Chevrolet Light Duty Truck 1973 Service Manual page 48

1A 2 2 HEATER A N D AIR C O N D IT IO N IN G
has already melted aw ay-but the thermometer still reads
32°.
All this time, the ice has been soaking up heat, yet it
never gets any warmer no matter how much heat it
draws from the stored food. On the other hand, the cold
drain water got progressively warmer as it soaked up
heat. The addition of heat will make water warmer yet
won't raise the temperature of ice above the 32° mark.
If we fill one drinking glass with ice and another with
cold water, and put both glasses in the same room where
they could absorb equal amounts of heat from the room
air, we will find it takes much, much longer for the ice to
melt and reach room temperature than it did for the
water in the other glass to reach the same temperature.
Obviously, most of the heat was being used to melt the
ice. But it was the heat that apparently disappeared or
was transformed because it couldn't be located with a
thermometer. To describe this disappearing heat,
scientists chose the word "latent" which means hidden.
Latent Heat
So latent heat is nothing more nor less than hidden heat
which can't be found with a thermometer.
At first it was thought that latent heat was in the water
that melted from the ice. But that wasn't exactly the right
answer because, upon checking water temperature as it
melts from ice, it will be found that it is only a shade
warmer than the ice itself. It is not nearly warm enough
to account for all the heat the ice had absorbed. The only
possible answer is that the latent heat had been used up
to change the ice from a solid into a liquid.
Many substances can be either a solid, or a liquid, or a
gas. It just depends on the temperature whether water for
example was a liquid, or a solid (ice), or gas (steam) (fig.
28).
All solids soak up huge amounts of heat without getting
any warmer when they change into liquids, and the same
thing will happen when a substance changes from a
liquid into a gas.
Fig. 2 7 -M e ltin g Ice Remains at 32"
Fig. 2 8 -T e m p e ra tu r e D e te r m in e s T h e S ta te O f W a te r
Put some water in a teakettle, set it over a fire and watch
the thermometer as the water gets hotter and hotter, the
mercury will keep rising until the water starts to boil.
Then the mercury seems to stick at the 212° mark. Put
more wood on the fire, despite all the increased heat, the
mercury will not budge above the 212° mark (fig. 29).
No matter how large or hot you make the flame, you
can't make water any hotter than 212° at sea level. As a
liquid changes into a gas, it absorbs abnormally great
amounts of heat without getting any hotter.
Now we have two different kinds of latent heat, which
are quite a bit alike. To keep their identities separate,
the first one is called
latent heat of fusion,
the same as melting. The other kind is called
because that means the same as
of vaporization
evaporation.
Refrigeration
It may seem as though we have discussed heat instead of
Fig. 29--Boiling Water Never Exceeds 212° at Sea
which means
latent heat
l_evel
LIGHT DUTY TRUCK SERVICE MANUAL