Meaning and mechanism
When the temperature is increasing within a specific range, the average particle movement energy increases. In the melting and boiling ranges, energy is used to change the arrangement and interaction between particles.
When pure substances are heated to a constant pressure, approaching equilibrium, the temperature can remain relatively constant. This is due to a change in the way energy is utilized, rather than a change in temperature itself.
Relationships and conditions
Regions without phase change use Q=mcΔT, while regions where melting or boiling occurs use Q=mL. If the heating power is constant, the energy and time required for each region are proportional.
Melting point and boiling point are affected by pressure and the purity of the substance. Ideal heating curves can omit factors such as the heat capacity of the container, heat loss, and supercooling/superheating.
What to observe in the simulation
The horizontal section is not a section where heat disappears, but a section used for state changes.
Source: Standard physical property values (rounded values for educational purposes). One type of water, constant pressure 1 atm, c ice = 2.09, c water = 4.18, c steam = 2.01 J/(g·K), heat of fusion 334 J/g, heat of vaporization 2260 J/g. The specific heat is constant, there is no heat loss, and water vapor remains in the system. Actual pressure changes, superheating, supercooling, and evaporation losses are omitted. The particle spacing is a schematic diagram.
Key relationships and symbols
Q = mcΔT · Qmelting = mLf · Qvaporization = mLv
| Symbol | Meaning | Unit |
|---|---|---|
| c | specific heat | J/(kg·K) |
| Lf, Lv | Latent heat of fusion and vaporization per unit mass | J/kg |
| Q | Heat transferred | J |
Worked example
What is the input heat used for in the 100 °C horizontal section of the heating curve?
In the latent heat section, the state ratio changes and the temperature remains constant. After evaporation, the temperature of the water vapor rises again.
Common misconceptions
- Even when the temperature is within a specific, constant range, matter can continue to receive energy.