1. Latent Heats
of phase changes
§ 17.5

2. Heating Curve for Water
steam
Water boils
Liquid water
Ice melts
ice

3. Latent heat
Potential energy of phase change (energy required to change the phase of 1 kg of substance)
Water’s latent heat of fusion (melting):
335,000 J/kg
Water’s latent heat of vaporization:
2,255,000 J/kg

4. Whiteboard Work
During the Alpe d’Huez climb, how much sweat would Lance have needed to evaporate to keep his body temperature constant?
The heat q needed to vaporize a mass m of water is
q = m (2.255  106 J/kg).
Solve for mass m and substitute in the values.

5. Evaporation of a Liquid
More energetic jostling = higher temperature
An especially fast molecule at the surface may detach!

6. Evaporation of a Liquid
More energetic jostling = higher temperature
An especially fast molecule at the surface may detach!

7. Evaporation Evaporating molecules carry away energy KE PE
Remaining liquid cools (KE decreases)
Adding heat is how we convert liquid water to gas (steam, vapor)!

8. Conservation of Energy
in heating
§ 17.6

9. Energy Conservation
Heat transfer into something is out of something else
Specific heat lab
Melting ice