IGCSE CP14 – Temperature-Time Graph | Edexcel International GCSE Physics

Theory — Changes of State

When a substance changes state, energy is supplied or removed — but the temperature stays constant. This energy goes into breaking or forming the bonds between particles, not into increasing their kinetic energy.

Why Temperature Stays Constant During a Change of State

Temperature is a measure of the average kinetic energy of particles. During melting or boiling, the energy supplied is used to break intermolecular bonds (potential energy), not to speed up particles. Since kinetic energy doesn't change, temperature doesn't change — even though energy is still being transferred.

This is called latent heat — the "hidden" heat that causes a change of state without a temperature change.

The Temperature–Time Graph

Rising sections — substance is in one state, temperature rising as kinetic energy increases
Flat plateau at 0°C — ice melting to water. Energy breaks bonds between water molecules in the solid lattice
Flat plateau at 100°C — water boiling to steam. Energy overcomes the forces holding liquid molecules together

Q = m × L

Q = energy needed for change of state (J) · m = mass (kg) · L = specific latent heat (J/kg)

Specific Latent Heat Values for Water

Specific latent heat of fusion (melting): L = 334 000 J/kg (334 kJ/kg)
Specific latent heat of vaporisation (boiling): L = 2 260 000 J/kg (2260 kJ/kg)
Much more energy needed to boil water than to melt ice — boiling plateau is much longer

Cooling Curves — Solidification

When a substance cools, the reverse happens. Stearic acid (melting point ≈ 70°C) is a common example — heated to liquid, then allowed to cool. The temperature–time graph shows a plateau at 70°C as the liquid solidifies, releasing latent heat to the surroundings. Above and below the plateau, temperature falls steadily.

Procedure

Equipment

Crushed ice · Beaker · Immersion heater / Bunsen burner · Thermometer · Stopwatch · Electronic balance

1

Set up beaker with crushed ice

Fill a beaker with crushed ice. Ensure the thermometer is fully surrounded by ice. Check initial temperature is below 0°C.

💡 Use crushed ice rather than cubes — greater surface area means more uniform temperature throughout.

2

Start heating and stopwatch simultaneously

Apply gentle heat (Bunsen or immersion heater). Start the stopwatch. Record temperature every 30 seconds.

3

Record through melting and beyond

Continue recording as ice melts (temperature stays at 0°C), then as water warms up. Continue until water is well above room temperature, or until boiling if time allows.

4

Plot temperature–time graph

Plot temperature (y-axis) against time (x-axis). The flat region at 0°C is where melting occurs — all energy goes into breaking bonds.

🌡 Choose an experiment. Press ▶ Start — temperature is recorded automatically every 30 seconds.

Experiment

Current State

Phase

Ice (solid)

SOLID

Below melting point

Readings

Time elapsed0 s

Temperature−10.0 °C

Rate of change—

Readings taken0

Latent Heat

Plateau temp—

Plateau duration—

Energy at plateau—

Data Table

No readings yet.

Time / s	Temperature / °C	Change from prev / °C	Phase
Run the simulation to collect data.

Temperature–Time Graph

The flat plateau shows where energy is used for change of state, not temperature rise.

Questions

Question 1

A student heats a beaker of crushed ice. The temperature–time graph shows two flat plateaus and two rising sections. (a) What temperature is the first plateau at, and what change of state is occurring? (b) Why does temperature stay constant during this plateau even though energy is being supplied?

(a) The first plateau is at 0°C. This is where melting occurs — ice is changing state from solid to liquid. (b) The energy supplied during the plateau is used to break the bonds (intermolecular forces) holding water molecules in the fixed positions of the solid ice lattice. This increases the potential energy of the molecules, not their kinetic energy. Since temperature is a measure of average kinetic energy of the particles, and kinetic energy is not increasing, the temperature remains constant. All the energy goes into the change of state.

Question 2

The latent heat plateau for boiling (at 100°C) is much longer than the melting plateau (at 0°C) even though the same heater is used. Explain why.

The specific latent heat of vaporisation of water (2 260 000 J/kg) is about 6.8 times greater than the specific latent heat of fusion (334 000 J/kg). Vaporisation requires far more energy per kilogram because during boiling, molecules must completely overcome the intermolecular attractions to escape the liquid and become widely separated gas molecules — the molecules go from being close together (in the liquid) to very far apart (in the gas). During melting, molecules only need enough energy to break free from their fixed positions in the solid lattice — they remain close together as a liquid. Since more energy is required per kg for vaporisation, and the power of the heater is constant, the boiling plateau lasts much longer.

Question 3

In a cooling curve experiment with stearic acid, the temperature–time graph shows a plateau at 70°C as the liquid solidifies. Explain why the temperature stops falling at this point, and predict what the graph looks like above and below the plateau.

As liquid stearic acid cools, it releases thermal energy to the surroundings and its temperature falls. At 70°C (the melting/solidification point), the molecules begin to form bonds and arrange themselves into the solid lattice — this is solidification. As bonds form, latent heat is released to the surroundings. This released energy replaces the heat being lost to the surroundings, so the net temperature remains constant — the plateau. Above the plateau: the graph is a curve falling from ~75°C toward 70°C — liquid cooling. Below the plateau: the graph is a curve falling from 70°C — solid cooling, falling more steeply because the solid has a lower specific heat capacity than the liquid. The plateau itself is flat (or nearly flat) at exactly 70°C and lasts until all the liquid has solidified.