Latent Heat and Modes of Heat Transfer
A 2019 CSIRO study of Antarctic ice sheet loss measured 252 billion tonnes of ice melting per year. Using the latent heat of fusion for water ($L_f = 334$ kJ/kg = $3.34 \times 10^5$ J/kg), the energy absorbed by that melting is $Q = mL = 2.52 \times 10^{14}$ kg $\times\ 3.34 \times 10^5$ J/kg $= 8.42 \times 10^{19}$ J per year — equivalent to 24 years of the entire world's electricity consumption. All that energy goes into breaking molecular bonds in the ice, not into raising temperature: the ice stays at 0°C while it melts.
When water boils at 100°C in a kettle, does its temperature keep rising? Why or why not? Where is the added energy going? Predict.
Warm-up — during a change of state (e.g. melting), the temperature of a substance:
Know
- Latent heat: $Q = mL$ (energy absorbed/released during change of state)
- $L_f$ (fusion) and $L_v$ (vaporisation) for water
- Three modes: conduction, convection, radiation
Understand
- Why temperature is constant during a phase change
- The particle model explanation for each mode of heat transfer
- Which mode(s) operate in a given scenario
Can Do
- Calculate $Q$ using $Q = mL$
- Interpret a temperature-time heating curve
- Identify and explain each mode of heat transfer
Core Content
Fill a kettle with 500 g of water at room temperature and switch it on. The water temperature climbs steadily until it reaches 100°C — then it stops rising. The kettle keeps going, steam pours out, but the thermometer reads 100°C the entire time until the water is gone. You supplied the same heating power throughout, yet the temperature refused to rise during boiling. Where did all that energy go? It went entirely into tearing water molecules away from each other — into breaking bonds, not into speeding molecules up.
When a substance changes state (solid→liquid or liquid→gas), energy is absorbed to break intermolecular bonds — not to increase KE. Temperature stays constant while this is happening. This energy is the latent heat.
$Q = mL$
$L_f$ (fusion/melting) = $3.34 \times 10^5$ J/kg for water · $L_v$ (vaporisation) = $2.26 \times 10^6$ J/kg for water
Note that $L_v \gg L_f$: evaporating water requires ~7× more energy per kilogram than melting ice. This is why sweating is effective — a small amount of water evaporating carries away a large amount of heat.
Latent heat: $Q = mL$, where $L_f$ (fusion, water) $= 3.34 \times 10^5$ J/kg and $L_v$ (vaporisation, water) $= 2.26 \times 10^6$ J/kg. Temperature stays constant during a phase change because supplied energy breaks intermolecular bonds, not speeds particles up.
Pause — copy the highlighted equation and latent heat values into your book before moving on.
How much energy is needed to vaporise 0.5 kg of water at 100°C? ($L_v = 2.26 \times 10^6$ J/kg)
We just saw that latent heat is energy absorbed during phase changes at constant temperature. That raises a question: once thermal energy exists in a substance, how does it actually move from one place to another? This card answers it → three distinct mechanisms operate: conduction (particle contact), convection (bulk fluid flow), and radiation (EM waves) — only radiation works in a vacuum.
| Mode | Mechanism | Requires medium? | Examples |
|---|---|---|---|
| Conduction | Direct particle-to-particle collision transfer of KE; free electrons in metals | Yes (solid/liquid/gas) | Metal spoon in hot soup, touching a cold surface |
| Convection | Bulk movement of fluid carrying thermal energy; density differences drive circulation | Yes (fluid only) | Boiling water, sea breezes, heating rooms |
| Radiation | Electromagnetic waves (infrared) emitted by all objects above absolute zero | No (vacuum fine) | Sun → Earth, campfire warmth, thermal imaging |
Three modes of heat transfer: (1) Conduction — particle-to-particle KE transfer in solids (metals fast via free electrons); (2) Convection — bulk fluid flow driven by density differences; (3) Radiation — infrared EM waves emitted by all objects above 0 K, the only mode that works in a vacuum.
Add the highlighted three modes and their key features to your notes before the check below.
Convection can transfer heat through a vacuum.
The temperature remains constant during a change of state even though energy is being added.
Activities
Use $L_f = 3.34 \times 10^5$ J/kg and $L_v = 2.26 \times 10^6$ J/kg:
- Calculate $Q$ to melt 2 kg of ice at 0°C.
- Calculate $Q$ to vaporise 0.3 kg of water at 100°C.
- Calculate the mass of steam that condenses releasing 9.04 × 10⁵ J.
Identify the primary mode of heat transfer in each situation and explain why:
- Heat reaches Earth from the Sun
- A metal fork heats up in hot soup
- Air near a heater rises and cooler air takes its place
- Sitting near a campfire on a cold night
On the axes below (describe the shape in words), sketch a temperature-time heating curve for ice starting at −20°C and heated until it becomes steam at 110°C. Label all five segments (heating solid, melting, heating liquid, vaporising, heating gas) and state the equation that applies to each segment.
Which mode of heat transfer does NOT require a medium?
Calculate the total energy needed to convert 0.1 kg of ice at 0°C to water at 0°C, then heat the water to 100°C. ($L_f = 3.34 \times 10^5$ J/kg, $c = 4186$ J/kg·K)
Why do metals conduct heat much better than wood?
UnderstandBand 3(3 marks) 1. Explain, using particle theory, why the temperature of water does not rise while it is boiling, even though energy is being continuously supplied.
ApplyBand 4(3 marks) 2. A 1.5 kg block of ice at 0°C absorbs 5.0 × 10⁵ J of thermal energy. How much ice melts? ($L_f = 3.34 \times 10^5$ J/kg)
AnalyseBand 5(4 marks) 3. A thermos flask minimises heat transfer by: a vacuum between walls (no convection or conduction), silvered inner surfaces (reduces radiation). Explain how each design feature prevents the relevant mode of heat transfer.
Show all answers
Activity 2 Calculations
1. $Q = 2 \times 3.34\times10^5 = 6.68\times10^5$ J 2. $Q = 0.3 \times 2.26\times10^6 = 6.78\times10^5$ J 3. $m = 9.04\times10^5/2.26\times10^6 = 0.40$ kg
MT4 Calculation
$Q = mL_f + mc\Delta T = 0.1 \times 3.34\times10^5 + 0.1 \times 4186 \times 100 = 33400 + 41860 = 75260$ J
Short Answer — Model Answers
Q1 (3 marks): During boiling, the energy supplied breaks intermolecular bonds between water molecules rather than increasing their kinetic energy. Since temperature is a measure of average KE, and KE is not changing (bonds are being broken instead), the temperature remains at 100°C. The flat portion on a heating curve corresponds to this latent heat of vaporisation. Only when all liquid has vaporised does the steam's temperature begin to rise again.
Q2 (3 marks): $m = Q/L_f = 5.0\times10^5/3.34\times10^5 = 1.50$ kg. All of the 1.5 kg block melts (exactly). The ice just fully melts with no energy left over to raise temperature of the water.
Q3 (4 marks): (a) Vacuum layer: conduction requires physical contact between particles to transfer kinetic energy. A vacuum has no particles, so conduction cannot occur. Convection requires a fluid (liquid or gas) to carry energy via bulk flow; with no fluid in the vacuum, convection is also eliminated. (b) Silvered inner surfaces: all objects emit infrared radiation. Silver is a poor emitter and a good reflector of infrared. The silvered inner surfaces reflect most of the infrared back into the flask (or back into the surroundings), greatly reducing net radiation transfer. Together, the three eliminated modes mean the flask loses/gains very little heat.
The CSIRO's 2019 measurement of Antarctic ice loss (252 billion tonnes per year) makes the scale of latent heat visceral: every tonne of ice that melts absorbs $3.34 \times 10^5$ J — all at 0°C with zero temperature rise. The total energy absorbed by Antarctic melting alone ($8.42 \times 10^{19}$ J/year) equals 24 years of global electricity production. That energy goes entirely into breaking molecular bonds (latent heat of fusion), not into kinetic energy — which is why the temperature stays constant during melting. The kettle example you predicted at the start of the lesson follows exactly the same physics, just on a kitchen scale.