Water as a Solvent
Why is water a good solvent?
You are right, water is an excellent solvent. Although not all things
dissolve in water, it is often referred to as the "Universal Solvent". The
geometry or the structure of the water molecule is what makes is so good. In
compounds, the number of positive protons is equal to the number of negative
electrons, so it would appear that all compounds are electrically neutral.
But that is not the case. Water's geometry is such that the electrons are not
uniformly distributed throughout the molecule. So the end of the molecule
with greater electron density is slightly negative and the other slightly
positive. Water and compounds like it are said to be polar, and kind of
behave like a magnet. Water generally dissolves other substances that are
also polar, but not non-polar substances like oil. (To continue the magnet
analogy, magnet are not attracted to nonferrous metals like aluminum.) Water
is quite polar which explains many of its properties like its high melting
and boiling points, high surface tension, and why it expands when frozen. To
see for yourself, get a thin stream of water flowing from a faucet. Using a
piece of wool or silk, build up a static charge on a plastic comb by rubbing
the fabric briskly across the comb. Bring the comb near the stream of water.
The charged comb will actually deflect the stream of water!
Many non-polar substances are made soluble in water by use of detergents
and soaps, both of which are large molecule with a polar end and a non-polar
end. The polar end is attracted to the polar water molecule, and the
non-polar part to the non-polar greasy dirt.
Hope this helps!
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Update: June 2012