21.1 Water
21.2 Solutions
21.3 Acids, Bases, and pH
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?
When two hydrogen
atoms each share one
electron with an
oxygen atom, a neutral
molecule is formed.
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.
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.
Ammonia, NH3, is another
polar molecule. With one
lone pair and three
bonding pairs of
electrons.
This gives the ammonia
molecule a pyramid shape.
Methane, CH4, is a
nonpolar molecule.
Since there are no lone
pairs of electrons, the
electrons are shared
equally between
atoms.
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.
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.
The attraction
between water
molecules, called
Cohesion, helps
water travel from
roots to stems and
leaves.
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.
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.
In general, like
dissolves like:
◦ water dissolves polar
substances
◦ non-polar solvents
dissolve non-polar
substances
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