Re: what is the evaporating temperature of water?
- From: RP <no_mail_no_spam@xxxxxxxxx>
- Date: Sat, 8 Dec 2007 10:17:20 -0800 (PST)
On Dec 8, 12:07 am, bhargava <bhargava.pra...@xxxxxxxxx> wrote:
hi,
water evaporates from its bodies ( likes ocean, river etc..) at
temperatuer of 30 and odd degress of temp, but where as in boiling it
doesn't even evaporate at 100 C(celcius). Why is this so?Then, What
determines the evaporating temperature of water?
Hope you got the question.
thanks
A liquid's surface always evaporates. Boiling occurs when the external
pressure (usually gravity plus atmospheric pressure) is lower than the
saturated pressure of the liquid.
Saturated pressure is in turn the pressure that corresponds to 100%
RH, or a steady state condition in which the number of molecules
diving back into the surface per unit time is equal to the number of
molecules evaporating from it per the same unit time.
When this equilibrium doesn't exist, for instance when RH above the
surface is less than 100%, then the evaporation becomes visible in
that the water level will gradually drop. There will be more molecules
escaping than returning.
Above the surface the RH can be much less than 100% RH, but under the
surface RH is always at 100%. Water is thus always at saturation
within its own surfaces and thus always exerts an outward saturated
pressure on itself equal to saturated pressure. When the external
pressure is greater than saturated pressure, then the water is
prevented from expanding as it wants to. By heating the water to
boiling temperature the internal outward vapor pressure equals the
atmospheric pressure pushing back down on it, and the tendency of the
water to expand is no longer countered. We see this as boiling.
Evaporation from the surface was occurring the whole time, even before
boiling temperature and/or pressure was reached, and continues to
occur after boiling starts.
The difference in evaporation rate is due to the fact that the total
surface area of the water, when it is boiling, is immensely increased.
Every bubble provides a surface in addition to the surface on top of
the water. The number of molecules escaping is thus increased
exponentially, and evaporation is amplified proportionately. Boiling
and evaporation are thus not interdependent phenomena. A sealed jar of
water with no air in it, but having a pocket above the surface
containing only water vapor, can be made to boil by simply dropping
it, which removes the gravitational pressure, which in turn upsets the
equilibrium within the jar.
As a final note, just to help minimize further confustion, RH
(relative humidity) has nothing to do with the amount of water that
the "air can hold," even though it is common practice to speak of it
in those terms. In reality RH is just a proportional measure of the
partial vapor pressure to the saturated vapor pressure, and ideally
this is independent of any other gasses present or not present in the
space above the water.
.
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