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.
Understanding Physics: Latent Heat and Phase Changes
At the particle level, a change of state is a struggle between motion and attraction. In a solid, particles vibrate around fixed positions because attractive forces hold them close. As a solid melts, the particles gain enough energy to move past one another.
They are still close together in the liquid, so many attractions remain. When a liquid becomes a gas, particles must separate much farther. This requires energy to overcome most of those attractions.
That is why vaporization needs far more energy than melting for many substances. The energy is stored as a change in the particles' potential energy.
A sample can contain two phases at the same time. A glass with ice floating in water is a familiar example. If ice and liquid water are both present at normal air pressure, the mixture stays at its melting temperature until all the ice has melted.
At that point, further heating raises the temperature of the liquid. The reverse occurs during freezing.
Energy leaves the water while crystals form, yet the water remains at its freezing temperature until the liquid has become solid. This released energy can keep nearby surroundings warmer than expected.
Pressure changes the temperatures where phase changes happen. At high altitudes, air pressure is lower, so water boils below one hundred degrees Celsius. Food cooked in boiling water may take longer because the water is less hot.
A pressure cooker raises the pressure above the water. Its boiling temperature rises, allowing the food to cook at a higher temperature. Dissolved substances can affect phase changes too.
Salt lowers the freezing temperature of water, which is why salt can help melt road ice. It does not create heat. It changes the conditions under which ice and liquid water can exist together.
Latent heat has important effects in everyday systems. Sweat cools skin when it evaporates because it takes energy from the skin. A wet shirt feels cool for the same reason.
Refrigerators use a fluid that repeatedly evaporates inside the appliance and condenses outside it. Evaporation absorbs thermal energy from the cold compartment. Condensation releases that energy to the room.
Steam is dangerous because it can condense on cooler skin and release a large amount of energy in a small area. Ice is useful in cool boxes because it absorbs energy as it melts without quickly becoming warmer.
When solving phase change problems, first identify every stage the material passes through. A process may include warming a solid, melting it, warming the liquid, boiling it, then warming the gas. Treat each stage separately and combine the energy amounts at the end.
Use mass in kilograms when the latent heat value is given per kilogram. Keep track of whether energy enters or leaves the sample. Graphs need similar care.
A flat section does not mean heating has stopped. It shows that energy transfer continues while the proportion of each phase changes. The most common mistake is using a temperature change calculation during a flat section, where there is no temperature change to use.
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.