BeginnerTopic 14 of 105Thermodynamics & Fluids
Phase change
Heating a substance at constant pressure absorbs latent heat during melting or boiling.
In depth
Heating at constant pressure: subcooled liquid, saturated liquid, liquid-vapour mixture at constant T (latent heat), saturated vapour, superheated vapour. Saturation temperature depends on pressure (water boils at 100 °C at 1 atm, lower at altitude). Critical and triple points mark limits.
Key points
- Latent heat of fusion and vaporisation
- T-v and P-v diagrams
- Sublimation
ExampleAt 3000 m, water boils near 90 °C.
Phase change absorbs or releases large amounts of energy at constant temperature, which is central to boilers, refrigerators and power plants.
What is Phase change?
Heating a substance at constant pressure absorbs latent heat during melting or boiling.
Why is it important to understand phase change?
Phase change absorbs or releases large amounts of energy at constant temperature, which is central to boilers, refrigerators and power plants.
What is latent heat?
The energy absorbed or released during a phase change without a temperature change, such as the energy to boil water at atmospheric pressure.
Why does temperature stay constant during boiling?
The added energy breaks bonds rather than increasing molecular kinetic energy, so the temperature stays at the saturation value for that pressure.
How is Phase change used in practice?
Typical applications include calculating the energy to boil water in a steam generator, estimating the energy absorbed when ice melts in a cold box and designing an evaporator that boils refrigerant at a constant temperature.
What should I learn before Phase change?
Study "P-V diagrams" first (topic 13 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Phase change?
Latent heat of fusion and vaporisation. T-v and P-v diagrams. Sublimation.
Can you give an example of Phase change?
At 3000 m, water boils near 90 °C.
What comes after Phase change?
The next topic is "Property tables and steam tables", which builds directly on this one.
How difficult is Phase change?
It is classed as Beginner in Thermodynamics & Fluids (topic 14 of 105). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.