EASA/GCAA Part-66 · Module 2 — Physics · Thermodynamics

Specific & Latent Heat

Objective: heating a substance raises its temperature (specific heat, Q = mcΔT) — except during a change of state, when the heat goes into breaking bonds at constant temperature (latent heat, Q = mL). Heat ice through to steam and watch the two flat plateaus.

Beaker

Heating curve — temperature vs energy

Heater power 1.0 kW

Mass of water 0.20 kg

Temperature

-20°C

State

Ice

Energy added

0kJ

Now going into

—

Sensible heat: Q = m·c·ΔT · Latent heat: Q = m·L (constant temperature)

Add heat to a solid and its temperature climbs at a rate set by its specific heat capacity c (Q = mcΔT). But at the melting point the temperature stops rising even though you keep heating — all the energy goes into the latent heat of fusion, pulling the solid's bonds apart to turn it to liquid. Once melted, the temperature climbs again until the boiling point, where a second, much larger plateau appears: the latent heat of vaporisation, turning liquid to gas. That is why steam at 100 °C scalds far worse than water at 100 °C — it carries all that extra latent energy. For water, fusion needs 334 kJ/kg and vaporisation a huge 2260 kJ/kg.

Try this: Heat from ice and watch two flat plateaus — melting at 0 °C and boiling at 100 °C.
Try this: Notice the boiling plateau is much longer — vaporisation takes far more energy than melting.
Try this: Raise the heater power — the whole curve is walked through faster, but the plateau temperatures don't change.
Try this: Increase the mass — every stage needs proportionally more energy.