Change of State and Latent Heat

Change of State and Latent Heat

Matter, as studied at this level, is ordinarily found in one of three states: solid, liquid, or gas. A

substance can be made to pass from one of these states to another by supplying heat to it, or by removing

heat from it. The main changes of state, and their names, are:

• Melting (fusion): solid $\to$ liquid (heat absorbed)
• Freezing (solidification): liquid $\to$ solid (heat released)
• Vaporisation (boiling): liquid $\to$ gas (heat absorbed)
• Condensation: gas $\to$ liquid (heat released)
• Sublimation: solid $\to$ gas directly, without an intervening liquid state (heat absorbed) -- as seen with solid carbon dioxide ("dry ice") and with naphthalene (mothballs)

Temperature stays constant during a change of state

A striking and important fact about any change of state occurring at constant pressure is that the

temperature of the substance remains exactly fixed throughout the change, even while heat continues

to be supplied to it (or removed from it) steadily. While a solid is melting, its temperature stays

pinned at the melting point until the very last trace of solid has turned to liquid; only after that does

further heating raise the liquid's temperature above the melting point. In just the same way, while a

liquid is boiling, its temperature stays pinned at the boiling point until every last drop of liquid has

turned to vapour. This is exactly what the two flat plateaus in a substance's heating curve (see the

figure accompanying this chapter's calorimetry section) represent.

Latent heat

The heat that is absorbed or released during a change of state, without being accompanied by any change

in temperature, is called latent heat -- "latent" meaning hidden, because this heat exchange does not

show up as any reading change on a thermometer; instead, it goes entirely into doing the internal work of

rearranging (or completely separating) the substance's molecules from one arrangement into another. The

latent heat capacity (or specific latent heat) $L$ of a substance, for a particular change of state,

is defined as the quantity of heat required to change the state of unit mass of the substance, at

constant temperature. For a mass $m$ undergoing the change,

with SI unit $\text{J kg}^{-1}$.

Two specific latent heats are distinguished by name: the latent heat of fusion, $L_f$, for the

solid-liquid change, and the latent heat of vaporisation, $L_v$, for the liquid-gas change. For any

given substance, $L_v$ is very much larger than $L_f$ -- for water, $L_f \approx 3.34\times10^{5}\ \text{J kg}^{-1}$ while $L_v \approx 2.26\times10^{6}\ \text{J kg}^{-1}$, nearly seven times as large (see the

reference table below for further examples). The physical reason is that melting only has to loosen a

rigid, ordered crystal lattice into a disordered but still-connected liquid, whereas vaporisation has to

do far more work: pulling molecules that are still attracting one another completely apart from each …