Phase changes, latent heat, and thermal expansion

Heating a substance usually raises its temperature by an amount set by its mass and specific heat. During a phase change, though, such as ice melting or water boiling, the added energy goes into overcoming the attractions between particles instead, so the temperature stays constant until the change is complete. The energy needed per kilogram is called latent heat: the latent heat of fusion for melting (about 334,000 J/kg for ice) and the latent heat of vaporization for boiling (about 2,260,000 J/kg for water). The same amounts of energy are released when the substance freezes or condenses, which is why steam at 100 degrees C can cause worse burns than boiling water at the same temperature. A heating curve for water shows sloped sections where the temperature rises and flat plateaus at 0 and 100 degrees C (at normal atmospheric pressure) where it changes phase. Most materials also expand when heated, because their particles move more and spread slightly farther apart, so engineers leave expansion joints in bridges and railroad tracks. Water is an important exception: it expands as it cools from 4 to 0 degrees C and expands again when it freezes, so ice is less dense than liquid water and floats.

This explanation is AI-generated and can occasionally be wrong.

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