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21.1 Water
21.2 Solutions
21.3 Acids, Bases, and pH
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We live on a watery planet.
All life on Earth depends on this
combination of hydrogen and oxygen
atoms.
What are the properties of water that
make it so valuable?
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When two hydrogen
atoms each share one
electron with an
oxygen atom, a neutral
molecule is formed.
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Because negative charges
repel, the electrons pairs
around the oxygen atom
are located where they are
the farthest apart.
This results in a geometric
shape called a tetrahedron.
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A water molecule has a
negative end (pole) and
a positive end.
A molecule (like water)
with a charge separation
is called a polar
molecule.
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Ammonia, NH3, is another
polar molecule. With one
lone pair and three
bonding pairs of
electrons.
This gives the ammonia
molecule a pyramid shape.
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Methane, CH4, is a
nonpolar molecule.
Since there are no lone
pairs of electrons, the
electrons are shared
equally between
atoms.
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A hydrogen bond is a bond between the
hydrogen on one molecule to another atom on
another molecule.
Hydrogen bonds are relatively weak so they
constantly break and re-form.
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Frozen water (or ice)
has an organized
structure that
resembles a
honeycomb because
each water molecule
can form hydrogen
bonds with four
other water
molecules.
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The attraction
between water
molecules, called
Cohesion, helps
water travel from
roots to stems and
leaves.
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Water has a high
specific heat value
because of
hydrogen bonds.
In order for water to
boil, enough energy
must be added to
separate the
hydrogen bonds.
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Water dissolves
sodium chloride
(salt) to form a
solution of sodium
(+) and chlorine (-)
ions. Called
dissosociation.
Dissolving
As water molecules collide with sugar
crystals, attractions develop between the
water molecules and sugar molecules at
the surface of the solid.
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In general, like
dissolves like:
◦ water dissolves polar
substances
◦ non-polar solvents
dissolve non-polar
substances