches the same temperature as the cooled beaker, it would still have more convection currents, and cool even faster. But even this is uncertain, as there are countless unpredictable variables in the freezing of water, that a true conclusion can not be reached.
The reason it remains so widely unknown, is that it’s possible that even cold water freezes faster than hot water (WHAT HAVE I BEEN TRYING TO LEARN). From an extremely controlled study by scientists in 2016, there was no observations of the Mpemba effect in beakers. An explanation for this comes from the impossibly difficult way of measuring the variables for water freezing. They found if a slight variation of the positioning of the thermocouple — the instrument for measuring temperature, by even a centimeter, a slight Mpemba effect appeared. After the scientists reviewed past studies of the Mpemba effect, they all saw that the reported effects were in margins of error, and could definitely be incorrect.
However, recently Xi Zhang at the Nanyang Technological University in Singapore and other chemists have found a more plausible and modern solution.
They say the answer originates from the hydrogen bonds between water. A hydrogen bond occurs when the slightly negatively charged hydrogen in one molecule (of water) comes close the slightly more positive oxygen in another water molecule and bonds to it. They say that hydrogen bonds bring water molecules into close contact with each other, and the natural repulsion between molecules causes the covalent bonds to stretch and store energy. But from heating water, the covalent bonds stretch, and the covalent bonds shrink, causing the stored energy to be released, and therefore cool.
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