Freezing water

[chakka35]

I keep coming across the claim that hot water will freeze faster than room-temp water. To me it makes no sense.

The room-temp water starts to cool from there, whereas the hot water will take time to reach room temperature and, from then on, will always lag behind the room-temp water in the cooling process. Am I missing something?

[DTCwordfan]

try putting two identical trays of water in your freezer - one with boiling water and one with room temp and then see which freezes first. Most times the boiling one goes first...

The reason for this is because when boiled, the water absorbs thermal energy (heat) and as we see in nature, when energy is placed in an environment with significantly less of that energy form, the cold air is "thermophilic" meaning the energy in the hot water is attracted out of the hot water, into the cold atmosphere, until equilibrium is met (or in the case of a freezer that is adjustable through a thermostat, the water wouldnt stop cooling until it reaches the temperature on the thermostat). I quote,

The fundamentals of what happens is similar to how coil magnets work when varied levels of electric energy are applied. A heavier electric current causes a stronger magnetic pull in a similar way that a greater difference between freezing and boiling causes a stronger thermal energy transfer from the water into its surrounding atmosphere. Just like with magnets, if the magnets are too weak, or if the water you are freezing does not have a large difference in temperature from what temp it is starting at, it is possible that there may not be enough energy here to even trigger a reaction at all. Like when magnets are a certain distance away, they don't pull one another anymore.... when the difference in temperature between the liquid and the air around it goes below a certain point, think room temperature, you see nothing more than a very slow trickle of that thermal energy out of the water, if you see the water cool at all. The water at cooler temperatures has been "charged" with less thermal energy and also not heated warm enough to produce steam, thus cannot initialize a rapid, steamy transfer of heat that you would see with hotter water.

[DTCwordfan]

so no you are not a candidate for the mad-house.

Two loons in the lounge.

"Is the clock on the wall over there correct?"

"Yes, I think so."

"Well then, it shouldn't be here."

[chakka35]

Thanks, DT, I think we all know that the rate of transfer of heat from a hot body to a cold is proportional to the temperature difference between them (which is why you should put the milk in the tea if you want it to stay warm longer)
But that does not explain why the originally-hot water, once it has reached room temperature, then cools faster than the water that was at room temperature to begin with and is therefore ahead of it in the race for freezing.

[DTCwordfan]

in non-science term, the faster cooling rate involved from starting with boiling water continues down to forming ice, rather than just starting at room temp and going down.

[mibn2cweus]

Time is only one factor of many regarding when and how water freezes. The quality of the water, the final weight and volume of the ice, variations in ambient temperature and convection currents within and around the water, evaporation, pressure, is the water frozen solid throughout or only on the surface, all must be considered and taken into account.

With the complementary corresponding mix of contributing causal agents it can be found that the hare has indeed beat the tortoise to the finish line, but not by defying the laws of physics. First things first. Show me the ghost and then when we can begin to investigate and to possibly determine the actual nature of and mechanism behind the manifestation.

[jake-the-peg]

Where did you get that information fro DTcrosswordfan?

Do you have a link?

Because it doesn't make much sense to me.

It sounds as if it's coming from Netwons law of cooling which says that the rate of cooling is proportional to the temperature differemce.

But that doesn't explain the mpemba effect because as it cools at some point the water in the boiled container will reach the same temperature as the cooled container and so should behave as the cooled container did but that one of course had a head start.

http://en.wikipedia.org/wiki/Mpemba_effect

I believe there are still a number of potential explanations to this effect - which seems widely observed. I think the jury is still out on it

[chakka35]

Precisely my point, jake. And I wonder why DT thinks he should talk in non-scientific terms when this is the Science site.

Quite right, mib. When my current ice-trays are empty (a few G&Ts from now) I'll see if the ghost manifests itself.

[ludwig]

It sounds like an urban myth. It implies that because the initial rate of heat loss from the hot water is greater than the room temperature water, some sort of heat loss momentum is maintained which causes the 'Hare to overtake the tortoise' as mib puts it.

I'm no physicist, but that doesn't seem likely.