Physics · Grades 9–12
Heating versus changing state
Choose between mcΔT and mL using the physical process.
Learn the idea
Temperature rise
Use Q = mcΔT when the sample's specific heat is treated as constant and no phase change occurs.
Phase change
At the phase-change temperature, use Q = mL for the mass that changes state.
Heating curve
A constant-temperature section can still involve energy transfer while a pure substance changes phase under suitable conditions.
Assumptions
Real apparatus may heat up and lose energy. A simple calculation must say when these effects are neglected.
Worked examples
A 2 kg sample with c = 500 J/(kg·°C) warms by 6 °C. Find energy supplied, with no losses.
Q = 2 × 500 × 6 = 6000 J.
6000 J
A 0.2 kg sample melts at its melting point with L = 300000 J/kg. Find energy required.
Q = mL = 0.2 × 300000 = 60000 J.
60000 J
Check your understanding
During a constant-temperature melting plateau, which can still increase?
- Energy transferred into the sample
- Temperature necessarily
- Mass necessarily
Which process matches Q = mcΔT most directly?
- Melting at constant temperature
- Warming without changing state
- A photon travelling through vacuum
A 1 kg sample receives 800 J and c = 400 J/(kg·°C). Find temperature rise, without losses or phase change.
Answers and marking notes
Answers and marking notes
- Energy transferred into the sample
Energy supports the phase change rather than a temperature rise in this ideal model.
- Warming without changing state
The formula models sensible heating with stated specific heat.
- 2 °C
ΔT = 800/(1 × 400) = 2 °C.
Further reading and review references
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Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.