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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!