II. Properties of Water
*Water is the single most abundant
compound in most living things.
A. The Water Molecule
1. Polarity
Polar molecule – a molecule
in which the charges are
unequally distributed.
-Water is a neutral, bent
molecule. Its bent shape
makes the oxygen end
(pole) slightly negative & its
hydrogen end slightly positive.
-Water molecules are polar because
there is an uneven distribution of
electrons between the oxygen &
hydrogen atoms.
Hydrogen Bonding
2. Hydrogen Bonds
Hydrogen bonding – weak
attraction between polar
molecules, ex : water.
**Is the weakest type **
of bonding.
-Being polar, water
molecules are
attracted to each
other (hydrogens to
oxygens).
Cohesion vs. Adhesion
-Water is very cohesive.
Cohesion – an attraction
between molecules of
the same substance, ex :
spiders on water.
Adhesion – an attraction
between molecules of different
substances, ex : the meniscus
of water in a graduated
cylinder.
-Adhesion is responsible for
capillary action (water rising in
a narrow tube against gravity)
in plants.
B. Solutions & Suspensions
-Water is sometimes
part of a mixture – a
material composed of
2 or more elements or
compounds that are
physically mixed
but not chemically
combined, ex :
salt & pepper.
2 Types of mixtures made with water :
1. Solutions – mixture of 2 or
more substances in which the
molecules of the substances
are evenly distributed.
-Solutions involve solutes –
a substance that is
dissolved (salt) & solvents –
substances in which the
solute dissolves (water).
-Water is the universal solvent.
2. Suspensions – mixtures of
water and non-dissolved
materials, ex : blood.
C. Acids, Bases, and pH
pH scale – measurement
system used to indicate the
concentration of hydrogen
ions (H+) in solution; it
ranges from 0 to 14.
-A pH of 7 has an equal
amount of H+ & OHions, making it neutral.
Pure water is neutral.
-pH below 7 is acidic & pH
above 7 is basic or alkaline.
Higher pH = more basic solution.
Acids, Bases, and Buffers
Acid – any compound that forms H+
(hydronium) ions in solution, ex: stomach
acid, lemon juice, tomato juice, acid rain, etc.
-Anything below 7 on the pH scale.
Base – a compound that produces OH
(hydroxide) ions in solution, ex : bleach,
soap, ammonia, sea water, lye, etc.
-Anything above 7 on the pH scale.
Buffers – weak acids or bases that
can react with strong acids or bases to
prevent sharp, sudden changes in pH.
- Are used by body to maintain homeostasis
by keeping chemical reactions pH balanced.