Chapter 5 of 5
Phase Changes: The Hidden Heat
Add heat to ice and its temperature rises. But when it hits 0°C… the temperature stops rising. You're still adding heat, but the thermometer won't budge. Where's the energy going?
It's going into breaking molecular bonds. In ice, water molecules are locked in a rigid crystal structure. To melt, those bonds need to break. All the energy you add goes into snapping bonds instead of making molecules move faster. Since temperature measures molecular speed, the temperature plateaus during a phase change.
This "hidden" energy is called latent heat (from Latin latere, "to hide"). It works in reverse too: when steam condenses to liquid, it RELEASES all that latent heat.
That's why steam burns are worse than boiling water burns. Steam at 100°C and water at 100°C are the same temperature. But when steam hits your skin and condenses, it dumps its latent heat — 2,260 joules per gram — directly onto your skin before the resulting hot water even starts to cool. Double whammy.
-20°C — Ice
Watch the flat spots — that's where all your heat goes into breaking bonds instead of raising temperature. That's latent heat.
~This is why sweating works
Self-assessment
After reading this guide, how well do you understand heat and thermal energy?
No right answer. Both ends are valid.