If we supply heat uniformly to a body of fixed mass and record its temperature, we obtain a revealing graph: it alternates sloped stretches (temperature rising) with flat stretches (temperature staying still during a change of state).
Temperature-heat diagram for water: on the sloped stretches the temperature rises, on the flat stretches a change of state occurs at constant temperature.
How to read the diagram
- Sloped stretches: holds. The slope is , so the higher the specific heat, the flatter the stretch (ice and water have different slopes).
- Flat stretches: holds. The temperature is constant, the state is changing; the length of the stretch measures the latent heat.
The diagram makes a counter-intuitive fact visible: the flat stretches, especially the boiling one, are often the longest. Taking water from ice to vapour requires much more heat to boil it than to heat it through all degrees of the liquid stretch (see the example from ice to vapour).
Links
Topics: Thermology Concepts: Latent heat and changes of state · Specific heat and heat capacity Methods: Temperature-heat diagram
Related exercises: Worked exercise — Coffee and steam exchange · Problem — Pot on the stove · Problem — The heating pad