Re: cooling water

[Tim Folkerts <Tim.Folkerts@valpo.edu>]

LUDWIK KOWALSKI wrote:

> The issue of "hot water freezing faster" can be resolved by an experiment.
> Put a glass of 50 C water into a freezer and measure the time it takes
> Put a glass of 30 C water into a freezer and measure the time it takes
> to reach the temperature of -1 C, for example. Then do the same with 
> 5 C water, under identical conditions. 


This could be good project for students.  As in most expeiments, 
"under identical conditions" is easier said than done.  

A typical freezer oscillates in temperature about a fixed point, so 
doing one first and then the other could lead to variations.  A typical 
freezer would also have temperature gradients from one spot to another, 
so putting both in at the same time also is problematic.  Finally, the  
glass of water itself interacts with the freezer to change the 
experiment: the evaporating water changes the humidity, warms the 
freezer, and could set up convection.  You could do it outside on a 
cold day, but then you don't have control of the temperature or wind.  
Furhtermore, the size, shape, and insulation of the container will also 
be important.  ( And there are probably more parameters that I haven't 
thought of.)

The point is that there are many variables.  I'm sure that in most 
cases the cold water will freeze faster, but I'll also bet that in 
SOME cases, the hot water will freeze faster.  Like I said, 
it's a good open ended question that could serve as a project for 
students.




---   Tim Folkerts


********************************************************
Timothy J. Folkerts               Tim.Folkerts@valpo.edu
Dept. of Physics & Astronomy      219-464-6634
Valparaiso University
Valparaiso, IN  46383