What Are Heating and Cooling Curves?
When you heat a substance, you expect its temperature to rise — but something interesting happens at certain points: the temperature stops changing even though you keep adding heat. This is the foundation of heating and cooling curves.
A graph that shows how the temperature of a substance changes as it is heated at a constant rate over time.
A graph that shows how the temperature of a substance changes as it loses heat at a constant rate over time.
These graphs are powerful tools in chemistry because they reveal the melting point, boiling point, and the energy behaviour of a substance as it changes state. The x-axis represents time (which relates to energy added or removed), and the y-axis represents temperature.
Think of heating a substance like filling a bath with water. Most of the time the water level rises steadily — but if the drain is open at the same rate you fill it, the level stays flat for a while. In a heating curve, the "flat parts" are where energy is being used to break bonds rather than raise the temperature.
The Three States of Matter — A Quick Reminder
Before exploring heating and cooling curves in detail, it helps to recall the key features of the three states of matter and how particles are arranged in each.
| State | Particle Arrangement | Movement | Shape & Volume |
|---|---|---|---|
| Solid | Closely packed, regular lattice | Vibrate in fixed positions | Fixed shape, fixed volume |
| Liquid | Close together, irregular | Flow and slide past each other | No fixed shape, fixed volume |
| Gas | Far apart, random | Move rapidly and freely | No fixed shape, no fixed volume |
When a substance changes state, its chemical identity does not change — water, ice, and steam are all H₂O. Only the arrangement and movement of particles changes.
The transitions between states have specific names:
- Melting: solid → liquid
- Freezing: liquid → solid
- Vaporisation / Boiling: liquid → gas
- Condensation: gas → liquid
- Sublimation: solid → gas (directly)
- Deposition: gas → solid (directly)
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