condensation temperature, to convert that gas to liquid at the same temperature, is called the
latent heat of condensation L c. For a given substance, the latent heat of condensation has the
same value as the latent heat of vaporization. For a sample of mass m of a gas at its
condensation temperature, the amount of heat that must flow out of the sample to convert the
entire sample to liquid is given by:
Q = mL
c
It is important to note that the actual values of the freezing temperature, the boiling temperature,
the latent heat of melting, and the latent heat of vaporization are different for different
substances. For water we have:
Phase Change
Temperature
Latent Heat
Melting
MJ
0 334
.
Freezing
0°C
kg
Boiling or Vaporization
MJ
2 26
.
Condensation
100°C
kg
Example 36-1
How much heat does it take to convert 444 grams of H2O ice at –9.0°C to
steam (H2O gas) at 128.0 °C ?
Discussion of Solution
Rather than solve this one for you, we simply explain how to solve it.
To convert the ice at –9.0 °
°
C to steam at 128.0 C, we first have to cause enough heat to flow into
the ice to warm it up to the melting temperature, 0 °
C. This step is a specific heat capacity
problem. We use
Q = m
∆
c
T
1
ice
where ∆T is [0 °
°
C – (–9.0 C)] = 9.0 C° .
Now that we have the ice at the melting temperature, we have to add enough heat to melt it. This
step is a latent heat problem.
Q = mL
2
m
After Q + Q flows into the H
°C. Next, we have to find
1
2
2O, we have liquid water at 0
2
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