Latent heat and change of state
When a substance melts or boils, its temperature stays constant even though heat is being supplied. This heat, called latent heat, is used to break the bonds between particles rather than to raise the temperature. The temperature stays the same throughout the change of state.
The specific latent heat L of a substance is the heat needed to change the state of 1 kg of the substance without a change in temperature. Its SI unit is J kg⁻¹.
- Specific latent heat of fusion (Lf): for melting/freezing. For ice, Lf ≈ 3.34 × 10⁵ J kg⁻¹.
- Specific latent heat of vaporisation (Lv): for boiling/condensing. For water, Lv ≈ 2.26 × 10⁶ J kg⁻¹.
Key formula / 关键公式
Q = mL
Q = heat absorbed or released (J); m = mass (kg); L = specific latent heat (J kg⁻¹).
During the change of state, θ (temperature change) is zero.
Reading a heating curve
On a temperature-time graph, the sloped parts show temperature rising (using Q = mcθ), while the flat plateaus show melting or boiling (using Q = mL). Lv is larger than Lf because vaporisation must fully separate the particles.
Worked example / 例题
How much heat is needed to completely melt 0.50 kg of ice at 0 °C? (Lf = 3.34 × 10⁵ J kg⁻¹)
Q = mL = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J = 167 kJ. The temperature stays at 0 °C during melting.
Remember / 记住
- Temperature does not change during melting or boiling.
- Use Q = mL for state change, Q = mcθ for temperature change.
- Lv > Lf for the same substance.