Does hot water freeze faster than cold?

Hi,
what is wrong with this title? Well it should sound like a nonsense, NO, of course not! Hot water freezes slower than the cold water simply because it takes some time for the water temperature to drop to the same value as the first water.


Actually you could find out that this is entirely not true and it has been a big mystery for a long time.

Of course I must add that such a water, in glass bottle for example is put into the freezer on the same place as the second bottle because if you put the hot one next to the side of the freezer, then of course it will freeze sooner.

While I did not try this experiment yet, it seems that there were already thousands of people who have done so before me and they have many hypothesis about what really happens there.

It has been actually known for years, even Aristotle pointed this out. This effect is called Mpemba effect, after the guy who wrote about it in 20th century, he discovered it when he was just a kid, with ice cream.

The graph below shows the temperatures of two water samples.

We of course must assume that this was very well conducted experiment but it works for illustration.

Now something had to happen when the water was hot. That is for sure because after about 4,000 seconds the water is same right?

Well it does not need to be, first we must know the factors that change for example how much the water holds its temperature.

Those are: its volume and its inside.

Its volume? Well 10 milliliters of water will lose their temperature almost “immediately” so this is important factor. How could it change (of course we assume that on start the volume is the same with the same water from tap)? Well water evaporates and it evaporates more quickly when it is hot. This is true the water will relly lose significant amount of its volume to have some effect. We can actually test if this is the only factor. We can close the container so it wont happen. This has been tested and even that it looked as a great hypothesis it was falsificated from what I read.

Another thing could be that when you have water and you heat it, its inside will change causing it to have different “hold” of temperature so it drops more quickly, gases dissolve better in cold liquids so there are less in the hot water.

What other factor could play here? One guy on physics stack exchange said that without the ice around the bottles everything worked perfectly fine but it seems that this is not actually the case while it could change it.

The problem here is that even if you have some process which will get it extra fast down, at one point it will be same anyway.

Another one could be convection. The top will be always hotter since the way water works. Since it loses heat primarily through surface it could lose its heat faster than we would expect because the top would be always warmer while the cold water would not be able to make such a great convection.

I came up with another solution. Even if you close the glass bottle so there is the same amount of water, the water will still evaporate and the side of the bottle will get wet with the condensated water. There will be bigger surface which will get cold while the water which condensated fell down again, cooling it by little bit. Also of course that when you put it freezer which has ice on bottom, it will melt through it and since water has higher thermal conductivity, it will get colder from the bottom.

What else? Maybe you can come up with something but I did not find the complete answer yet!

Dragallur

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