Thermal · Experiment

Phase Changes & Latent Heat

Heat ice steadily and its temperature climbs — until it hits 0 °C, where it stalls. All the incoming energy goes into melting, not warming. Those flat plateaus are latent heat.

Controls

Phasesolid
Temperature−20 °C
Heat added0 J
Latent heat (fusion)334 J/g
Latent heat (vaporisation)2260 J/g
Heating solid ice — temperature rising toward the melting point.
About this experiment ›

What you are looking at

A sample is heated at a steady rate by the burner below. The beaker shows its state (solid, liquid, gas), the thermometer shows its temperature, and the graph plots temperature against the heat you've poured in. The striking feature is the two flat plateaus — stretches where you keep adding energy but the temperature refuses to rise.

Two ways heat is used

Within a single phase, added heat raises the temperature. How much depends on the specific heat capacity c:
Q = m c ΔT  (sloped parts of the curve)
But at a phase change, the heat goes into breaking bonds rather than speeding molecules up, so the temperature holds steady while the substance converts. The energy per gram to do this is the latent heat L:
Q = m L  (the flat plateaus)
Melting needs the latent heat of fusion L_f; boiling needs the latent heat of vaporisation L_v.

Why the plateaus differ in length

For water, L_f = 334 J/g but L_v = 2260 J/g — so the boiling plateau is far longer than the melting one. It takes far more energy to tear molecules completely apart into a gas than merely to loosen them from a solid into a liquid. That huge vaporisation energy is why steam burns are so severe and why sweating cools you so effectively.

Reading the curve

Notice the sloped segments even have different steepness: ice, liquid water and steam have different specific heats (2.09, 4.18 and 2.01 J/g°C), so the same heat raises their temperatures by different amounts — liquid water warms the slowest. Switch substances to see how the melting and boiling points, and the plateau lengths, change with the material.

Things to try

Heat water from ice all the way to steam and watch the temperature stall twice. Compare the short melting plateau with the long boiling one. Turn up the power to move faster, or add mass to lengthen every stage proportionally. Switch to iron and see melting and boiling points in the thousands of degrees.