Answer 1:
The interesting thing about molecules is that they
do not necessarily
have the same properties as the elements from
which they are made.
This principle applies to the state of matter
(gas, liquid, solid) and
also many other properties. The state of matter of
a molecule is
determined by intermolecular interactions -
interactions between the
molecules. If the molecules strongly attract one
another, they can
stick together. If molecules stick together a
little bit, they can
form a liquid and if the stick together a lot,
they will form a solid.
There are several types of intermolecular
interactions that cause
molecules to be attracted to each other.
London Dispersion Forces.
The London dispersion force is a type of Van
Der Waals force. These
interactions are weak and they are proportional to
the size of the
atom or molecule. For example, hydrocarbon chains
are made up of
carbon and hydrogen. Carbon is solid at room
temperature and hydrogen
is a gas. The smallest hydrocarbon, methane
(CH4) is a gas at room
temperature and pressure. So are the three next
largest straight-chain
hydrocarbons, ethane (C2H6),
propane (C3H8), and butane
(C4H10)).
However, the larger hydrocarbons, pentane
(C5H12), hexane
(C6H14), etc
are liquids at room temperature because they are
large enough for the
London dispersion force to be strong enough to
hold them together. For
the forces to be strong enough to make the
molecules solid, the
hydrocarbons must have at least 17 carbons. The
London dispersion
forces between two hydrogen or oxygen molecules
are not strong enough
to cause them to form a liquid. A water molecule
is bigger than a
hydrogen molecule (has a higher molecular weight),
so the London
dispersion force is stronger; however, the
molecular weight of water
is smaller than that of oxygen, so London
dispersion forces do not
tell the whole story.
Dipole interactions and Hydrogen bonding
Dipole interactions and Hydrogen bonding are the
second part of the
explanation. Dipole interactions are
Coulombic attractions between
partially positive and negative charges on molecules.
When molecules
are made up of elements with different
electronegativities*, some of
the atoms have higher electron densities than
others, which causes
some atoms to have partial positive charges and
some to have partial
negative charges. Oxygen is more electronegative
than hydrogen, so in
water the oxygen has a partially negative charge
and the hydrogens
have a partially positive charge. These charges
cause the oxygen of
one water molecule to be attracted to the hydrogen
of another
molecule. This interaction is much stronger than
London dispersion
forces. In fact, the interaction in water is
called hydrogen bonding
which is a particularly strong dipole interaction.
Hydrogen bonding is
strong enough to cause even a small molecule like
water to be a liquid
at room temperature and pressure. Hydrogen bonding
occurs between
molecules with hydrogen bonded to fluorine,
oxygen, or nitrogen and is
strong enough that it is generally described as a
separate category of
intermolecular interactions from dipole
interactions.
Therefore, the simple answer to your questions
is that water has an
extra type of intermolecular force at work,
hydrogen bonding, which
comes from the strong electric dipole on the water
molecule caused by
the electronegativity difference between oxygen
and hydrogen.
If you take chemistry in high school you will
learn about other types
of intermolecular interactions including
ion-dipole forces,
dipole-induced-dipole interactions, and
Coulombic attractions, etc.
*Electronegativity is a measure of the
tendency of an atom to attract
electrons when bonded to another atom. The higher the
electronegativity, the more an atom attracts
electrons in its
direction in a bond. Fluorine (3.98), oyxgen
(3.44), and Nitrogen
(3.04) have the highest electronegativities.
References:
Lide, David R. CRC Handbook of Chemistry and
Physics. Boca Raton: CRC,
2006. Print.
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