Temperature does not always rise when energy is added. During a phase change, energy can go into changing particle arrangement instead of particle kinetic energy. This lesson also closes the module with the three modes of heat transfer: conduction, convection, and radiation.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Why does sweating cool you down, and why can stepping out of a pool into a breeze make you feel even colder even if the air temperature hasn't changed much?
Type your prediction below. You will revisit it at the end.
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Wrong: Zero acceleration means an object is stationary.
Right: Zero acceleration means constant velocity — the object could be moving at constant speed in a straight line.
📚 Core Content
Latent heat is the energy absorbed or released during a change of state at constant temperature. The word "latent" means hidden — the energy goes into rearranging particles rather than showing up as a temperature change.
During melting or boiling, energy is still being transferred into the substance, but the temperature may stay constant because the energy goes into changing the arrangement of particles rather than increasing their average kinetic energy. When ice melts, water molecules break free from their fixed lattice positions. When water boils, molecules separate completely from each other to form a gas. Both processes require energy to overcome intermolecular forces. This energy increases the potential energy of the particle system, not the kinetic energy. Because temperature is a measure of average kinetic energy, the temperature remains flat during the phase change.
The amount of energy required depends on the substance and the type of phase change. Water is exceptional: it takes a huge amount of energy to turn liquid water into steam because hydrogen bonds must be almost completely broken. This is why steam burns can be so severe — the steam carries an enormous amount of latent heat that is released onto the skin as it condenses.
Different phase changes use different latent heat values. The energy required to turn a liquid into a gas is always much larger than the energy required to turn a solid into a liquid, because vaporisation involves complete separation of particles.
| Process | State change | Latent heat type | Approximate value for water |
|---|---|---|---|
| Melting | Solid → liquid | Latent heat of fusion ($L_f$) | $3.34 \times 10^5$ J/kg |
| Freezing | Liquid → solid | Latent heat of fusion ($L_f$) | $3.34 \times 10^5$ J/kg |
| Boiling / evaporating | Liquid → gas | Latent heat of vaporisation ($L_v$) | $2.26 \times 10^6$ J/kg |
| Condensing | Gas → liquid | Latent heat of vaporisation ($L_v$) | $2.26 \times 10^6$ J/kg |
Notice that the latent heat of vaporisation for water is roughly seven times larger than the latent heat of fusion. This reflects the much greater structural change involved in going from liquid to gas compared with solid to liquid. In the liquid state, particles are already somewhat free to move; in the gas state, they must become completely independent. Breaking all those intermolecular bonds requires far more energy than simply loosening the rigid structure of a solid.
On a heating curve, sloped sections show temperature change and flat sections show phase change. The shape of the graph tells you exactly what is happening to the particles at each stage.
A typical heating curve for ice being heated at a constant rate shows five distinct regions:
If the heating rate is constant, a flat section means energy is still being supplied but temperature is not increasing. The length of the flat section is directly proportional to the latent heat for that phase change. A longer flat section means more energy is required per kilogram.
Conduction transfers energy particle-to-particle through matter, especially solids. It is the dominant mode of heat transfer in materials where particles are locked in fixed positions.
Hotter particles transfer energy to neighbouring particles by collisions and interactions. In solids, atoms vibrate about fixed lattice points. When one end of a solid is heated, those atoms vibrate more vigorously and bump into their neighbours, passing kinetic energy along the chain. In metals, this process is dramatically accelerated by free electrons. These electrons can move through the metal lattice and carry kinetic energy over long distances before colliding with atoms and transferring energy. This is why metals are such good conductors of heat — and why a metal spoon handle in hot soup becomes too hot to hold within seconds.
Non-metals such as wood, plastic, and wool lack free electrons. Heat must propagate slowly through lattice vibrations alone. These materials are called thermal insulators. Even at the same temperature, a metal door handle feels colder than a wooden one in winter because metal conducts heat away from your hand much faster, lowering the skin temperature more rapidly.
Convection is bulk movement in fluids; radiation is energy transfer by electromagnetic waves. These two mechanisms operate very differently from conduction.
Convection occurs in liquids and gases when warmer, less dense regions rise and cooler, denser regions sink. This creates circulation currents that transport heat throughout the fluid. A classic example is a pot of water on a stove: water at the bottom is heated, expands, becomes less dense, and rises. Cooler water sinks to take its place, gets heated, and rises in turn. This convection current distributes heat efficiently. Convection cannot occur in solids because the particles are fixed in place and cannot flow past one another.
Sea breezes along the Australian coast are driven by convection. During the day, land heats up faster than the ocean. The air above the land warms, rises, and cooler sea air flows inland to replace it. At night, the land cools faster, reversing the circulation. This daily convection cycle is a direct consequence of the different specific heat capacities of land and water.
Radiation does not require a medium. It occurs because all objects above absolute zero emit electromagnetic radiation, including infrared. The hotter the object, the more radiation it emits, and the shorter the average wavelength. The Sun transfers energy to Earth across 150 million kilometres of empty space almost entirely by radiation. Similarly, you feel warmth from a campfire even if you are not touching the flames — that warmth is infrared radiation.
Unlike conduction and convection, radiation can travel through a vacuum. It is also the only mechanism by which Earth loses heat to space. Dark, matte surfaces absorb and emit radiation efficiently; shiny, light surfaces reflect it. This is why wearing light-coloured clothing in the Australian summer reduces the radiant heat absorbed from the Sun.
Many situations involve more than one heat-transfer mode at once. Recognising which mechanisms are active is key to explaining real-world thermal phenomena.
Sweating cools the body because evaporation requires latent heat of vaporisation. When sweat changes from liquid to gas on the skin, it extracts approximately 2.26 MJ of energy per kilogram from the body. This removes thermal energy and lowers skin temperature. A breeze enhances this cooling in two ways: it removes humid air from the skin surface, allowing more evaporation to continue, and it increases convective heat transfer by moving cooler air across the skin. Radiation is also always present — the body emits infrared radiation to the surroundings continuously.
Stepping out of a swimming pool into a breeze feels dramatically colder than standing in still air because the wet skin has a much higher evaporation rate. The latent heat demand is constant, and the wind continuously supplies fresh dry air to sustain it. This combined effect of evaporation (latent heat), convection (air movement), and radiation (infrared emission) explains why wind chill on wet skin can feel like a temperature drop of 10°C or more even when the actual air temperature has barely changed.
Visual Break — Decision Flowchart
✏️ Worked Examples
Problem type: Type 18 — Latent heat of fusion.
Scenario: How much energy is required to melt 0.50 kg of ice at 0°C? Use latent heat of fusion $L_f = 3.34 \times 10^5\ \text{J/kg}$.
The ice were initially at −10°C. Describe the two stages of heating now required, naming the equation used for each stage and the total energy needed.
Problem type: Type 18 — Interpreting a flat section on a heating curve.
Scenario: A heating curve shows a flat section at 100°C while energy is still being supplied at a constant rate. Explain what is happening physically at the particle level, and state which equation applies.
If the same mass of substance had a much smaller latent heat of vaporisation, sketch how the heating curve would differ. Would the flat section be longer or shorter? Explain.
Problem type: Type 18 — Combining specific heat and latent heat in a multi-stage problem.
Scenario: Calculate the total energy required to convert 0.20 kg of ice at −10°C into steam at 100°C. Use the following values: c_ice = 2100 J/kg·K, $L_f = 3.34 \times 10^5$ J/kg, c_water = 4180 J/kg·K, $L_v = 2.26 \times 10^6$ J/kg.
The mass were doubled to 0.40 kg but the final temperature was only 50°C liquid water (no boiling). Calculate the new total energy and identify which stage now dominates.
🏃 Activities
Type your answers below.
Complete the table in your book.
Earlier you were asked why sweating cools you and why wind can make that effect stronger.
The full answer: evaporation requires latent heat of vaporisation, which is taken from the body, lowering its thermal energy and skin temperature. Wind helps remove the humid layer of air next to the skin, supporting further evaporation, and also increases convective heat transfer away from the skin. Radiation continues in the background. These multiple mechanisms work together to produce a powerful cooling effect.
Now revisit your prediction. Which mechanisms are working together in this cooling process?
Annotate your prediction in your book with what you now understand differently.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
✅ Check Your Understanding
1. Latent heat is energy used to:
2. During a flat section of a heating curve, the substance is:
3. The latent heat of fusion applies to:
4. Convection occurs mainly in:
5. Radiation is different from conduction and convection because it:
6. A breeze cools wet skin especially effectively because it:
7. Explain why temperature stays constant during a phase change even though energy is still being supplied. 3 MARKS
8. Calculate the energy needed to vaporise 0.20 kg of water if the specific latent heat of vaporisation is $2.26 \times 10^6\ \text{J/kg}$. 3 MARKS
9. Compare conduction, convection, and radiation, giving one example of each. 4 MARKS
1. B — latent heat changes state at constant temperature.
2. D — a flat section indicates phase change.
3. A — fusion is the solid-liquid phase change.
4. C — convection occurs in liquids and gases.
5. B — radiation does not need a medium.
6. A — wind supports evaporation and convection.
Activity 1 — Heating Curve Sort:
Activity 2 — Transfer Modes:
Activity 4 — Energy to Melt and Warm:
Stage 1 (melting): Q = mLf = 0.30 × 3.34 × 105 = 100 200 J
Stage 2 (warming): Q = mcΔT = 0.30 × 4180 × 25 = 31 350 J
Total Q = 100 200 + 31 350 = 131 550 J ≈ 132 kJ
Q7 (3 marks): During a phase change, energy is still being transferred into the substance, but it is used to change the arrangement of particles and overcome intermolecular forces rather than increase their average kinetic energy. Because temperature depends on average kinetic energy, the temperature remains constant. The supplied energy is latent heat.
Q8 (3 marks): $Q = mL = 0.20 \times 2.26 \times 10^6 = 4.52 \times 10^5\ \text{J}$.
Q9 (4 marks): Conduction is heat transfer particle-to-particle through matter, especially solids, such as a metal spoon warming in soup. Convection is heat transfer by bulk fluid motion, such as hot water rising in a saucepan or sea breezes along the coast. Radiation is energy transfer by electromagnetic waves, such as heat from the Sun reaching Earth through the vacuum of space.
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