Beginner · 10 min
Melting, freezing, and boiling
Explain energy transfer during changes of state and calculate melting energy.
Temperature can stay constant
Matter can change between solid, liquid, and gas. Melting and boiling require energy; freezing and condensation release energy to the surroundings. During an equilibrium change of state of a pure substance at constant pressure, the temperature stays constant while the state changes. The transferred energy changes the internal arrangement and interactions of particles rather than simply raising temperature.
Worked example
Ice already at its melting point can absorb energy and become water at the same temperature. Heating the resulting water further is a separate stage.
Energy per unit mass
Specific latent heat is the energy transferred per unit mass during a change of state. Use Q = m × L, with compatible units. A value in J/kg needs mass in kilograms; a value in J/g needs grams. This calculation covers only the change of state. If a material first warms to its melting or boiling point, include the warming energy separately.
Worked example
Using a rounded melting energy of 330 J per gram of ice: Energy to melt 4 g already at its melting point = 4 × 330 = 1,320 J
Separate warming from melting
- Sketch a temperature–time graph for ice warming to its melting point and then melting at a constant energy input rate. Mark the flat melting section.
- Using 330 J per gram, calculate the energy to melt 5 g of ice already at its melting point. Explain why starting with colder ice would need extra energy.