Latent Heat
Phases of Matter

The properties of solids, liquids, and gases differ.
How does matter change state?

Solid


Liquid

Gas
Watched Pot

You want to cook a pot of potatoes in boiling water.
To use energy most efficiently, you should
•
•
•
•
A) Heat it up quickly and leave it at a full boil
B) Heat it up slowly and turn it up to a full boil
C) Heat it up quickly and turn it down to simmer
D) Heat it up slowly and leave it at a low boil
Energy of Phase

Internal energy can be
stored as bonds between
atoms and molecules.
• Solids have the strongest
bonds
• Energy required to break
them to a liquid
Heat of Transformation
heat of fusion
heat of vaporization

As a system reaches the
point of a phase change heat
no longer changes the
temperature.

The heat is used to change
the phase.
Latent Heat

The heat of transformation
per unit mass is called the
latent heat (L).
• Measured in J/kg
Q  Lm

It takes as much energy to
melt 1 g of ice as it does to
raise the temperature from
0 C to 80 C.
Material (f or v)
Mercury (f)
Lead (f)
Uranium (f)
Copper (f)
Water (f)
Latent Heat
11.3 kJ/kg
24.7 kJ/kg
82.8 kJ/kg
205 kJ/kg
334 kJ/kg
Oxygen (v)
Water (v)
213 kJ/kg
2300 kJ/kg
Phase Diagrams

For each temperature and
pressure a material is in a
specific state.

The triple point is uniquely
defined for each material.
• P3 for water is used to
define the Kelvin

The critical point is where
there is no difference
between liquid and gas.
Phase diagram for water
Sublimation

At low pressures a solid can
go directly to a gas and vice
versa.

This process is called
sublimation.
• Dry ice to gaseous carbon
dioxide
• Water vapor to frost

There is a latent heat of
sublimation
Phase diagram for carbon dioxide
Summer Cooler


200 g of ice at -10 C is
added to 1.0 kg of water at
15 C.

Find the amount of heat
needed to bring the ice to 0
C and melt it.
• Qi = mcDT + mLf
• Qi = 70.9 kJ
Is there enough ice to cool
the water to 0 C?

Find the heat needed to cool
the water down to 0 C.
• Qw = mcDT
• Qw = 62.8 kJ

Yes, there’s enough ice.
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