MadSci Network: Physics
Query:

Re: When water vaporizes what happens to the kinetic+potential energy?

Date: Sun Jan 17 10:39:39 1999
Posted By: Larry Lurio, Staff, Center for Materials Science and Engineering, IMM-CAT
Area of science: Physics
ID: 915762551.Ph
Message:

Dear Laura,
The energy of a molecule of water is composed of two parts, a kinetic
energy due to its motion, and a potential energy due to the forces of
the other water molecules acting on it.  The farther away the molecules
are from one another, the less each one acts upon others; thus water
molecules in a gas have less potential energy than molecules in liquid.
When a molecule of water evaporates it must do work to pull away from
all the other water molecules.  Hence its potential energy increases
(because it does work), and this is compensated by a decrease in its
kinetic energy.  The total energy of the molecule remains constant.
You might wonder how water can cool by evaporating if the energy of
any molecule stays the same when it evaporates.  The reason is this:
although any particular molecule keeps its same total energy whether it
is in a liquid or a gas, not all molecules have the same energy; some
have more total energy, some have less, and the difference is chiefly in
the amount of kinetic energy each possesses.  The molecules with the most
energy (and thus the most kinetic energy) are most likely to evaporate.
Hence if the most energetic molecules leave the liquid and the least
energetic ones stay behind, energy will flow from the liquid to the vapor.
I hope this helps.

Larry


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