water, is called latent heat of vaporization. All
substances that change from liquid to a vapor or gas go
Heat absorbed, or given up, during the conversion
through this stage. The amount of heat required to
of a substance from one physical state to another has
convert 1 pound of water at 212°F into steam, or to
another name. This is called latent heat. (Latent is taken
convert steam to water, is 970.4 Btu.
from the Greek language meaning hidden.) Latent heat
has two forms: latent heat of fusion and latent heat of
vaporization. When latent heat is added to or subtracted
In refrigeration and air conditioning, the total heat
from a substance, and the physical change takes place,
of a substance, or of the air in a room, is all the heat
there is no change in the sensible heat or temperature of
present; that is:
Total heat = sensible heat + latent heat.
LATENT HEAT OF FUSION.--If heat is applied
to a piece of ice at a temperature of 0°F, the temperature
In discussions, the term heat content is sometimes
of the ice will gradually rise. This change in
used. This term has the same meaning as total heat.
temperature, which can be measured by a thermometer,
is called sensible heat. No change in state takes place,
only a change in the temperature of the ice.
If a vapor is not in contact with a boiling liquid,
As more sensible heat is added, the temperature of
either because the liquid has been completely converted
the ice finally reaches 32°F. Now, as more heat is
to vapor or because the vapor has been separated from
absorbed by the ice, the ice melts or changes state, but
the boiling liquid, further application of heat produces a
the temperature of the liquid remains constant at 32°F
rise in temperature of the vapor, which is called
until the ice is completely melted. The heat added
superheating. To meet the definition for superheat,
during the process of melting the ice at 32°F to water at
there must be no liquid present.
32°F is the hidden or latent heat of fusion.
This process also works in reverse. When water is
chilled to 32°F and more heat is taken away to form it
into ice at 32°F, this heat is also latent heat of fusion.
As stated earlier, heat can be neither created nor
destroyed; it can only be transferred from one object to
What is demonstrated here is one of the most
another. There are three laws that govern heat transfer:
important laws in physics; heat can never be destroyed.
It can only be transferred from one substance to
· Heat always flows from a hot object to a cooler
another. So the same amount of heat required to melt
the ice into water must be removed from the water to
· The greater the difference in temperature, the
convert it back to ice.
faster the heat flows.
The latent heat of fusion for pure water at 32°F and
· Heat will continue to flow until all temperatures
at sea level is 143.33 Btu per pound.
LATENT HEAT OF VAPORIZATION.--As the
The transfer of heat can be accomplished in one of three
last of the ice melts, the temperature of the water begins
to rise. The temperature causing the rise is sensible
heat. When the temperature of water reaches 212°F, the
temperature stops rising and another change takes
place. More heat is added and the water boils or
In radiation, heat is transferred through an empty
changes to steam, but there is no change in temperature.
space (a vacuum) or through the atmosphere. Radiation
This too is hidden heat. As the last of the water
does not heat the air through which it passes; it heats
vaporizes and more heat is added, the temperature will
only the objects upon which it falls. Not only the sun,
again rise, and again we are dealing with sensible heat.
but other objects such as stoves, flames, light bulbs,
The heat added to, or taken away, in the process of
machines, and even the earth itself radiate heat. Even
changing water to steam (vapor), or from steam back to
our bodies radiate heat.