MadSci Network: Chemistry

Re: Why does plain water evaporate faster than saltwater?

Date: Tue Nov 30 20:05:51 1999
Posted By: Dan Patel, Undergraduate, Chemistry Major/Math Minor, University of Houston
Area of science: Chemistry
ID: 943484734.Ch

        There are several approaches we could take to explain why pure 
water evaporates faster than saltwater, and some of them can become rather 
complicated.  It would probably be easiest to explain what exactly is 
happening to the water and salt molecules themselves.
        Salt is what we call a nonvolatile substance.  This means that it 
will not easily evaporate.  Water is a slightly volatile substance, meaning 
that if left standing it will evaporate (go from the liquid to the gas 
        When we have pure water, there is nothing to prevent it from 
evaporating.  That is, on the surface of the water there are only water 
molecules, and we know that evaporation takes place on the surface.
        When we have saltwater, the surface now contains both salt and 
water. Salt does not like to evaporate (because it is nonvolatile), so it 
will stay in the solution.  The water will still evaporate, but not as 
quickly because now salt takes up part of the surface area at the top of 
the solution.  Since the water molecules in salt water don't have as much 
surface area to evaporate from as the water molecules in pure water, the 
water in salt water will take longer to evaporate.
        We can also look at the forces between the salt and water in 
saltwater.  We call these intermolecular forces and they result from the 
attraction of the positive and negative parts of a water molecule to the 
positive and negative ions in salt.  Salt is made up of ions, which are 
just atoms with either a positive or negative charge.  This charge comes 
about when an atom has more or less electrons than it does protons.  Sodium 
and chloride ions make up salt, and when we put salt in water, these ions 
separate from each other (we call this dissociation) and the salt 
dissolves.  The chloride ions, which are negatively charged, are attracted 
to the partial positive charge on the hydrogen atoms in a water molecule 
while the positively charged sodium ions are attracted to the partial 
negative charge on the oxygen atom in a water molecule.
        When the water in saltwater tries to evaporate it has a harder time 
because now it has sodium and chloride ions holding it back.  Remember that 
salt doesn't like to evaporate so it tries to keep the water in the 
solution, too.  In order for the water in saltwater to evaporate it needs 
more energy than pure water, so it will take a longer time to evaporate.
        Pure water, on the other hand, does not have to worry about 
intermolecular forces with ions.  It does have something we call "hydrogen 
bonding," which is basically a weak force between the negative and 
positively charged parts of a water molecule, but hydrogen bonding is not 
as strong as the forces between the water and ions in saltwater.  In 
hydrogen bonding, the hydrogen atoms (which have a partial positive charge) 
of one water molecule are attracted to the oxygen atom (which has a partial 
negative charge) of another water molecule.
        The electrons in the bond between hydrogen and oxygen in water are 
not shared evenly.  They spend more time closer to the oxygen atom, which 
is what gives it the partial negative charge.  To get a good picture of how 
hydrogen bonding works, you can look up the following website:

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