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Name: Fred
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 1999-2001


Question:
water boils at a lower temp. at higher altitudes does altitude affect the temp. at which water will freeze?


Replies:
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.
Assistant Director
PG Research Foundation, Darien, Illinois



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