[Phys-L] Re: water and internal energy

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

----- Original Message -----
From: "Anthony Lapinski" <anthony_lapinski@PDS.ORG>


> I tell my students that steam burns at 100°C are more harmful than water
> burns at 100°C because the steam has more internal energy (latent heat --
> mL). Similarly, water at 0°C has more internal energy that ice at 0°C.
> Phase changes (+Q) increase the internal PE of the atoms, but not the KE
> (since the temperature does not change).
>
> So here's my dilemma. How do you discuss this conceptually to students,
> especially when talking about water? Ice is clearly LESS dense than water
> because of its expansion (increased volume) upon freezing. But if water at
> 0°C has more internal energy, the WATER molecules be farther apart and
> thus become less dense. But they're not! What causes this? I'm looking for
> an "easy" explanation for my students to understand that the expansion has
> LESS internal energy.
>
> For all other substances, the solids are MORE dense than the liquids.
> Water is a most unusual substance!

This is a wonderful subject for doing some integration of topics (even some
biology).  Using the behavior of water below 4 degrees, you can then detail
(through buoyancy and convection) the process by which ponds and lakes end
up freezing from the top down, ultimately to insulate the deep 4 degree
water from the much colder air above.  This then permits the existence of
northern aquatic life which might otherwise be 'flash frozen' in solid ice
were it not for this 'most unusual' property.

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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