Re: How does running water freeze?

((I know that as water is running it cannot freeze (movement requires 
energy and 
the temperature of the water needs to be higher than freezing inorder to 
move) 
How is it that at night when outdoors it is below freezing some water 
rushing 
into the drainpipes doesn't freeze while others (not too far from the 
same 
location) do freeze? also on that night in particular when I noticed this 
there was water that obviously ran down a hill and froze on the way down, 
if 
water is rushing at therefore won't freeze how did that water freeze?))



Hello,

This is a very interesting question.  The answer has to do with one of 
the complexities in way that water freezes (or melts):  This concept is 
called the latent heat of fusion. This is the amount of energy that must 
be removed from liquid water to allow it to freeze. Water requires 80 
calories per gram to change from liquid to solid at the freezing 
temperature, with no additional change in temperature. I will illustrate 
this in the following example:

Start with 100 grams of water at 20 degrees C (68 deg F) and place it in 
the freezer at home.  Let's look at the amount of heat energy that must 
be transferred (removed) from the water for it to turn to ice:  For every 
degree of cooling down to the freezing temperature (0 deg. C or 32F), 1 
calorie per gram must be extracted.

Therefore, 100 gm x 1 cal/gm/degC x 20 deg. = 2000 cal

This is the heat energy that must be extracted just to cool the water to 
0 deg.  Now, an extra 80 cal/gm must be extracted to turn this water into 
ice WITH NO ADDITIONAL TEMPERATURE CHANGE:

100 gm x 80 cal/gm = 8000 cal

The reason for this example is to show you that a much greater amount of 
cooling must take place to freeze the water than to reduce its 
temperature. This is why some water may not immediately freeze, even when 
the outside temperature is below freezing.  

When water is flowing, the temperature and the amount of latent heat will 
be fairly constant throughout its volume due to mixing.  This means that 
both the temperature-change-energy AND the latent heat energy must be 
removed from the entire volume before freezing can take place.

In a stagnant pool of water, the outside 'layer' will be exposed to 
freezing environment.  This layer can 'give up' its latent heat and turn 
to ice without freezing the layers below.

When water is flowing, the mixing has a much greater effect in slowing 
the freezing process than the additional energy (kinetic energy) that the 
water contains from the movement.

Best Regards

Jay Shapiro