Hot water freezes before cold water!

In fact, under general experimental conditions, hot water will freeze faster than cold water. This phenomenon is counterintuitive, and even many scientists are surprised. But this is true and has been observed and studied in many experiments. Although after Aristotle, Bacon and Descartes

He wrote this passage to support one of his wrong views, which is called "antithesis". Anti-peristalsis is defined as "the increase of one feature because it is surrounded by another opposite feature." For example, when the surrounding environment suddenly gets cold, a warm body will get hot. 」 。 The most powerful refutation is that Wojciechowski's experiment found that Mpemba effect was still observed in a closed container without mass loss.

dissolved gas

Another explanation is that the dissolved gas in hot water is discharged, which changes some properties of water, and these changes can explain this effect. The lack of dissolved gas may change the heat transfer capacity of water, or change the heat required for freezing per unit mass of water, or change the freezing point. It is right that hot water retains less dissolved gas than cold water, and boiling water drives away most of the dissolved gas. The question is whether the Mpemba effect can be affected to a considerable extent. As far as I know, there is no theoretical work to support this explanation.

An experiment indirectly supports this explanation. Mpemba effect can be seen when water containing gas is used for experiments, but it can't be seen after removing water containing gas, but its significance to Mpemba effect is not completely clear.

It should be noted that the density of water reaches the minimum at 4℃, so below 4℃, the density of water will decrease due to the decrease of temperature, forming a "hot top". This complicates the situation.

surroundings

Hot water may change the surrounding environment, which will cool down quickly in the future. An experiment reports that the experimental data will change with the change of refrigerator size [7]. Therefore, it can be considered that not only water, but also the environment around water is very important.

For example, if the container for water is placed on a thin layer of frost, the container for hot water will melt the frost and directly hit the bottom of the refrigerator, while the container for cold water will continue to sit on the cold frost. Therefore, hot water and cooling system have good heat exchange. If the melted frost freezes again and becomes an ice bridge between the refrigerator and the container, the heat exchange may be better.

Obviously, even if this argument is true, its application is quite limited, because most scientists will be very careful when doing experiments, and will not put the container on the frost, but on the thermal insulator or the cooling basin. So this explanation may be suitable for family experiments, but it is not suitable for most published experimental results.

Overcooling

Finally, [supercooling] may be very important for Mpemba effect. Supercooling occurs when water is not at 0℃, but freezes at a lower temperature. Supercooling occurs because "water freezes at 0℃" is a statement about the lowest energy state of water-below 0℃, water molecules "want" to arrange into ice crystals. This means that they should stop moving randomly as they do in liquid state and replace them with ordered solid lattices. However, they don't know how to arrange them, but need a small number of irregular objects or nucleation sites to inform them. Sometimes when the water drops below 0℃, the nucleation site cannot be seen. At this time, water does not freeze when it is below 0℃. This phenomenon is not uncommon. One experiment found that hot water was only slightly supercooled (about -2℃) and cold water was supercooled more (about -8℃)[ 12]. If it is true, this can explain the Mpemba effect, because cold water needs to do more work-that is, the temperature needs to be lower to freeze.

However, this cannot be considered as the only explanation of "that". First of all, as far as I know, this result has not been independently confirmed. There are only a few experiments in the above experiment [12], so this discovery may be a statistical fluke.

Second, even if this result is true, it can't fully explain this effect, but it just shifts the problem to another place. Why is there less supercooled hot water? After all, once water cools to a lower temperature, it is generally expected that water will not remember what temperature it is used to. One explanation is that hot water dissolves less gas, which will affect the supercooling phenomenon. The problem is that people will expect that because hot water has less gas, that is, fewer nucleation points, it should be supercooled more, not less. Another explanation is that when the temperature of hot water drops to 0℃ or below, its temperature distribution changes greatly compared with that of cold water. Because the temperature shear leads to freezing [26], the hot water is less supercooled, so it freezes first.

Thirdly, this explanation can't work in all experiments, because many experiments measure the time when the water temperature reaches 0℃ [7, 10, 13] (or the time when the water surface forms thin ice [17]). Some documents say that the "real Mbamba effect" is that hot water freezes completely at first, but other documents have different definitions. Because the strict time of supercooling itself is unpredictable (see the example in [26]), many experiments choose not to measure the freezing time of the sample, but to measure the time when the top of the sample reaches 0℃ [7, 10, 13]. Supercooling cannot be applied to these experiments.