On its way to becoming frozen, doesn't hot water have to become cold at some point?

Let's say that one bottle has water at 80 degrees and the other contains 60 degree water. As the temperature gradient between the water bottle and the outside air decreases (the water gets colder), the rate of change in temperature decreases. So the time it takes to get from 80 degrees to 60 degrees is less than the time to get from 60 to 40. However, the total time to get from 80 to 0 is still more than the time to get from 60 to 0.

Note that the quoted article was talking about hot water freezing faster than lukewarm water because of evaporative cooling. That may work fine in a shallow uncovered pan (which the article talks about doing an experiment with), but if the water is in a closed water bottle there isn't going to be any evaporative cooling because it can't evaporate if there is nowhere for the vapor to go.

The folks before me have hit it regarding rate of cooling.
Here is a video of boiling water to frozen in a moment. Outside air needs to be well below 0C.
http://www.youtube.com/watch?v=urPngC2dNQ4

moonspots wrote:

dan2see wrote:...Well you know how your breath fogs in winter? In that cold, the fog would actually solidify into a breath balloon, ...Well the yells gelled as breath balloons, with the words frozen right inside them. They fell into the snow, and kinda layed around there. Then in spring, when they thawed, the racket of all those melting words was really annoying.

That's pretty funny! If you're not careful, someone from warm country might believe this and pass it on, the beginning of another "internet legend".

That's OK, it's true. I grew up there!

My sisters and brothers shared the same experience. What else could explain those pretty apples in our cheeks?

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Living on a farm, we had to be more independent in basic life-skills. None-the-less, we invented re-cycling for fuels:

My job, and my brother's, was to chop some fire-wood every day for the wood-stove in the kitchen. My mother's job was to light the stove in the morning, to thaw the kettle and keep the house warm. My father's job was to climb up to the roof, where the wood-smoke came out of the chimney and froze during the night. He'd saw it off, and bring the smoke-logs back to Mom who'd toss them back into the wood-stove and burn again.

woodsxc wrote:On its way to becoming frozen, doesn't hot water have to become cold at some point?

If you Google "Mpemba effect" you'll find several explanations of why your argument is invalid. The hidden assumption is that the state of the system is entirely described by one scalar (water temperature). While, apparently, a satisfactory complete explanation of the so-called Mpemba effect is yet to be found, it is generally accepted that various forms of non-uniformity are at work.

However, if the water cools slowly and is constantly mixed, then the cooling process approaches the quasi-static condition that is frequently mentioned in introductory thermodynamics textbooks. My guess is that reproducing the Mpemba effect in two water bottles carried in a backpack would be rather difficult, especially if they are properly insulated. I haven't done the experiment, though.

EDIT: good summary. Interestingly, evaporative cooling seems not to be always required.

Years ago this idea was brought up, and never had a satisfactory proof. The way it was presented to me, was that if you wanted to wash you car in winter, you should use cold water. The supposed explanation had to do with the amount of gas dissolved in cold vs. hot water from the tap.

http://www.gi.alaska.edu/ScienceForum/ASF6/650.html
http://www.livescience.com/mysteries/08 ... cubes.html
http://wiki.answers.com/Q/Does_hot_wate ... cold_water

Maybe it would be a thornier issue is you specified that the hot and cold water had the same mass (not volume) at the start.

Ignoring all the theoretical BS being thrown around, the answer is no, hot water will not freeze faster than cold water in the conditions anyone outside a lab cares about.

mconnell wrote:the conditions anyone outside a lab cares about.

Except, maybe, those Tanzanian ice-cream makers.

dan2see wrote:I grew up in the country near North Bay Ontario. Every winter the weather went down to -40 and stay that way for 3 weeks.

Well you know how your breath fogs in winter? In that cold, the fog would actually solidify into a breath balloon, and then fall into the snow like a light-weight soft rock.

The children played outside. We played tag, or snow-hockey, or just clowned around. But you know how kids yell when they play outside? Well the yells gelled as breath balloons, with the words frozen right inside them. They fell into the snow, and kinda layed around there. Then in spring, when they thawed, the racket of all those melting words was really annoying.

We used to light a small campfire so that we could bring the breath balloons over, and hold them over the fire to thaw them out so that we could talk to each other. Interesting though, if you through them into the fire, they would thaw so rapidly that the words would come out sounding like a munchkin.

Well, I will settle this for sure in a few weeks. I'm going to the White Mountains this week and if I have time between climbing and surviving the extreme cold, I'll do the experment with my water bottles.

If I don't have time, I'll do it in two weekends when I go out for one of my weekend day hikes.

I'll keep you up to date with what I find.

My two cents.

Hot water freezes all the way through faster, where as cold water creates a layer of ice on the in side sides of the bottle which creats an insulation, where the water in the middle stay non-fozen longer. Hot water turns to all slush at the same time and does not create and insulating layer of ice on the inside of the bottle, which will render drinking it useless once the slush is to thick or forzen all the way thourgh. At least with cold water, 3/4 of the water is still drinkable.

Anyone experience the same thing?