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

[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

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[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...