“Chemistry”
This article is no longer available online so an archived copy has been provided for your
reference. - SWS
Source:
Clinton Community College
Web Site:
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/che
mistry/chemistr.htm
Accessed:
May 24, 2014
Water
Water covers approximately 71% of Earth's surface. Life evolved in water. Living things are 70-90%
water. In nature, water is a solvent for many kinds of chemical reactions.
Water is a polar molecule because the oxygen atom is much more electronegative than the
hydrogen atoms, resulting in unequal sharing of electrons. As a result, it forms hydrogen bonds with
other polar or charged particles.
Cohesion and Adhesion of Water Molecules
The hydrogen bonds between adjacent water molecules are very weak. As a result, they form and
break rapidly, often lasting only a few trillionths of a second. At any
instant in time, a large proportion of water molecules are bonded to
nearby water molecules, giving water a cohesive property.
Water molecules are also attracted to other polar substances
causing them to adhere to many kinds of materials. The meniscus
shown below forms when water adheres to the sides of the glass
container.
The photograph shows water on the roof of a waxed car. The
water molecules cling to each other but not to the waxed surface
because the wax is nonpolar.
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This water strider is able to remain on the surface of water
because of hydrogen bonding between the molecules. The
insect is light and it's weight is spread over the water so that
there is not much weight at any one point.
Ions and Polar Molecules Dissolve in Water
The partial positive and negative charges on a water molecule produce attractions with ions and
other polar molecules. The attraction between water molecules and ions may be strong enough to
separate the ions, causing the ions to become suspended (dissolved) in the water.
Below: Note that the orientation of the water molecules is depends on the charge of the ion.
The ability of water to flow freely while hydrogen-bonded to other molecules and ions makes it an
excellent transport medium.
In the example above, the salt (NaCl) becomes dissolved in the water, forming a solution. A
solution is composed of a substance dissolved in another substance. The substance dissolved is
the solute and the substance that dissolves the solute is a solvent. In this example, the solvent is
water and the solute is salt. A solution in which water is the solvent is called an aqueous solution.
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Temperature Change of Water
A given amount of heat energy will change the temperature of water less than it will change the
temperature of most other kinds of substances. It takes a relatively large amount of heat will raise
the temperature of water a small amount. This property is due to hydrogen bonding. Normally,
adding heat energy to a substance causes increased motion of the molecules. The hydrogen bonds
between water molecules, however, cause resistance to increased motion; additional heat energy is
needed to break the bonds. Similarly, a large amount of heat is released as water cools.
It takes 1 calorie of heat to raise the temperature of 1 gram of water 1 degree C. To raise the
temperature of an equal volume of air 1 degree C takes 0.0003 calories. Water requires 3000 times
more energy than air.
This property protects organisms from rapid temperature changes. On a larger scale, ocean currents
carry an enormous amount of heat energy and have a major impact on climate.
Freezing and Evaporation of Water
Water has kinetic energy because the molecules are constantly in motion. The molecules of hot
water have greater movement than those of cold water, thus, hot water has more energy. As the
temperature of water decreases, there is less energy for breaking hydrogen bonds. At 0 degrees C,
the hydrogen bonds do not break and ice forms. During the freezing process, as the particles are
prevented from moving by hydrogen bonds, their kinetic energy is released in the form of heat.
When zero degree water
changes to ice, 80
calories/gram are
released from the water.
Thus, in order to convert
1 gram of water at 0
degrees C to ice at 0
degrees C, 80 calories
must be removed.
Similarly, 536 calories/gram must be added to change 100 degree water to water vapor without
changing the temperature. Once vaporized, additional calories will raise the temperature of the
vapor. At 0 degrees C, water requires 597 calories/gram to evaporate. The high heat of vaporization
occurs because hydrogen bonds must be broken before water molecules can escape the liquid and
it takes energy to break the bonds. Even at low temperatures, only the most energetic (hottest)
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molecules are able to leave the liquid and become vapor. The result of removing the most energetic
molecules is that the average amount of energy of those that remain behind is less.
A large amount of energy is also needed to evaporate water as is indicated by the broken line in the
diagram above. The high heat of vaporization enables organisms to use evaporation as a cooling
mechanism because each gram of water that evaporates from the surface of an organism at 25
degrees C removes 580 calories of heat.
Density of Water
Water is most dense at 4 degrees and as it warms, it becomes less dense due to increased
molecular motion associated with temperature increases.
Hydrogen bonds hold water molecules farther apart than they would be without the bonds. The
normal motion of liquid water molecules causes some of the hydrogen bonds between water
molecules to break, enabling them to become packed closer together. As water gets colder than 4
degrees C, there is less movement of the molecules and therefore less breaking of hydrogen bonds.
Increased hydrogen bonding results in a greater average distance between water molecules. The
water becomes less dense because a given volume contains fewer molecules. As ice forms, the
number of hydrogen bonds becomes maximal (4 per water molecule) causing ice to be less dense
than water.
Ice floats on water because it is less dense than water. Other compounds are more dense at lower
temperatures and the solid form does not float on the liquid form.
Ionization of Water
We learned earlier that atoms that gain or lose electrons become ions. Hydrogen is the smallest
atom, composed of one electron and one proton. It can lose its electron to become an ion.
A hydrogen ion, however, is a proton because there are no remaining electrons. The words
"proton" and "hydrogen ion" are often used interchangeably in discussion of biological topics.
Water molecules can ionize. When two water molecules form a hydrogen bond, the proton (H+) of
one water molecule may be removed and transferred to the other molecule forming H3O+ (a
hydronium ion). The molecule that lost the proton becomes OH-.
H2O → H3O+ + OH-
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The ionization of water therefore produces equal numbers of hydrogen ions (H +) and hydroxide ions
(OH-) and the hydrogen ions are attached to nearby water molecules forming hydronium ions (H 3O+).
The process is reversible; the hydrogen ion (H+) and hydroxide ioni (OH-) can combine to form
water.
Acids and Bases
Some molecules form ions when they are dissolved in water. For example, the HCl molecule comes
apart (it dissociates) and produces H+ and Cl-. The electron that was normally with the H remains
with the Cl. The H now has a positive charge because it no longer has an electron. Similarly, the Cl
has a negative charge because it has the electron from the H atom.
HCl → H+ + ClAcids are substances that dissociate to produce hydrogen ions and a negative ion (anion). HCl is
therefore an acid.
Bases are substances that combine with hydrogen ions, thus lowering the concentration of
hydrogen ions. Substances that produce hydroxide ions (OH-) are bases because hydroxide ions are
capable of combining with hydrogen ions to form water, thus lowering the concentration of hydrogen
ions. Bases are therefore proton acceptors.
OH- + H+ → H2O
When NaOH is dissolved in water, an electron from the sodium atom remains with the OH. This
produces a sodium ion (Na+) and a hydroxide ion (OH-). NaOH is therefore a base.
NaOH → Na+ + OHMost bases produce hydroxide ions and a cation when dissolved in water.
Water molecules have a slight tendency to dissociate, forming both H + and OH- as shown below.
Water is neutral when it ionizes because the number of H + equals the number of OH-.
H2O → H+ + OH5
pH
The measure of the strength of an acid or base is called the pH. It is a measure of the concentration
of hydrogen ions (H+).
The pH scale is a logarithmic scale; each decrease of 1 pH unit corresponds to a 10-fold increase in
the concentration in hydrogen ions. The pH can be calculated using the equation below. It is the
negative log (base 10) of the hydrogen ion concentration.
pH = -log10[H+]
pH
[H+]
[OH-]
[H+] X [OH-]
0
1 X 10-0
1 X 10-14
1 X 10-14
1
1 X 10-1
1 X 10-13
1 X 10-14
2
1 X 10-2
1 X 10-12
1 X 10-14
3
1 X 10-3
1 X 10-11
1 X 10-14
4
1 X 10-4
1 X 10-10
1 X 10-14
5
1 X 10-5
1 X 10-9
1 X 10-14
6
1 X 10-6
1 X 10-8
1 X 10-14
acid
7
1 X 10-7
1 X 10-7
1 X 10-14
neutral
8
1 X 10-8
1 X 10-6
1 X 10-14
base
less than 7. A base has a pH greater than 7. A pH of 7
9
1 X 10-9
1 X 10-5
1 X 10-14
is neutral. A substance that has the same number of
10
1 X 10-10
1 X 10-4
1 X 10-14
11
1 X 10-11
1 X 10-3
1 X 10-14
12
1 X 10-12
1 X 10-2
1 X 10-14
Water is therefore neutral. The table above shows that
13
1 X 10-13
1 X 10-1
1 X 10-14
it has the same concentration of hydroxide ions as
14
1 X 10-14
1 X 10-0
1 X 10-14
The brackets above are used to indicate concentration.
It is measured in moles per liter. Water has a hydrogen
-7
ion concentration of 0.0000001 X 10 moles per liter.
The pH of pure water is therefore 7.
The product of hydrogen ion concentration and
hydroxide ion concentration is always 1 X 10-14, so if
the number of one kind of ion is known, the other can
be calculated. For example, a solution with a pH of 4
(or 1 X 10-4 hydrogen ions) will have 1 X 1010
hydroxide ions because 10-4 X 10-10 = 10-14. The
table shows the concentration of hydrogen and
hydroxide ions at different pH levels.
The pH scale ranges from 0 to 14. An acid has a pH
hydrogen ions and hydroxide ions is neutral.
The equation below shows that when water ionizes, it
produces one hydrogen ion and one hydroxide ion.
hydrogen ions.
H2O → H+ + OHThis equation can also be written as shown below because the hydrogen ion becomes attached to
another water molecule producing H3O+.
2H2O → H3O+ + OH6