On a heating curve, slopes show a temperature change and flat sections show a change of state. The length of a flat section gives the latent heat, and the steepness of a slope gives the specific heat capacity.
This lesson is part of heat. It combines using specific heat capacity and using specific latent heat.
What does each section mean?
Read the graph in four parts.
| Section | Temperature | Process | Energy goes into |
|---|---|---|---|
| Sloping, solid | Rises | Warming the solid | Kinetic energy of particles |
| Flat, first | Constant | Melting | Potential energy of particles |
| Sloping, liquid | Rises | Warming the liquid | Kinetic energy of particles |
| Flat, second | Constant | Boiling | Potential energy of particles |
Worked example: an idealised dataset
A 300 W heater warms 0.20 kg of ice from −10°C, with no heat loss. These original readings are idealised.
| Time (s) | Temperature (°C) |
|---|---|
| 0 | −10 |
| 14 | 0 |
| 237 | 0 |
| 377 | 50 |
| 517 | 100 |
The flat section from 14 s to 237 s is melting at 0°C. The liquid section from 237 s to 517 s rises from 0°C to 100°C.
Calculating from the data
Latent heat: the energy supplied during melting is 300 × (237 − 14) = 300 × 223 = 66 900 J. Dividing by 0.20 kg gives l = 3.3 × 10⁵ J kg⁻¹.
Specific heat capacity of water: the temperature rises 100°C in 280 s. The energy supplied is 300 × 280 = 84 000 J. Then c = Q ÷ (mθ) = 84 000 ÷ (0.20 × 100) = 4200 J kg⁻¹ °C⁻¹.
The mistake that costs marks
The slip is to say “no heat is supplied” on the flat section, or to say the energy raises the kinetic energy of the particles. The heater is still on, and the energy raises their potential energy.
| Statement | Problem | Better |
|---|---|---|
| “The heater stops on the flat part” | Heater still supplies power | Heat is supplied but used for melting |
| “Particles speed up during melting” | Temperature is constant | Potential energy increases |
| Reading l from the slope | The slope gives c | Use the flat section’s duration |
Use a sentence frame: “From A to B the temperature is constant, so the substance is melting, and the energy supplied increases the potential energy of the particles.” The graph evidence comparison lab lets you compare curves like this.
Check yourself
The same heater melts the ice but the flat section lasts 446 s instead of 223 s. What does this tell you about the mass?
Answer
The energy supplied is 300 × 446 = 133 800 J, which is double the earlier 66 900 J. Since l is fixed, the mass must be double: 0.40 kg.
A longer flat section means more material to melt at the same power.
What to study next
Finish with the heat practice set. If the flat sections are unclear, revisit using specific latent heat.
To have a teacher go through graph questions with you, see online one-to-one Physics tuition.