MadSci Network: Chemistry

Re: Do liqiuds cool as they evaporate?

Date: Tue Nov 14 00:44:24 2000
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Chemistry
ID: 973726613.Ch

Greetings, Ashley:

You should be able to find initial information on the
subject of evaporation in any good encyclopedia.  Some of
them, such as Encyclopaedia Britannica, are accessible via
the Internet (  Be sure to check
out some of the cross-links to related topics, and/or to
specify a "search" or two (example to type in the search
box:  evaporation AND cooling).

With respect to conducting experiments, this particular
phenomenon is relatively easy to test.  Nevertheless, a
measure of caution is always necessary.

One key fact about evaporation is:  The amount that can
occur is directly related to the surface area of the liquid
that is exposed to the air.  Suppose that only a tiny amount
of cooling happens to the liquid; how can you measure it
with an ordinary thermometer?

The trick is to use a "wick".  This is generally a length of
fibrous substance that you partly use to surround the little
bulb at the end of a thermometer.  Then you dip the rest
of the wick into a container of liquid.

CAREFULLY!  Some readily available liquids are poisonous,
flammable, and even explosive.  If you dare to test a liquid
like gasoline, for example, use small amounts only, and do
it outdoors!  Other readily available liquids are water,
vinegar, rubbing alcohol, and motor oil and kerosene.  A few
liquids you may be able to obtain after some searching, but
probably not with any greater difficulty than getting an
adult involved:  sulfuric acid, carbon tetrachloride, and
dimethyl sulfoxide, for example.  PAY CLOSE ATTENTION TO ALL
WARNING LABELS!  Be sure to pick a wick material that can
withstand all these liquids!  (You may be able to find one
made from glass fibers.)

The experiment consists first of recording the temperature
before placing the wick in the liquid.  Allow a minute to
pass, giving the liquid a chance to move up the wick and
into the air, taking advantage of the large surface area it
provides.  (Something else to research is HOW all those
wick-fibers provide vast amounts of surface area for the
evaporation of a liquid.)

Make recordings of the temperature every minute, for fifteen
minutes or so.  By then any change in temperature that is
going to happen will have happened.  It is not necessary to
test all your different liquids at the same time. 
Good luck!

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