The Universal
Solvent
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| Water is valued as a solvent in part because so many types of molecules will dissolve in it. |
Why Is Water the Universal
Solvent?
Why water dissolves so many different chemicals
by
Anne Marie Helmenstine, Ph.D.
Water is known as the
universal solvent.
Here is an explanation of why
water is called the universal solvent and what properties make it good at
dissolving other substances.
Chemistry Makes Water
A Great Solvent
Water
is called the universal
solvent because more substances dissolve in water than in
any other chemical.
This has to do with the
polarity of each water molecule.
The hydrogen side of each
water (H2O) molecule carries a slight positive electric charge, while the
oxygen side carries a slight negative electric charge.
This
helps water dissociate ionic compounds into their positive and negative ions.
The
positive part of an ionic compound is attracted to the oxygen side of water
while the negative portion of the compound is attracted to the hydrogen side of
the water.
Why Salt Dissolves in
Water
For
example, consider what happens when salt dissolves in water.
Salt is sodium chloride,
NaCl. The sodium portion of the compounds carries a positive charge, while the
chlorine part carries a negative charge.
The two ions are connected by
an ionic bond. The hydrogen and oxygen in the water, on the other hand, are
connected by covalent bonds.
Hydrogen and oxygen atoms
from different water molecules are also connected via hydrogen bonds.
When salt is mixed with
water, the water molecules orient so that the negative charge oxygen anions
faces the sodium ion, while the positive-charged hydrogen cations face the
chloride ion.
Although
ionic bonds are strong, the net effect of the polarity of all the water
molecules is enough to pull the sodium and chlorine atoms apart.
Once
the salt is pulled apart, its ions become evenly distributed, forming a
homogeneous solution.
If a lot of salt is mixed
with water, it won't all dissolve.
In
this situation, dissolution proceeds until there are too many sodium and
chlorine ions in the mixture for water to win the tug-of-water with undissolved
salt.
Basically,
the ions get in the way and prevent the water molecules from completely
surrounding the sodium chloride compound.
Raising
the temperature increases the kinetic energy of the particles, increasing the
amount of salt that can be dissolved in the water.
Water Doesn't Dissolve
Everything
Despite
its name as the "universal solvent" there are many compounds water
won't dissolve or won't dissolve well.
If the attraction is high
between the opposite charged ions in a compound, then the solubility will be
low. For example, most of the hydroxide exhibit low solubility in water.
Also, non-polar molecules
don't dissolve very well in water, including many organic compounds, such as
fats and waxes.
In summary, water is called
the universal solvent because it dissolves the most substances, not because it
actually dissolves every single compound.
Anne Marie Helmenstine, Ph.D. is an author and consultant with a broad
scientific and medical background.
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