Latent Heat

Definitions

ð Change of Phase: a change from one state (solid or liquid or gas) to another without a change in chemical composition

ð Heat of Fusion (L_F): Heat required to change 1.0 kg of a substance for the solid to the liquid state

ð Heat of Vaporization (L_V): Heat required to change a substance from the liquid to the vapor phase

ð Latent Heats: Values for L_F and L_V

Temperature as a Function of Heat

Note the constant temperature regions

Latent Heats at 1 atm

Notes

These are also the amount of heat released by a substance when it changes phase

Examples:

When steam changes to water, 2260 kJ/kg of heat is released, when the water freezes it releases 333 kJ/kg

Going the other way, it takes 33 kJ/kg to change ice to water, and 2260 kJ/kg to change water to steam

Note also that the heat required in a change of phase depends, in addition to the latent heats but also upon the mass

Additional Comments-Notes

Heat is required to raise the material (water, in the diagram illustrated below) to its melting point

To go from the solid to liquid phase requires heat, the heat of fusion. For water it is about 80 (79.7 in table above) kcal/kg

(It is all water after 100 kcal has been added. Subtract from this the heat that is needed to bring it to its melting point or transition point - 20)

Water stays liquid until 200 kcal has been added. This is the transition point.

An additional 539 kcal is needed (740 - 200 with rounding) is needed to convert it to vapor - this is the heat of vaporization (see above table)