Change of State
When matter changes state (melting, freezing, boiling, condensation), energy is absorbed or released, often at constant temperature.
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Heating and cooling curves
If you heat a pure substance steadily, temperature rises in stages: sloping regions where temperature increases within a state, and flat regions during phase changes (e.g. melting, boiling) where temperature stays constant while energy goes into changing the state.
✔ Key takeaway: During a phase change, energy changes the state, not the temperature.
Melting and boiling points
Each pure substance has characteristic temperatures at which it changes state at a given pressure: the melting point (solid ↔ liquid) and the boiling point (liquid ↔ gas). At these points, both states can coexist while energy is absorbed or released.
✔ Key takeaway: At melting and boiling points, two states coexist and energy is exchanged without temperature change.
Link to simulator: Watch flat regions on the heating curve and the particle animation as the state changes.
💭 Think about this:
Why does the temperature of boiling water remain almost constant even when you keep heating?