Below the boiling
point, atmospheric pressure on the liquid surface is greater
than the pressure that bubbles of vapor can develop in the
liquid, so these bubbles are prevented from forming. Evaporation
takes place only at the liquid-gas interface. But at the boiling
point, the vapor pressure becomes as great as the total pressure
on the liquid surface. Bubbles of vapor begin to form inside
the liquid as well as at its surface, which leads to the rapid
agitation that we call boiling.
What would happen to the equilibrium vapor
pressure of a liquid if some nonvolatile
solute molecules or ions were added? In the pure liquid
water, every molecule that approaches the surface has a certain
chance of escaping into the vapor phase, depending on its
kinetic energy. If a nonvolatile material such as sugar is
added so that one molecule in ten is sugar and not water,
then only 90% of the molecules
that formerly were potential escapees have a possibility of
getting out of the liquid.
The average escaping tendency of water molecules from a
given amount of solution is reduced,
but the rate of condensation
is unaffected since no sugar
molecules are present in the vapor. Condensation
gets ahead of vaporization, so more vapor condenses.
When vapor-liquid equilibrium is established once more, we
find that the equilibrium vapor pressure is only 90% as great
as it was originally
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