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[Phys-L] Re: water and internal energy



Regarding Anthony's question:

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?

Don't forget that the potential energy between a pair of water
molecules has a very strong steric component to it. Under such
crowded conditions changes in the relative orientations of the
molecules is a more important contribution to the change in potential energy than changes in the distance between their centers of mass
(whose contribution is near a minimum as a function of distance
anyway).

I'm looking for an "easy" explanation for my students to understand
that the expansion has LESS internal energy.

Not only is the freezing phase transition accompanied by a bulk
expansion but even in the liquid phase at low temperatures (less than
3.98 deg C) the liquid expands upon cooling as ever larger clusters
of molecules tend to acquire more of the relative steric geometry of
the solid lattice (tetrahedrally coordinated oxygens with the
hydrogens asymmetrically positioned on the nearest neighbor
inter-oxygen links) over short ranges.

For all other substances, the solids are MORE dense than the
liquids.

Not *all* other substances, but certainly the vast majority of them.

Water is a most unusual substance!

Yes it is.