The Chemical Basis for Life
Unique Properties of Water
Lesson Objectives
• Describe the distribution of Earth’s water and outline
the water cycle.
• Identify water’s unique properties that support life on
Earth.
• Identify the chemical structure of water, and explain
how it relates to water’s unique properties.
• Define solution, and describe water’s role as a solvent.
• State how water is used to define acids and
bases, and identify the pH ranges of acids and
bases.
• Explain why water is essential for life processes.
Introduction
• Water is biologically important…
• H2O is a simple molecule, but it has some
very unique properties…
WATER, WATER EVERYWHERE
• Common chemical substance on Earth
• ‘Water’ refers to it in a liquid state
• Also can ba a solid (ice) or gas (water
vapor)
Where Is All the Water?
• Water covers about 75% of Earth’s
surface
– 97% of this water is salt water
– Only 3% is freshwater
• Most of the freshwater is frozen in glaciers and the
polar ice caps
• About 2% of water is stored in
underground spaces
How Water Recycles
• Water is continuously recycled
• Cycle keeps repeating
CHEMICAL STRUCTURE AND
PROPERTIES OF WATER
• Water is tasteless, odorless, and transparent
• The transparency of water is important for
organisms that live in water.
– sunlight can pass through it
– sunlight is needed by water plants and other water
organisms for photosynthesis
Chemical Structure of Water
• Chemical formula H2O
• Nucleus of the oxygen atom (8 positively charged
protons) attracts electrons much more strongly than
do the hydrogen nuclei (1 positively charged proton).
This results in a negative electrical charge near the
oxygen atom (due to the ”pull” of the negatively
charged electrons toward the oxygen nucleus) and a
positive electrical charge near the hydrogen atoms.
• A difference in electrical charge between different
parts of a molecule is called polarity.
SOLUTIONS
• Homogeneous mixture composed of two or
more substances
• One substance is dissolved (solute) in another
substance (solvent), forming a mixture that has
the same proportion of substances throughout
• The ability of a solute to dissolve in a particular
solvent is called solubility.
• Many chemical substances are soluble in water;
thus water is known as the universal solvent.
Sticky, Wet Water
• Water has unusual properties due to its hydrogen
bonds; water molecules tend to stick together.
• If you drop a tiny amount of water onto a very smooth
surface, the water molecules will stick together and
form a droplet, rather than spread out over the
surface; this is called cohesion.
• Water also sticks to other substances because of its
unique bonding properties; this is called adhesion.
Cohesion
• Cohesion is the attraction of one molecule
to a similar molecule. Water molecules
form droplets because of this principle.
Adhesion
• When a molecule is attracted to a different
molecule, it is called adhesion. Water
sticks to other substances.
Surface Tension
• Water's surface, molecules pulled from side to
side and down only
– Result: skin of water at the surface in which the
molecules are held tightly together.
• Surface tension: measurement of the amount
of force required to break this skin on the
surface of water.
http://www.youtube.com/watch?v=45yabrnryXk
Capillarity
• Check out the demonstration of capillarity!
Use cohesion and
adhesion to explain
what is happening
& WHY!
Quick Review of Info…
• Water– cohesion, adhesion, polarity,
solvency
• https://www.youtube.com/watch?v=DAilC0
sjvy0
• Surface Tension & Capillary Action
• https://www.youtube.com/watch?v=wOOY
1szbcX4
Specific Heat
• Water has a high specific heat; takes a lot of
energy to raise or lower its temperature
– Specific heat is a measure in joules of how much
energy it takes to raise the temperature of 1 gram
of a substance by 1°C.
• Every substance has its own specific heat
capacity, the specific heat capacity of water is
much higher than that of other common liquid
substances.
Density of Ice and Water
• Water in a solid state has a lower density than water in a liquid state;
water expands when it freezes.
– Hydrogen bonds cause water molecules to line up less efficiently in ice
than in liquid water. As a result, water molecules are spaced farther
apart in ice, giving ice a lower density than liquid water.
– A substance with lower density floats on a substance with higher
density. This explains why ice floats on liquid water, whereas many
other solids sink to the bottom of liquid water.
• In a large body of water, such as a lake or the ocean, the water with
the greatest density always sinks to the bottom. Water is most
dense at about 4° C (39.2°F).
– In climates with cold winters, this layer of 4° C water insulates the
bottom of a lake from freezing temperatures.
– Lake organisms such as fish can survive the winter by staying in this
cold, but unfrozen, water at the bottom of the lake.
WATER AND LIFE
• Humans are composed of about 70% water
• Water’s ability to dissolve most biologically
significant compounds—from inorganic salts to
large organic molecules—makes it a vital solvent
inside organisms and cells.
• As a result, virtually all life processes depend on
water.
• Without water, life as we know it could not exist
WATER ACTS LIKE A BUFFER
• Defined: substance that helps to moderate any
changes in pH that result from the addition of
acids or bases.
• Importance: most chemical processes that
occur in living organisms are highly sensitive to
pH, and drastic changes in pH can cause some
serious trouble.
• Water, can act like a buffer if there is a sudden
change in pH.
• HOW you ask????
• Water disassociates into hydrogen (H+)and
hydroxide (OH-) ions constantly.
– dissociated water molecules are what give water its
buffering ability.
• If we add an acid to solution, some of the free
OH- ions will bind to the newly added H+ ions,
which will moderate the decrease in pH.
• If we add a whole bunch of base to the solution,
some of the added base will bind to the free H+
ions in solution, which will moderate the
increase in pH.
• HOWEVER, WATER CANNOT BUFFER
EXTREMELY DRASTIC CHANGES IN pH
• Think of it as a mathematical equation with
positive and negative integers:
• For example:
-13 + 2 = -11
So if you take a base with a pH of 13 (filled with
OH- ions) and increase the amount of positive H+
ions in the solution the base’s pH will drop down
to 11 making it less alkaline
Acids and Bases
• Pure water is the solvent in solutions
called acids and bases.
• In pure water (such as distilled water), a
tiny fraction of water molecules naturally
breaks down, or dissociates, to form ions.
– An ion is an electrically charged atom or
molecule. The dissociation of pure water into
ions is represented by the chemical equation:
2 H2O → H3O+ + OH-
Acids, Bases and pH
• The hydronium ions in pure water are balanced by
hydroxide ions, so pure water is neutral (neither an
acid nor a base). This gives pure water a pH of 7.
• If a solution has a higher concentration of hydronium
ions and lower pH than pure water, it is called an acid.
• If a solution has a lower concentration of hydronium
ions and higher pH than pure water, it is called a
base.
The pH Scale
• Ranges from 0 to 14
• The pH scale is a negative logarithmic
scale; as the ion concentration
increases, the pH value decreases. In
other words, the more acidic the
solution, the lower the pH value.
• Each one-point change in pH reflects
a ten-fold change in the hydronium ion
concentration and acidity.
– For example, a solution with a pH of 6
is ten times as acidic as pure water
with a pH of 7.
• Acids have a pH less than 7, and
bases have a pH greater than 7.
Acids
• Defined as a hydrogen ion donor
• Acids have a sour taste and may sting or
burn the skin.
• Testing solutions with litmus paper is an
easy way to identify acids. Acids turn blue
litmus paper red.
Bases
• Defined as a hydrogen ion acceptor
• Bases have a bitter taste and feel slimy to
the touch. They can also burn the skin.
• Bases, like acids, can be identified with
litmus paper. Bases turn red litmus paper
blue.
Quick Review of How Acids and
Bases Form
• https://www.youtube.com/watch?v=6zGH
CyFhZk0
Acids and Bases in Organisms
• Enzymes do their job within a specific range of pH and are
needed to speed up biochemical reactions.
– For example, the enzyme pepsin, which helps break down proteins
in the human stomach, requires a very acidic environment in order
to function.
– Strong acid is secreted into the stomach, allowing pepsin to work.
• Once the contents of the stomach enter the small
intestine, where most digestion occurs, the acid must be
neutralized. Enzymes that work in the small intestine
need a basic environment.
– An organ near the small intestine, called the pancreas, secretes
bicarbonate ions (HCO3-) into the small intestine to neutralize the
stomach acid.
Neutralization
• What do you think would happen if you mixed an acid
and a base?
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• ‘‘Cancel
each other out”, causing a neutralization
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reaction
– For example, when the base sodium hydroxide (NaOH) and
hydrochloric acid (HCl) react, they form a neutral solution of
water and the salt sodium chloride (NaCl). This reaction is
represented by the chemical equation:
NaOH + HCl → NaCl + H2O.
– In this reaction, hydroxide ions (OH-) from the base combine
with hydrogen ions (H+) from the acid to form water. The
other ions in the solution (Na+) and (Cl-) combine to form
sodium chloride.
Lesson Summary
• Most of Earth’s water is salt water located on the planet’s
surface. Water is constantly recycled through the water cycle.
• Water molecules are polar, so they form hydrogen bonds. This
gives water unique properties, such as a relatively high boiling
point.
• A solution is a homogeneous mixture in which a solute
dissolves in a solvent. Water is a very common solvent,
especially in organisms.
• The ion concentration of neutral, pure water gives water a pH of
7 and sets the standard for defining acids and bases. Acids
have a pH lower than 7, and bases have a pH higher than 7.
• Water is essential for most life processes, including
photosynthesis, cellular respiration, and other important
chemical reactions that occur in organisms.