Water
Living organisms
depend on the unique
properties of water.
H2O Molecule
• 2 atoms of hydrogen: Atomic # = 1
• How many protons and electrons?
• 1 atom of oxygen: Atomic # = 8
• How many protons and electrons?
Electron Configuration
of Water
Space-Fill Model of Water
Water Molecule
Polarity
• Oxygen is a bigger atom with 8
protons and pulls more forcefully
on the electrons than hydrogen
does.
• This creates positive (+) and
negative (-) poles on the molecule.
Polar Covalent Bonds
• Bonds that form when electrons
are shared unevenly.
• Areas of the molecule are slightly
charged, although water
molecules remain neutral.
Hydrogen Bonds
• Hydrogen (+) of
one molecule is
attracted to a
nearby atom (-)
of a different
molecule of water
or other
substance
Hydrogen Bonds are Weak
• Electrons are not
shared, gained, or
lost
• Temporary bonds
form
• Bonds are easily
broken
Hydrogen Bonds form in
Water and DNA
Cohesion
Adhesion
Surface Tension
Special Properties of
Water
• Cohesion: Attraction of same
molecules to each other
• Adhesion: Attraction of different
molecules to each other
• Surface Tension: The force needed to
break the surface of a liquid due to
cohesion
Polar and Non-Polar
Molecules
Polar molecules repel
non-polar molecules
• Water is
polar
• Oil is nonpolar
The Cell Membrane has Polar
and Non-Polar Regions
Capillary Action:
How Water Moves up Plants
• Plant tubes (xylem)
are made of
cellulose which is
polar.
• Water molecules
are attracted to the
sides of thin tubes
of xylem.
Soap and Water
• The surface of water has high tension
due to the strong force of cohesion.
• Soap molecules have two ends. One
end is attracted to water and the other
end repels water.
• Soap separates the water molecules
and breaks the surface tension.
What Happened to the
Pepper?
• Pepper floats on the water.
• Soap break the surface tension of water
by getting “between” water molecules.
• Pepper scatters as water’s hydrogen
bonds are broken near the soap.
How many drops of water can
you place on a penny?
 http://programs.northlandcollege.edu/bi
ology/Biology1111/animations/hydroge
nbonds.html