Mindboggling

[unixronin]

Last week's weather was really ugly at times, and in particular, we had one night when it rained, then snowed, then rained, then snowed ...  It ended up with the driveway covered several inches deep in slush too heavy for the snowblower to shift.  Then the slush froze, leaving an ice crust thick enough to drive on.  I wound up, in desperation, hooking up the hosepipe to a hot-water faucet to get the ice off the driveway.  (This wasn't as absurdly energy-intensive as it sounds.  The ice was spongy, and melted very rapidly under a strong jet of hot water.

While I was doing this, one of our neighbors said, "You know hot water freezes faster than cold water, right?  Try it in your freezer sometime, put an ice tray of hot water and one of cold in there, and the hot water will freeze first."

I was so gobsmacked I just didn't know what to say.  I've heard some ridiculous urban legends in the past, but this one ... I just don't know where to start.  Where on earth does anyone get such a totally wrong-headed idea?  It falls down on even the very simplest of examination — the hot water's got to get cold before it can freeze, and that's going to take time, so the only way the hot water can freeze faster is if some cold water freezes faster than other water, but how do you know you didn't get magic fast-freezing water out of the cold tap in the first place?  To freeze before the water that started out cold, with both trays being cooled at the same rate, the hot water would have to cool to the starting temperature of the cold water in negative time.

Snopes is silent on this one.

The only way I can think of that someone could "prove"this to themself is if they inadvertently put the tray of hot water right under the cold-air inlet to the freezer compartment, so that it's being much more strongly cooled than the other.

Comments

[brownkitty.livejournal.com]

I believe the reasoning is that hot water will create a breeze inside the freezer, so hot water freezes faster by convection.

How the breeze is supposed to continue, I have no idea.

[unixronin.livejournal.com]

Even if there were convection at work, it couldn't possibly make the hot water cool enough faster to make up for the cold water's headstart. Unless the "hot" tray is being cooled more strongly than the "cold" tray throughout, it can never catch up to the "cold" tray without violating the laws of thermodynamics.

[tomatoe333.livejournal.com]

I'd be willing to bet that an ice tray filled with cold water would still freeze faster than one filled with hot, but that the time difference would be very small.

Reasoning: You have to remove a LOT more heat to make the water go from liquid at freezing point to solid at freezing point than you have to remove to drop the temperature of the water, hot or cold, to the freezing point.

[skellington.livejournal.com]

http://math.ucr.edu/home/baez/physics/General/hot_water.html

[unixronin.livejournal.com]

Interesting ... but this looks like it's an effect that they can reproduce some of the time under some conditions, and no-one can really figure out why. I have to speculate that in some way they're haven't yet recognized to take into account, the conditions of the two are not equal. I see they've tried repeating it with degassed water, and that they've calculated evaporation can't be enough to explain it. I wonder what happens if both are stirred to neutralize the difference of convection currents in the water?

It's so counter-intuitive and ass-backwards it makes my brain hurt just thinking about it.

[skellington.livejournal.com]

Yeah.

I'd heard the "old wives tale" when I was a kid, went through the same thought process you did, and did an experiment in the kitchen freezer. (2 trays of ice, one with hot, one with cold.) The hot took longer to freeze and I wrote it off.

But it is a commonly known fact that it does occur, and there's plenty of scientific evidence now to support it.

[tabbifli.livejournal.com]

Your freezer self-defrosted. :)

[databeast.livejournal.com]

actually a kid fom some south african high school got some award aboutt tyears ago for proving that *in certain circumstances* hot water does indeed freeze faster than cold, he even got the name of the effect named for hm

And yeah, its fairly obvious once you realize what's happening... the evaporation from the hot water is sucking out heat faster than regular radiation/conduction would, basically a convection process.

gets there like 2% faster than room temperature water, or something marginal like that.

Mpemba effect, google reveals.

[perspicuity.livejournal.com]

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

they often mention supercooled fluids, but don't mention one of the cool things about them.

if a fluid is below its freezing/frozen point, and it's given an impulse (like tapping/snapping the container with a pencil), it will suddenly and massively start crystalizing :) in the case of water, instant slush. why? well, if there's no nucleation site, and the water is very very still, its kinda lazy :)

ever see those liquid hot packs with the "coin" inside - you snap the coin the pack goes from liquid to slushy/solid in seconds? and puts out 100+ dF in the process for a while? http://www.howstuffworks.com/question290.htm

#

[unixronin.livejournal.com]

Yeah, supercooling is a fun effect.

[unixronin.livejournal.com]

And yeah, its fairly obvious once you realize what's happening... the evaporation from the hot water is sucking out heat faster than regular radiation/conduction would, basically a convection process.

But the various articles say they've already verified evaporation isn't enough ... and besides, if that was the only factor, then the accelerated cooling should slow down as the temperature differential stops. It should never catch up; all other factors being equal, the water that started out colder should always have a headstart.

If the hot water loses enough mass by evaporation that you end up cooling a sufficiently smaller mass of water that it freezes first, that IMHO is cheating. It's like saying that water freezes faster if you pour half of it out. Sure, technically it does ... but no faster than if you only half-filled the container in the first place.

[databeast.livejournal.com]

heh, but remember, this is a piece of 'lay science' we're talking about here, Where the observed effect is what matters, not understanding of the observed effect.

(corollary would be if two new car designs are put together in a competition for fuel efficiency, both claim to have new aerodynamic designs, but one uses lighter materials.. as far as the general observer is concerned, it's got the better fuel efficiency, even if its for the wrong reasons)

but yeah, the Mpemba effect is validated, and does give plenty of credence to the idea that non only scientists can have repeatable results in their obvservations...

[tabbifli.livejournal.com]

Actually, I do know where this belief comes from in the US, and it's got nothing to do with all the tests and supercooled and de-gassed and evaporated and whatever experiments listed above. It's pretty stupid, actually.

Non-self defrosting freezers.

After iceboxes, when inset freezers first started coming about, they were frequently coated in a thick covering of frost (much like your driveway, really). You can't defrost these freezers (or at least you shouldn't) by taking a knife or an icepick to them, and it's a hassle and three halves to unplug the damn thing - it involves unplugging the whole darn fridge.

On the other hand, if you put an ice cube tray full of HOT water in the freezer, you start melting off all the frost, and it sucks the heat right out of the hot water, just to freeze up solid, quick as that. Clean freezer and fresh ice, what do you think?

As it turns out, the freezer actually DID freeze the hot water faster than the cold while it was defrosting - faster ice cubes, defrosted freezer - and the ice box frosted over about every 2 or 3 days, depending on what you put in them. As a result, it got rather firmly embedded in the American mindset, from about the mid-40s onward, that hot water freezes faster than cold.

I really wish I'd known about this particular trick in the first apartment we lived in back in Pittsburgh, too. I burned out a hairdryer defrosting that damn freezer.

[olafthunderfoot.livejournal.com]

i used to defrost mine with a small hammer!