Conceptual Questions
- Under normal conditions (atmospheric pressure) what are the temperatures at which you can have liquid water?
- What are the temperatures that you can have ice?
- What range of temperatures for ice water?
- What range of temperatures for boiling water?
Learning Objectives
- Understand and calculate heat capacity to find energy needed to make phase changes.
- Describe phase changes like freezing, melting, evaporation, and condensation.
Latent heat of fusion
- How much energy does it take to freeze a gram of water?
- What is the temperature of the water before and after freezing?
Latent heat of vaporization
- How much energy does it take to boil a gram of water?
- What is the temperature of the water before and after boiling?
Heat capacities
| Water | 4.18 J per g per Kelvin |
| Ice | 2.11 J per g per Kelvin |
| Steam | 2.00 J per g per Kelvin |
Phase change
During a phase change, the temperature stays the same.
Latent heat
The amount of energy added or removed from material to change phase.
Latent heats
| Evaporation | 2.26 kJ per gram |
| Melting | 0.334 kJ per gram |
Q = m l
Where Q is the energy absorbed or released, m is the mass of the substance undergoing a phase change, and l is the latent heat of melting or boiling.
Relation to enthalpy
You may have seen these quantities referred to as enthalpy of fusion or the enthalpy of vaporization.
Example
How much thermal energy must be removed from 1 kilogram of 0C water to create 1 kilogram of 0C ice?
Q = m l
m is 1 kilogram of water, l for freezing is 334 joules per gram.
Q = 1000 g \cdot 334 J/g = 334 kJ