Evaporation and Vapor Pressure
Evaporation is different in different liquids why is this?
Also why does evaporation cause a cooling effect?
The evaporation of a liquid depends upon its vapor pressure -- the higher
the vapor pressure at a given temperature the faster the evaporation --
other condition being equal.
The higher/lower the boiling point the less/more readily will a liquid
The amount of energy required to convert one mole of liquid to one mole of
vapor, the more energy is required for evaporation. You can find some
interesting trends in these behaviors by perusing a handbook such CRC/s
Handbook of Chemistry & Physics.
Water is a unique liquid with regard to its vaporization properties. It must
absorb ~10 k.cal/mol of heat at 25 C to convert one mole = 18 gm = 18 ml of
water to vapor and of course must release the opposite amount of heat for
the opposite process.
This is a HUGH energy lever that has a large impact on weather and climate!!
Estimate how much energy is released in a thundershower of 1 inch of rain
over 1 mile squared!!!
Evaporation in different liquids is different because of differences in
weights of the molecules of the liquid and (generally more importantly)
differences in attraction of molecules for each other within the
liquid. For example, water (H2O, molecular weight 18) has a much much
higher boiling point than ethane (C2H6, molecular weight 30) even though it
has a lower weight because of hydrogen bonding between water
molecules. But when you look a series of chemicals with similar
composition (for example the alkanes- methane, ethane, propane, butane,
pentane, hexane), as you get higher weights you generally get higher
Evaporation takes energy from whatever source and converts it into breaking
the attractions between molecules- as the now gaseous molecules float off,
they carry this energy with them.
In a material at a certain temperature, each molecule has a different
"temperature". What we measure is an average value. In evaporation, some
of the fastest-moving (i.e. hottest) molecules on the surface manage to
break free of the liquid. Because only the hottest molecules, the ones with
the most energy, actually evaporate, only the cooler molecules are left
behind in the liquid. This is why evaporation is a cooling process.
The molecules of different liquids hold on to each other with different
amounts of force. For liquids that hold tight, a great deal of energy is
required for a molecule to break free. Such a liquid will take a long time
to evaporate. Liquids that do not hold on very tightly will evaporate
quickly. Temperature also affects evaporation rate. A hot material has
more energy per molecule. It is easier for a molecule to get the extra
energy required to evaporate. At lower temperatures, evaporation is more
Evaporation is the process of molecules leaving a condensed liquid state and
going into a gaseous state. The liquid state arises because molecules
attract each other, making the condensed state lower in energy than the
gaseous state. To form the condensed state from the gaseous state, the
molecules need to give off energy in the form of heat. To form the gaseous
state from the condensed state, the reverse must ccur, that is, the system
must absorb heat.
That's why evaporation has a cooling effect. The reason that different
liquids evaporate differently is that the attractions between molecules are
different for different substances.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012