Temperature and States of Matter
When matter is heated or cooled, it often does things other than simply expand-
ing or contracting, or exerting increased or decreased pressure. Sometimes it
undergoes a change of state. This happens when solid ice melts into liquid water,
or when water boils into vapor, for example.
THAWING AND FREEZING
Imagine it is late winter in a place such as northern Wisconsin, and the temper-
ature of the water ice on the lake is exactly 0°C. The ice is not safe to skate on,
as it was in the middle of the winter, because the ice has become “soft.” It is
more like slush than ice. It is partly solid and partly liquid. Nevertheless, the
temperature of this ice, both the solid and liquid parts, is 0°C.
As the temperature continues to rise, the slush gets softer. It becomes propor-
tionately more liquid water and less solid ice. But its temperature remains uni-
form at 0°C. Eventually all of the ice melts into liquid. This can take place with
astonishing rapidity. You leave for school or work one morning and see the lake
nearly “socked in” with slush, and return in the evening to find it almost entirely
thawed. Now you can get the canoe out! But you won’t want to go swimming.
The liquid water near the surface will stay at 0°C until all of the ice is gone.
Now consider what happens in late autumn. The weather, and the water,
grows colder. The surface temperature of the water finally drops to 0°C. It
begins to freeze. The temperature of this new ice is 0°C. Freezing takes place
until the whole lake surface is solid ice. The weather keeps growing colder (a lot
colder, if you live in northern Wisconsin). Once the surface is entirely solid ice,
the temperature of the ice begins to fall below 0°C, although it remains at 0°C
at the boundary just beneath the surface where solid ice meets liquid water (and
may be considerably above 0°C deeper down because the lake is fed by subter-
ranean springs). The layer of ice gets thicker. Exposed ice at the surface can get
much colder than 0°C. How much colder depends on various factors, such as the
severity of the winter and the amount of snow that happens to fall on top of the
ice and insulate it against the chill of the air.
The temperature of water does not follow exactly along with the air temper-
ature when heating or cooling takes place in the vicinity of 0°C. Instead, the
water temperature follows a curve something like that shown in Fig. 1-8. At A,
the air temperature is getting warmer; at B, it is getting colder. The water tem-
perature “stalls” as it thaws or freezes. Many other substances also exhibit this
property when they thaw or freeze.
CHAPTER 1 Background Physics
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