Hot water freezes faster than cold – and now we know why

Water_FlameMany moons ago Deskarati asked the question ‘Why does hot water freeze quicker than cold‘. We never really got to the bottom of it. But now it looks like we might have an answer – Deskarati.

Hot water seems to freeze faster than cold water, known as the Mpemba effect. The effect was named after the Tanzanian student who in 1963 noticed that hot ice cream mix freezes faster than a cold one.  The effect was first observed by Aristotle in the 4th century BC, then later Francis Bacon and René Descartes. Mpemba published a paper on his findings in 1969.

Theories for the Mpemba effect have included: faster evaporation of hot water, therefore reducing the volume left to freeze; formation of a frost layer on cold water, insulating it; and different concentrations of solutes such as carbon dioxide, which is driven off when the water is heated. Unfortunately the effect doesn’t always appear, and cold water often freezes faster than hot water. Until now, no one had ever worked out exactly why hot water freezes more quickly than cold.

solid waterNow a team of physicists from the Nanyang Technological University in Singapore, led by Xi Zhang, have found evidence that it is the chemical bonds that hold water together that provide the effect. Each water molecule is composed of one oxygen atom bonded covalently to two hydrogen molecules. These bonds involve atoms sharing electrons and are well understood. The separate water molecules are also bound together by weaker forces generated by hydrogen bonds. These forces occur when a hydrogen atom from one molecule of water sits close to an oxygen atom from another.

The team now suggest it is these bonds that cause the Mpemba effect. They propose that when the water molecules are brought into close contact, a natural repulsion between the molecules causes the covalent bonds to stretch and store energy. When the liquid warms up, the hydrogen bonds stretch as the water gets less dense and the molecules move further apart.

The stretching in the hydrogen bonds allows the covalent bonds to relax and shrink somewhat, which causes them to give up their energy. The process of covalent bonds giving up their energy is essentially the same as cooling, and so warm water should in theory cool faster than cold. The team’s calculations suggest that the magnitude of the covalent bond relaxation accounts for the experimental differences in the time it takes for hot and cold water to freeze. Via Hot water freezes faster than cold – and now we know why

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2 Responses to Hot water freezes faster than cold – and now we know why

  1. Phil Krause says:

    It still doesn’t do it for me. Imagine two bowls of water, both have identical histories except one is 30 minutes behind the other. Both have been heated up to the same temperature and are cooling down to be frozen. The warmer one will still have to pass through the same temperature as the cooler one. How can it ever overtake the one with a 30 minute advantage? Also I have seen this theory debunked a couple of times.

    • Deskarati says:

      I found the following in the paper refereed to in the post (which can be found here ) and it mentions your problem, which is at least reassuring that the researchers were aware of it.

      The Mpemba effect [1], named after Tanzanian student Erasto Mpemba, is the assertion that warmer water freezesfaster than colder water, even though it must pass the lower temperature on the way to freezing. There have been reports of similar phenomena since ancient times, although with insufficient detail for the claims to be replicated. As indicated by Aristotle [2]: “The fact that the water has previously been warmed contributes to its freezing quickly: for so it cools sooner”. Hence many people, when they want to cool water quickly, begin by putting it in the sun. Although there is anecdotal support for this paradox [3], there is no agreement on exactly what the effect is and under what circumstances it occurs.

      Observations [1, 4] in Figure 1, show the following facts: a) hot water freezes faster than the cold water under the same conditions; b) the temperature θ drops exponentially with cooling time (t) and duration (∆t) for water transiting into ice varies with experimental conditions (volume, exposure surface, etc. For example, freezing 35 °C water takes about 90 min in (a) but 35 min in (b)); c) the skin is warmer than sites near the bottom in a beaker of water being cooled. Besides, blocking heat transfer from the skin with a film of oil drastically slowed cooling. The fact that the temperature of the skin remains higher than the in the bulk of the water throughout the process of cooling is in accordance with findings that the heat capacity of the supersolid skin is higher than the body as the H-O bond there is shorter and stronger [5].

      temp

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