Altitude, Temperature and Pressure
water boils at a lower temp. at higher altitudes does
altitude affect the temp. at which water will freeze?
The reason that altitude affects the boiling point is that the air pressure
is less the higher the altitude. Since the volume change upon going from
the liquid state to the gaseous state (in other words, boiling) is so large,
pressure makes a big difference. Expanding against a high pressure takes a
lot more energy than expanding against a low pressure, so you have to put a
lot more energy (as heat) into the system to boil at lower altitudes.
The volume change upon melting is much smaller than the volume change upon
boiling. In fact, the volume decreases when ice melts. So, higher
pressures favor the liquid state over the solid state. As a consequence,
the melting point (which is the same thing as the freezing point) of water
will actually DECREASE as the pressure increases, opposite the direction the
boiling point moves. The size of the change, however, will be VERY small
compared to the change in the boiling point.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012