Re: cooling water

[Doug Craigen <dcc@cyberspc.mb.ca>]

David Dockstader wrote:
> This issue clearly revolves around a subterfuge.          The same water is
> not being frozen under the same conditions, but the changes are deliberately
> not mentioned in the wording of the question, leaving it to the reader of
> the question to assume that identical conditions prevail.  

Nobody is trying to trick you here, they are telling you exactly what 
somebody else told them about what they did and then observed.  This is a 
recurring theme in phys-l ... how many assumptions do *we* insert into people's 
claims when we assess "real" problems?  If a person says that "unless something 
keeps pushing an object, it will stop", are they wrong?  Are they playing tricks 
on us because they are omitting non-ideal assumptions in their wording?    I 
think not.  Rather, it is us who look foolish if we contradict somebody's 
observations and then have to tell them about all the things they forgot to 
mention when their observations turn out to be correct.

In this case, the typical telling of the story is quite simple.  According to 
many people who claim to have done it, or perhaps have an uncle (brother, friend 
....) who has done it, if you leave two buckets of water outside in the winter - 
one filled with hot water and one filled with cold - the one filled with hot 
water will freeze faster.

Now, notice that we are told very few things.  Both buckets are stated to be full 
initially, but we are not told the amount of ice at the end - and we of all 
people should realize they may not be the same.  We are told one has hot water 
and one has cold.  We are not told how hot or how cold, or where they came from. 
 Is the water from kettles?  From hot and cold water taps?  Furthermore, we are 
not told that the water is identical, other than the temperature.  Once again, we 
of all people should know better than to read this assumption in to the claim.

When somebody brings this claim to me I directly ask them if they get as much ice 
out at the end.  I could also ask them if it still makes a difference if you wet 
the bottoms of each bucket.  And so on.  Far better to do this than to challenge 
what for all I know they have done in their freezer with ice cube trays.  If they 
know it works, there isn't much you can do to recover your credibility after 
dismissing it.

This last winter I built an ice fort in our back yard with the kids.  In the 
process of freezing hundreds of blocks of ice in buckets I played around with 
this particular claim a bit, and under a variety of conditions (temperature, 
snowfall, wind, bucket on ice, bucket on cement, bucket in snowbank).  
Unfortunately, with such a mild winter (hardly ever below -20) I didn't get much 
chance to study really cold conditions.  I can tell you that there is room for 
different outcomes which are based on following the directions given.

|++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|
| Doug Craigen                                                 |
|                                                              |
| If you think Physics is no laughing matter, think again .... |
|    http://www.cyberspc.mb.ca/~dcc/phys/humor.html            |
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