The thermometer waits.
Start with 1 kg of ice at −15 °C. Watch the temperature reach zero, then follow the shrinking ice. What keeps changing while the thermometer holds still?
FIELD NOTE 09 / PHASE CHANGE LAB
When heat changes matter.
A flame, a pot, and a thermometer that sometimes stands still.
0.0 min
Open pot · about 1 atm
The cutaway reveals the sample. Amounts follow the model; fill levels, ice shapes, flame and mist are illustrative. Real water vapour is invisible; a white plume consists of condensed droplets.
- Sample temperature
- −15.0 °C
- Propane consumed
- 0.0 g
- Heat reaching the pot
- 1.35 kW
Warming the ice
SOLIDEnergy increases the temperature of the ice and pot.
Ice 1.000 kgWater 0.000 kgEscaped 0.000 kg
The three amounts add up to the original sample.
Change it now
The heating curve
°C · elapsed minutesSample temperatureSaved temperature
Watch for plateaus at 0 °C and 100 °C while the phase fractions change.
Follow the energy
kJ · elapsed minutesFuel energyDelivered to potNet to roomCarried by vapour
Fuel energy splits between heat delivered and heat bypassing the pot. The room trace can be negative when the surroundings warm the sample.
Read the complete energy account
Delivered heat + heat from the room = retained enthalpy change + enthalpy carried away by vapour. The pot’s sensible heat is included.
The fuel behind the flame
Grams of propane · elapsed minutesPropane consumedSaved fuel use
The slope follows burner input power. Turning the burner off stops fuel consumption while heat can still move between the pot and room.
Keep a run, change the burner or heat transfer, then Reset to compare. Use the same starting sample for a matched comparison.
MOMENTS IN THIS RUN
- Press Play to begin.
Ready. Play begins the experiment. All heat-transfer controls work while it runs.
Make a prediction. Change one thing. See what happens.
Model notes & references ↗FOLLOW THE SCIENCE
Heat keeps arriving.
The temperature waits.
Temperature tells part of an energy story. First, energy warms the ice. At the melting point, ice and water can coexist while added energy changes their proportions. Once the ice is gone, further energy warms the liquid. At the boiling point, another change of state begins.
The flat portions of the heating curve are the clue: a sample can absorb considerable energy while its temperature stays almost constant. In an open pot, boiling also carries mass and energy away. The water level falls even though the thermometer holds steady.
A FEW QUESTIONS TO TRY
Let’s see what happens.
More flame. Hotter water?
Once boiling begins, increase the burner power. Compare temperature, the rate of vapour loss, and the slope of the fuel trace.
Make more of the fuel.
Increase the fraction of burner heat reaching the pot. The fuel flow stays the same at fixed input power, but more energy reaches the sample each second.
“Reset with ice” starts a fresh sample. The other actions change the current run without pausing or rewinding. Save a run before resetting if you want its temperature and fuel traces to remain visible.
Warming and changing state are different jobs.
SENSIBLE HEAT
The thermometer moves.
A temperature change requires approximately Q = m c ΔT. Liquid water’s specific heat is about 4.18 kJ/(kg·K), so warming 1 kg from 0 to 100 °C takes about 418 kJ, before allowing for the pot and heat losses. Real heat capacity varies with temperature. NIST: properties of liquid water ↗
LATENT HEAT
The phase changes.
Melting 1 kg of ice near 0 °C requires about 333.5 kJ. The added energy changes the phase fraction while ice and liquid coexist. Freezing reverses the process and releases that latent energy. PTB / BIPM: enthalpy of fusion ↗
Boiling takes the larger energy investment.
Turning 1 kg of boiling water into vapour near 100 °C takes about 2,257 kJ—roughly 5.4 times the energy needed to warm that kilogram from 0 to 100 °C. More burner power mainly increases the boiling rate at fixed pressure. NIST: saturated water and steam ↗
The molecules are still water.
Melting and boiling do not split water into hydrogen and oxygen. They change how H₂O molecules are arranged and how they interact. During a phase change, energy is used in that transformation instead of producing the temperature rise you would expect within a single phase.
Fuel energy is only the beginning.
A burner releases chemical energy. Some reaches the pot; some bypasses it. The pot and its contents also exchange heat with their surroundings. A cold sample can gain energy from a warm room, while a hot sample loses energy to it. The energy account keeps those paths separate.
Here, propane consumption is fuel energy divided by an assumed lower heating value of 46.4 MJ/kg. This convention excludes recovering the condensation heat of water in the combustion exhaust. Burner-to-pot coupling is an adjustable teaching parameter, not a rating for a particular appliance. U.S. Department of Energy: fuel energy conventions ↗
Why does energy leave with the vapour?
The vapour carries the energy associated with warming the water as well as vaporizing it. The amount retained in the pot can therefore fall during boiling, even while heat is entering. Open the energy account below the plot to follow the balance. Escaped vapour is not returned to the pot when you turn the burner off.
Continue exploring energy storage in The ground remembers ↗, or heat loss and insulation in A house holds the heat ↗.
The mathematics, assumptions & references
A well-mixed pot and an open-system balance
Q̇pot = η Pfuel · Q̇room = UA(Troom − T)
dEretained/dt = Q̇pot + Q̇room − ṁvapour hvapour
dEretained/dt = Q̇pot + Q̇room − ṁvapour hvapour
The pot and retained sample share one temperature. E is their retained enthalpy relative to ice and the pot at 0 °C. The surrounding pressure is fixed near one atmosphere; melting and boiling temperatures are rounded to 0 °C and 100 °C. Mass changes only when boiling. Sub-boiling evaporation, condensation returning to the pot, convection within the water, supercooling, nucleation and superheating are omitted.
| Quantity | Value |
|---|---|
| Ice specific heat | 2.10 kJ/(kg·K) |
| Liquid-water specific heat | 4.18 kJ/(kg·K) |
| Enthalpy of fusion | 333.5 kJ/kg |
| Enthalpy of vaporization near 100 °C | 2,257 kJ/kg |
| Pot heat capacity | 700 J/K; heated together with its contents |
| Propane lower heating value | 46.4 MJ/kg, rounded reference value |
| Room exchange coefficient UA | 0–10 W/K; fixed until adjusted |
How a temperature plateau appears
Eice = (m cice + Cpot)T
Emelting = m Lfusion fliquid
Eliquid = m Lfusion + (m cwater + Cpot)T
Emelting = m Lfusion fliquid
Eliquid = m Lfusion + (m cwater + Cpot)T
During sensible heating or cooling, the model integrates the lumped heat-balance equation exactly for each constant setting. At a phase boundary, it resolves the arrival time within the step. While melting or freezing, the temperature remains zero and net input changes the liquid fraction. During boiling, ṁvapour = Q̇net/Lvap when Q̇net is positive. If it becomes negative, the remaining liquid cools below the boiling point.
For each kilogram leaving at 100 °C, the model subtracts hvapour = Lfusion + cwater × 100 + Lvap from the total open-system budget, using the same ice-at-zero reference. The retained liquid loses its corresponding liquid enthalpy as that mass leaves. This prevents double-counting latent energy or keeping vanished water’s heat capacity in the pot.
Reading the plots
Fuel energy is the time integral of burner input. Delivered energy is the integral of ηP. Their difference is burner energy that bypassed the pot. “Net to room” is the negative of integrated room input, so it can be negative while the room warms a cold sample. “Carried by vapour” is escaped enthalpy in the stated reference, not just latent energy. The exact numerical account is available as a table. Fuel mass is Efuel/LHV. No heating continues after the sample has fully vaporized.
The maximum substep is five seconds, with exact event handling at phase transitions and an empty pot. Playback stops at sample completion or two model hours. Temperature, energy, remaining mass and time persist when live controls change. Starting mass and temperature apply only at Reset, which explicitly begins a new sample; Reset keeps current heat-transfer settings, playback state and any saved comparison. A saved comparison keeps its original conditions and is not recomputed.
The pot illustration does not define a physical volume or an exact burner geometry. Ice shapes, water level, flames and mist visualize the model state schematically. A real gas flame, pot bottom and sample have different temperatures; this model combines the pot and sample into one thermal node. The fixed UA approximates room exchange and does not resolve convection or radiation separately. These are simulated experiments, not measured cooking times or appliance performance.