Friday, May 20, 2005

Weird Science: Hot Or Cold Freezing H2O

Ever hear this statement? - hot water freezes faster than cold water. Ever shake your head at the person that said it, move away discreetly and wonder where they escaped from and whether you should call the police? Sorry to dispel this urban myth so quickly but - hot water will not freeze faster than cold water given the same starting conditions and a sealed environment. If you don’t believe it, (Never Never Land exists too and the jungles of South America are teeming with hidden dinosaurs, but I digress) try it and see. But don’t use ice cube trays (quick evaporation of boiling water in the modern no-humidity freezer will leave so little of the boiled water left in a small ice cube tray as to make the experiment moot - a small mass of water will freeze before a large mass of water), use larger, preferably smooth walled metal containers for the experiment instead.

But my Grandpappy told me it was true! Yeah, well maybe your Grandpappy was taking some medication that was a little too strong and not fully tested by the FDA. But before you get all upset at me, let’s dissect the wording of this wild claim for additional clues. Firstly, the sentence states "hot water freezes faster". What a curious way to put that. It could be interpreted as water, which was previously heated, will freeze faster than water which was not previously heated. And here is where Grandpappy may have been on the correct medication, just a bit misunderstood, because previously boiled water "may" freeze faster than cold water if they both start off at the same temperature for the experiment.

Allow me to explain. Water which has been boiled will have driven out some of the air bubbles (which are present in water) and because air bubbles cut down thermal conductivity (why do you think windows filled with inert gases are better insulators than ones without?) they can inhibit freezing. This may allow the previously boiled water to freeze more quickly than water which was not previously boiled - again, provided they start off at the same temperature for the experiment.

Which brings us to the next item of why hot water pipes tend to burst before cold water pipes when they freeze. As a test you could turn off your furnace in the middle of winter until the pipes burst, but I wouldn’t suggest that you do - (but if you must know first hand, please conduct this experiment - then feel free to email me your results). And yes, it is true that hot water pipes tend to (notice the "tend to" which means not in all cases) burst before the cold water pipes and here is why. Previously heated water (which has driven out some air bubbles) forms denser ice than water which contains more air bubbles. And because solid (frozen) H2O occupies more volume than its liquid form, it can burst pipes - and the denser the ice, the larger the volume, which gives it a slightly better chance of busting a pipe first.

Oh, and Grandpappy most likely didn’t tell you this about the hot water pipes bursting first, because he never had them - he got his water from the well out back - you know the one he had to walk two miles uphill in both directions to get to - at three o’clock in the morning after milking the cows, tilling the fields and planting ten acres of corn with his bare hands.

1 comment:

  1. I never put hot water in my ice trays. The last time I did that the water froze so fast that the ice was still hot.

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