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Lab #23.5
Heat: Warming by Freezing
Purpose
In this activity, students will
demonstrate that heat is
released when freezing occurs.
Discussion
We know that it is necessary to
add heat to liquefy a solid or
vaporize a liquid. In the
reverse process, heat is
released when a gas condenses or
a liquid freezes. Thermal energy
that accompanies these changes
of state is called latent heat
of vaporization (going from gas
to liquid or liquid to gas), and
latent heat of fusion (going
from liquid to solid or solid to
liquid).
Water, which
normally freezes at 0°C (32°F),
can be found under certain
conditions in a liquid state as
low as –40°C (–40°F) or more.
This supercooled water (liquid
water below 0°C) often exists as
tiny cloud droplets, common in
clouds where snow or ice
particles form. Freezing in
clouds depends on the presence
of ice-forming nuclei, most of
which are active in the –10°C to
–20°C range. Ice-forming nuclei
may be many different substances
such as dust, bacteria, other
ice particles, or silver iodide
used to “seed” clouds during
droughts. Silver iodide is
active at temperatures as high
as –4°C.
Cold clouds
containing large amounts of
supercooled water and relatively
small amounts of ice particles
can be dangerous to aircraft.
The skin of the aircraft, well
below freezing, provides an
excellent surface on which
supercooled water suddenly
freezes. This is called aircraft
icing, which can be quite severe
under certain conditions.
The heat pack
provides a dramatic example of a
supercooled liquid. What you
observe in the heat pack is
actually the release of the
latent heat of crystallization,
which is analogous to the
release of latent heat of
vaporization or the latent heat
of fusion. The freezing
temperature of the sodium
acetate solution inside the heat
pack is about 55°C (130°F), yet
it exists at room temperature.
The heat pack can be cooled down
to as low as –10°C before it
finally freezes.
It only takes a
quick click to activate the heat
pouch. You will notice that the
internal trigger button has two
distinct sides. If you use your
thumb and forefinger and squeeze
quickly, you will not need to
worry about which side is up.
After observing
the crystallization of the
sodium acetate and the heat
released, you might want to try
it again and measure the heat of
crystallization. The package has
a mass of about 146 grams. The
packaging and the trigger
mechanism have a mass of about
26 grams. Thus, the sodium
acetate solution inside the
package has a mass of about 120
grams.
Required
Equipment
Crystal Heat
Packs, hot plate, large pan or
pot of boiling water
Download
Student Worksheet
Teacher Notes
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