Lesson Overview
Properties of Water
2.2 Properties of Water
WATER, H20
• The “blue planet”
• There is something very
special about water and the
role it plays in living things.
• Imagine our world without it!
Lesson Overview
Properties of Water
You should know:
How does the structure of water contribute to its unique properties?
How does water’s polarity influence its properties as a solvent?
Why is it important for cells to buffer solutions against rapid changes in pH
• Discuss the unique properties of water
• Differentiate between solutions and suspensions
• Explain what acidic solutions and basic solutions are
Lesson Overview
Properties of Water
The Water Molecule
• Water is one of the few compounds found in a liquid state over most of
Earth’s surface.
• Like other molecules, water (H2O) is neutral. The positive charges on its 10
protons balance out the negative charges on its 10 electrons.
1 Oxygen: _______ Protons, _________ Electrons
2 Hydrogen: _______ Protons, _________ Electrons
TOTAL: ________Protons
________ Electrons
water is neutral, no charge
Lesson Overview
Properties of Water
Polarity
O wants 10 e- to be happy , so if
it joins with hydrogen, they can
share!
8 protons
( )
Because of the angles of its
chemical bonds: (covalent bonds)
( )
1 proton
-oxygen atom- on one end of the
molecule
-hydrogen atoms-on the other end
( )
1 proton
Lesson Overview
Properties of Water
Polarity
• 8 protons in O more attractive
to electrons than a single
proton in H…so shared
electrons hang out more often
near the O nucleus.
• As a result, the oxygen end of
the molecule has a slight
negative charge and the
hydrogen end of the molecule
has a slight positive charge.
()
( )
()
Lesson Overview
Properties of Water
Polarity
I am water and
I am polar!
A molecule in which the charges
are unevenly distributed is said to
be “polar,” because the molecule
is a bit like a magnet with two
poles.
( )
( )
The charges on a polar molecule
are written in parentheses, (–) or
(+), to show that they are
weaker than the charges on
ions such as Na+ and Cl–.
( )
Lesson Overview
Properties of Water
Hydrogen Bonding
• Because of their partial positive and
negative charges, polar molecules,
such as water, attract each other.
• The attraction between a hydrogen
atom on one water molecule and the
oxygen atom on another is known as a
hydrogen bond.
(+)
(+)
(-)
(+)
(-)
(-)
(+)
• Every molecule of water connected
to another molecule of water by a
hydrogen bond!
(-)
(+)
(+)
(+)
Lesson Overview
Properties of Water
Hydrogen Bonding
Water is able to form multiple
hydrogen bonds, which
account for many of its
special properties.
Hydrogen bonds are not as
strong as covalent or ionic
bonds.
Can form in other compounds
besides water.
Lesson Overview
Properties of Water
The Water Molecule
How does the structure of water contribute to its unique properties?
Answer:
Because water is a polar molecule, it is able to form multiple
hydrogen bonds with other water molecules, which account for
many of water’s special properties.
Lesson Overview
Properties of Water
Cohesion
Cohesion is an attraction between
molecules of the same substance.
Because a single water molecule
may be involved in as many as
four hydrogen bonds at the same
time, water is extremely cohesive.
Water “sticks” together, until its
hydrogen bonds are broken.
Lesson Overview
Properties of Water
Cohesion
Causes water molecules to be drawn
together, which is why drops of water form
beads on a smooth surface.
Cohesion also produces surface tension,
explaining why some insects and spiders
can walk on a pond’s surface.
surface tension is a characteristic of
water = a “skin-like” layer on top of the
water.
Ex: Belly flop hurts so bad b/c have to break
surface tension
-Spiders can “walk” on water, distribute their
weight
-Paper clip can float on water, even though
more dense
Lesson Overview
Properties of Water
Adhesion
Adhesion is an attraction between
molecules of different substances.
The surface of water in a graduated
cylinder dips slightly in the center,
forming a curve called a meniscus,
because the adhesion between water
molecules and glass molecules is
stronger than the cohesion between
water molecules.
Lesson Overview
Properties of Water
Adhesion
Adhesion between water and
glass also causes water to rise
in a narrow tube against the
force of gravity = capillary
action.
Capillary action is one of the
forces that draws water out of
the roots of a plant and up into
its stems and leaves.
Cohesion holds the column of
water together as it rises.
Lesson Overview
Properties of Water
Heat Capacity
Because of the multiple hydrogen bonds between water molecules, it
takes a large amount of heat energy to cause those molecules to move
faster and raise the temperature of the water.
Water’s heat capacity, the amount of heat energy required to
increase its temperature, is relatively high.
Large bodies of water, such as oceans and lakes, can absorb large
amounts of heat with only small changes in temperature. This protects
organisms living within from drastic changes in temperature.
****At the cellular level, water absorbs the heat produced by cell
processes, regulating the temperature of the cell.
Lesson Overview
Properties of Water
Solutions and Suspensions
Water is not always pure; it is often found as part of a mixture.
A mixture = material composed of two or more elements or compounds
that are physically mixed together but not chemically combined.
Living things are in part composed of mixtures involving water.
Two types of mixtures that can be made with water:
1) solutions
2) suspensions
Lesson Overview
Properties of Water
Solutions
Sodium and chloride ions in salt crystal are attracted to the polar water molecules.
Ions break away from crystal, surrounded by water molecules.
The ions gradually become dispersed in the water, forming a type of mixture called
a solution.
Solution = type of mixture in which ALL the components are evenly distributed
Lesson Overview
Properties of Water
Solutions
All the components of a solution are evenly distributed throughout the
solution.
In a saltwater solution:
Table salt is the solute—
the substance that is dissolved
Water is the solvent—
the substance in which the solute dissolves
does the dissolving
Lesson Overview
Properties of Water
Solutions
• Water’s polarity gives it the ability to dissolve both
ionic compounds and other polar molecules.
• Water easily dissolves salts, sugars, minerals,
gases, and even other solvents such as alcohol.
Lesson Overview
Properties of Water
Suspensions
Some materials do not dissolve when placed in water
--separate into pieces so small that they do not settle out
-- movement of water
Such mixtures of water and nondissolved material are known as
suspensions.
Some of the most important biological fluids are both solutions and
suspensions.
Ex: Blood is mostly water. It contains many dissolved compounds, but
also cells and other undissolved particles that remain in suspension as
the blood moves through the body.
Lesson Overview
Properties of Water
Solutions and Suspensions
How does water’s polarity influence its properties as a solvent?
Water’s polarity gives it the ability to dissolve both ionic compounds and
other polar molecules.
http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve
.html
Lesson Overview
Properties of Water
Acids, Bases, and pH
• Special circumstances when you put something in water and it
causes the solution to:
• Taste sour
• Turn Litmus paper Red
• Litmus paper---a special paper made from lichens.
= AN ACID
• Other times, when you put something in water, it causes the
solution to:
• Taste bitter
• Turn Litmus paper Blue
= A BASE
Lesson Overview
Properties of Water
Another test: The pH Scale
• Another type of treated paper is pH paper.
• Can tell you if a substance is acidic or basic, ac acid or a base.
• pH = Potential Hydrogen
• The more H+ released in a solution, the more acidic the solution.
• The less H+ released in a solution, the more basic the solution
• (H+ means Hydrogen ions, Hydrogen lost its e-)
Lesson Overview
Properties of Water
Another test: The pH Scale
• Chemists devised a measurement system called the pH scale to
indicate how acidic or how basic a solution.
• Essentially, measures the amount of H+ in the solution.
• 1-14 scale
• pH < 7 = ACID
• pH > 7 = BASE
Lesson Overview
Properties of Water
Buffers
The pH of the fluids within most cells in the human body must generally
be kept between 6.5 and 7.5 in order to maintain homeostasis. If the pH
is lower or higher, it will affect the chemical reactions that take place
within the cells.
One of the ways that organisms control pH is through dissolved
compounds called buffers, which are weak acids or bases that can
react with strong acids or bases to prevent sharp, sudden changes in
pH.
Lesson Overview
Properties of Water
Buffers
Adding acid to an unbuffered solution causes the pH of the unbuffered
solution to drop. If the solution contains a buffer, however, adding the
acid will cause only a slight change in pH.
Lesson Overview
Properties of Water
Acids, Bases, and pH
Why is it important for cells to buffer solutions against rapid changes
in pH?
Buffers dissolved in life’s fluids play an important role in maintaining
homeostasis in organisms.