Freezing water of different starting temp.

[feyn]

I have questions to a classic experiment : you have 2 equal containers filled with water. IN one the temperature of the water is 98° Celsius, in the other the water has 25° Celsius. If you put both in the freezer the one with the hotter water freezes first.
Now i do understand that one looses its temperature quicker due to the evaporating water (hotter water evaporates more then lukewarm water), but at some point both will have the same temperature, so why is one still loosing its temperature quicker ? Is it cause it has lost more water, so there is less to freeze ? So little water can´t have that much consequences, can it ?

 

[Studiot]

Hello feyn and welcome to Physics Forums.

We already have at least one thread about this subject.

https://www.physicsforums.com/showthread.php?t=616899

 

[blue spruce]

Go to: http://www2.binghamton.edu/physics/docs/Royal%20Society%20Mpemba%20effect%20Final%20%2010%20Aug%2012%20%201.pdf

for the answer.

Blue Spruce

 

[fluidistic]

If you have 2 recipees, one with say 100 ml of water and the other with 90 ml of water at the same temperature and you start to freeze them. It is not obvious at all that the 90 ml will freeze first. I've noticed that the fewer amount of water you have, the lesser it will reach in temperature before freezing (overcooling). I could never avoid over cooling (even with vibrations) when freezing water.
This could imply that evaporation of water actually enlarge the time it will take for the water to freeze. (I did not test for high temperatures).
For very small drops of water, I could go as low as -17°C easily without them to freeze. In fact I could not get them to freeze at 0°C or even close to it. The bigger drops would freeze before the smaller ones (around -4°C was common).
This effect is something I've never read when reading about the Mpemba effect. On the contrary, they all seemed to assume that a lesser quantity of water would freeze before a greater quantity. So they say that evaporation helps the hot water to freeze before the colder water. Well, I do not trust this at all. I've played enough in the lab with water and I'm sure that this is wrong.

 

[PJC01]

First of all, it is important to note that this experiment is a simplified model and may not accurately represent real-world scenarios. In reality, there are many factors that can affect the freezing time of water, such as the size and shape of the containers, the temperature and air flow inside the freezer, and the purity of the water.

That being said, the reason why the hotter water freezes faster in this experiment is because of the principle of heat transfer. Heat always flows from a hotter object to a colder object until they reach thermal equilibrium, meaning they have the same temperature. In this case, the 98°C water has a higher temperature than the 25°C water, so heat will flow from the hotter water to the colder water. This means that the hotter water will lose heat at a faster rate, causing it to cool down and freeze faster.

Additionally, as you mentioned, the hotter water also evaporates at a faster rate. This is because the molecules in the hotter water have more energy, making them more likely to escape into the air. As the water evaporates, it takes away some of the heat energy with it, causing the remaining water to cool down even faster.

It is also important to consider the specific heat capacity of water. This is a measure of how much heat energy is needed to raise the temperature of a substance. Water has a high specific heat capacity, meaning it takes a lot of heat energy to raise its temperature. So, even if the two containers have the same amount of water, the hotter water will still have more heat energy to lose before reaching the same temperature as the colder water.

In conclusion, while the amount of water may play a small role in the different freezing times, the main reason why the hotter water freezes faster is due to the principles of heat transfer and evaporation. It is also important to keep in mind that this is a simplified experiment and may not accurately represent real-world scenarios. More complex factors may come into play in a real-life situation.