Latent heat explains why adding energy to a substance does not always raise its temperature. During melting, freezing, boiling, or condensing, energy is used to change the arrangement of particles rather than increase their average kinetic energy. This is why a heating curve has flat sections where temperature stays constant.
Understanding latent heat helps explain cooking, weather, refrigeration, and why steam burns can be so severe.
On a heating curve, sloped regions show a single phase warming up, such as solid ice becoming warmer or liquid water approaching its boiling point. Flat regions show phase changes, such as melting or vaporization, where the added heat changes potential energy between particles. The heat needed in sloped regions is found with Q = mcΔT, while the heat needed during phase changes is found with Q = mL.
The latent heat of vaporization is usually larger than the latent heat of fusion because separating particles into a gas requires much more energy than loosening them into a liquid.
Key Facts
- Heat for warming within one phase: Q = mcΔT.
- Heat for a phase change: Q = mL.
- Latent heat of fusion is the energy per kilogram needed to melt or freeze a substance at its melting point.
- Latent heat of vaporization is the energy per kilogram needed to boil or condense a substance at its boiling point.
- During a phase change, temperature remains constant because added energy changes particle spacing and bonding rather than average kinetic energy.
- For water at 1 atm, melting occurs at 0°C and boiling occurs at 100°C.
Vocabulary
- Latent heat
- Latent heat is energy absorbed or released during a phase change without a temperature change.
- Heat of fusion
- Heat of fusion is the energy per unit mass required to change a substance between solid and liquid at its melting point.
- Heat of vaporization
- Heat of vaporization is the energy per unit mass required to change a substance between liquid and gas at its boiling point.
- Heating curve
- A heating curve is a graph of temperature versus heat energy added that shows warming regions and phase change plateaus.
- Specific heat capacity
- Specific heat capacity is the heat energy needed to raise the temperature of 1 kilogram of a substance by 1°C or 1 K.
Common Mistakes to Avoid
- Using Q = mcΔT during melting or boiling is wrong because ΔT = 0 during a phase change, so the correct relationship is Q = mL.
- Thinking temperature rises while a substance melts is wrong because the added energy breaks or loosens intermolecular attractions instead of increasing average kinetic energy.
- Confusing heat of fusion with heat of vaporization is wrong because fusion applies to solid liquid changes, while vaporization applies to liquid gas changes.
- Forgetting unit consistency is wrong because mass must usually be in kilograms when L is in J/kg, and mixing grams with J/kg gives an answer off by a factor of 1000.
Practice Questions
- 1 How much heat is needed to melt 0.250 kg of ice at 0°C? Use Lf = 3.34 × 10^5 J/kg.
- 2 A 0.100 kg sample of water at 100°C is completely vaporized into steam at 100°C. How much heat is required? Use Lv = 2.26 × 10^6 J/kg.
- 3 Explain why the temperature of a pot of boiling water stays near 100°C even when the stove continues to add heat.