Water has unusual properties:
the key is that the electrons of
each covalent bond are not shared
equally between oxygen and
hydrogen atoms.
Oxygen pulls electrons much more
strongly than does hydrogen.
This sharing of electrons causes the
oxygen end of the molecule to have
a slight negative charge,
while the end with the two hydrogen
atoms is slightly positive.
A molecule in which opposite ends
have opposite electric charges is
called a polar molecule.
Water is a compound consisting of
polar molecules.
Water molecules are attracted to
one another in a specific way.
The slightly negative oxygen end
of one molecule attracts the
slightly positive hydrogen ends
of adjacent water molecules,
causing the molecules to
become arranged.
A type of weak attraction
between the hydrogen atom
of one molecule and a
slightly negative atom within
another molecule is a type
of chemical bond called a
hydrogen bond.
Cohesion and Adhesion
►Each
hydrogen bond between
molecules of liquid water lasts for a
short time.
►Most
of the molecules are involved in
hydrogen bonding with other
molecules because new hydrogen
bonds form as fast as old ones break.
•The tendency of molecules of the same
kind to stick to one another is called
cohesion.
•Cohesion is much stronger for water
than for most other liquids.
•Water molecules are also attracted to
certain other molecules. The type of
attraction that occurs between unlike
molecules is called adhesion.
Water's Ability to Dissolve Other
Substances
►When
you stir table salt into a glass of
water, you are forming a solution, a
uniform mixture of two or more
substances.
►The substance that dissolves the other
substance and is present in the greater
amount is the solvent (in this case,
water).
Acids and Bases
► Sometimes
atoms give their electrons up
altogether instead of sharing them in a
chemical bond. This process is known as
disassociation.
► Water, for instance, dissociates by the
following formula:
►H2O
H+ + OH–
► The hydrogen atom gives up a negatively
charged electron, gaining a positive charge,
and the OH compound gains a negatively
charged electron, taking on a negative
charge.
► The H+ is known as a hydrogen ion and
OH– ion is known as a hydroxide ion.
► The
disassociation of water produces
equal amounts of hydrogen and hydroxide
ions.
► However, the disassociation of some
compounds produces solutions with high
proportions of either hydrogen or
hydroxide ions.
► Solutions high in hydrogen ions are known
as acids, while solutions high in hydroxide
ions are known as bases.
► Both types of solutions are extremely
reactive—likely to form bonds—because
they contain so many charged particles.
►The
technical definition of an acid
is that it is a hydrogen ion donor,
or a proton donor, as hydrogen
ions are consist of only a single
proton.
►Acids put H+ ions into solution.
► The
definition of a base is a little more
complicated: they are H+ ion or proton
acceptors, which means that they remove
H+ ions from solution.
► Some bases can directly produce OH– ions
that will take H+ out of solution. NaOH is an
example of this type of base:
►NaOH
Na+ + OH–
►A
second type of base can directly take H+
out of an H2O solution. Ammonia (NH3) is a
common example of this sort of base:
► NH3 + H2O
NH4+ + OH–
The pH Scale
► The
pH scale, which ranges from 0 to 14,
measures the degree to which a solution is
acidic or basic. If the proportion of
hydrogen ions in a solution is the same as
the proportion of hydroxide ions or
equivalent, the solution has a pH of 7,
which is neutral.
The pH Scale
►The
most acidic solutions (those
with a high proportion of H+) have
pHs approaching 0,
► while the most basic solutions
(those with a high proportion of
OH– or equivalent) have pHs closer
to 14.
► Water
has a pH of 7 because it has equal
proportions of H+ and OH– ions.
► Some acids are more acidic than others
because they put more H+ ions into solution.
Stomach fluid, for example, is more acidic
than saliva.
► When
sodium hydroxide (NaOH)
disassociates, it forms only hydroxide ions,
making it a base and giving it a pH above 7.
► Like acids, bases can be strong or weak
depending on how many hydroxide ions
they put in solution or how many hydrogen
ions they take out of solution.
Temperature Moderation
►Because
of hydrogen bonding,
water has a better ability to
resist temperature change than
most other substances.
Thermal energy
►Thermal
energy is the total
amount of energy associated
with the random movement of
atoms and molecules in matter.
Temperature
►Temperature
is a measure of the
average energy of random motion
of the particles in a substance.
When two substances differ in
temperature, thermal energy in
the form of heat is transferred
from the warmer substance to the
cooler one
When you heat a substance—such
as a metal pan or water—its
temperature rises because its
molecules move faster.
But in water, some of the thermal
energy that is absorbed goes to
break hydrogen bonds.
As a result, the water absorbs the
same amount of thermal energy
but undergoes less temperature
change.
This releases thermal energy in
the form of heat, so there is less of
a drop in temperature than in
metal.
Low Density of Ice
► Density
is the amount of matter in a given
volume.
► A high-density substance is more tightly
"packed" than a low-density substance.
► In most substances, the solid state is more
dense than the liquid state. Water is just the
opposite—its solid form (ice) is less dense
than the cold liquid form.
•The substance that is dissolved
and is present in a lesser amount is
the solute.
•When water is the solvent, the
result is called an aqueous solution
Acids, Bases, and pH
►In
aqueous solutions, a very small
percentage of the water molecules
themselves break apart into ions.
►The ions formed are positively charged
hydrogen ions (H+) and negatively
charged hydroxide ions (OH-).
►A hydroxide ion is a combination of an
oxygen atom and a hydrogen atom
that carries a 1- charge..
For the chemical processes of
life to work correctly, the right
balance of H+ ions and OHions is critical
Some chemical compounds
contribute additional H+ ions to
an aqueous solution while others
remove H+ ions from it.
A compound that donates H+
ions to a solution is called an
acid.
A compound that removes H+
ions from an aqueous solution is
called a base.
The pH Scale
►The
pH scale describes how
acidic or basic a solution is.
►The scale ranges from 0 (most
acidic) to 14 (most basic).
►Each pH unit represents a
tenfold change in the
concentration of H+ ions.
Buffers
►Because
the molecules in cells are very
sensitive to concentrations of H+ and
OH- ions, even a slight change in pH
can be harmful to organisms.
►Many biological fluids contain buffers,
substances that cause a solution to
resist changes in pH.