Topic 3 of 6
Changes of state
A substance can gain or lose internal energy while its temperature stays constant during a change of state.
Consider a pure substance undergoing the stated transition at constant pressure. Its temperature remains at the transition temperature while the amounts in the two states change. The particles continue moving; constant temperature does not mean zero motion.
State changes and the direction of energy transfer
For a pure substance at fixed pressure, temperature stays constant during each stated transition. Energy moves into or out of the substance.
Four processes, with two transfer directions
- Melting: solid to liquid
- Energy enters the substance. Particles become able to move past one another instead of remaining near fixed positions. Their arrangement and interactions change, increasing the potential-energy contribution to internal energy.
- Solidification: liquid to solid
- Energy leaves the substance. Particles settle into an arrangement in which they vibrate about fixed positions. The potential-energy contribution decreases. The released energy can increase internal stores of the surroundings.
- Boiling: liquid to gas
- Energy enters the substance. Particles become widely separated as the liquid changes to gas. Energy is needed to separate particles against their attractions, so their potential energy increases.
- Condensation: gas to liquid
- Energy leaves the substance. Particles become close together in a liquid, and their potential energy decreases. The energy transferred out can increase the surroundings' internal stores.
During each transition at the stated conditions, the average random kinetic energy remains unchanged because the temperature remains unchanged. The energy change is associated with the changing particle arrangement and potential energy.
Melting does not break the atoms or molecules into smaller particles. It changes how they are arranged and how freely they move relative to their neighbours. A liquid's particles are still close together; becoming widely separated is characteristic of the gas state.
Read a temperature plateau as a state change
The following idealised values describe an unspecified pure sample at a constant pressure. Its melting point is 50°C, and energy continues to enter throughout the six minutes. These are supplied model values, not measurements of a named material.
| Time / min | Temperature / °C |
|---|---|
| 0 | 20 |
| 1 | 35 |
| 2 | 50 |
| 3 | 50 |
| 4 | 50 |
| 5 | 65 |
| 6 | 80 |
Energy continues entering during the melting plateau
Supplied idealised data for an unspecified pure substance at constant pressure. Its melting point is 50 °C.
0-2 min: the solid warms from 20 to 50 °C.
2-4 min: solid changes to liquid at 50 °C. Energy transfer continues.
4-6 min: the liquid warms from 50 to 80 °C.
- 0 to 2 min: the solid warms from 20°C to 50°C. Its average particle kinetic energy increases.
- 2 to 4 min: melting takes place at 50°C. The amount of liquid increases and the amount of solid decreases. Internal energy increases while average random kinetic energy stays unchanged.
- After 4 min: the sample is liquid and its temperature rises. Average particle kinetic energy increases again.
Worked explanation
Why does the reading stay at 50°C?
At 3 min, the sample is in the melting interval. Continued energy input changes the particle arrangement and potential energy as more solid becomes liquid. It does not increase average random kinetic energy during this transition, so the temperature remains at 50°C.
The flat section does not show that the heater has stopped or that the particles have stopped moving. Temperature and total internal energy are different quantities.
Explain the reverse process
If energy is removed from the same liquid, it first cools to its solidification temperature. While it solidifies at the same fixed pressure, energy continues to leave and the amount of solid increases, but its temperature stays constant until the transition is complete.
The same reasoning applies to boiling and condensation at the boiling/condensation temperature for that pressure. Boiling requires continuing energy input while liquid becomes gas; condensation transfers energy out while gas becomes liquid. A constant temperature does not mean the transfer has ended.
Measure temperature and time consistently
Keep the thermometer's sensing region in the sample, clear of the heated vessel, and use the same timing origin for all readings. Read at the chosen intervals while recording whether the sample is solid, liquid or a mixture. Do not infer state only from a single temperature reading.
A thermometer that responds slowly can lag behind a rising sample temperature and round off a change in slope. Poor placement can measure a locally hotter region rather than the sample represented by the model. Repetition does not by itself fix either cause.
State the plateau conditions. This explanation concerns a pure substance undergoing a transition at fixed pressure. It does not claim that every mixture melts at one sharp temperature or that all warming must include a flat interval.
Optional check A pure sample melts at 50 degrees C at constant pressure while energy continues to enter. Which particle explanation accounts for its constant temperature?
Latent heat gives the energy needed for a stated amount to change state. Before calculating, distinguish boiling from evaporation: both can make vapour, but they occur differently.