Outline 2-2: Properties of Water
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2-2 Properties of Water
The Water Molecule
The Water Molecule
Like all molecules, a water molecule is neutral.
Polarity
Oxygen atoms, with 8 protons, has a much
stronger attraction for electrons than does the
hydrogen atom with one proton.
Thus, the electrons are more likely to be near
oxygen than hydrogen.
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2-2 Properties of Water
The Water Molecule
As a result, the oxygen end of the molecule has a
slight negative charge and the hydrogen end has a
slight positive charge.
A water molecule is polar because there
is an uneven distribution of electrons
between the oxygen and hydrogen
atoms.
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2-2 Properties of Water
The Water Molecule
Water Molecule
+
+
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2-2 Properties of Water
The Water Molecule
Hydrogen Bonds
Because of their partial positive and negative
charges, polar molecules attract each other.
Attractions between the H atoms on one molecule
and the O atom on another water molecule is an
example of a hydrogen bond.
Hydrogen bonds are not as strong as covalent or
ionic bonds.
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2-2 Properties of Water
The Water Molecule
A single water molecule may be involved in as
many as four hydrogen bonds at a time.
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2-2 Properties of Water
The Water Molecule
Some special properties of water that are due to
its ability to form hydrogen bonds:
1. Cohesion
This is an attraction between molecules of the
same substance.
a. Cohesion explains why drops of water form
beads on a smooth surface.
b. Cohesion also explains why some insects and
spiders can walk on water.
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2-2 Properties of Water
The Water Molecule
2. Adhesion
This is an attraction between molecules of
different substances.
a. Adhesion explains why the water in the center
of a cylinder dips lower than the edges.
b. It also is the reason water will rise in a narrow
tube. This effect is called capillary action.
c. Adhesion and cohesion together explain why
water goes up in roots & stems of plants.
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2-2 Properties of Water
Solutions and Suspensions
Solutions and Suspensions
A mixture is a material composed of two or more
elements or compounds that are physically mixed
but not chemically combined.
Two types of mixtures:
1. Solutions
2. Suspensions
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2-2 Properties of Water
Solutions and Suspensions
Solutions
All the components of a solution are evenly
distributed throughout the solution.
In a salt–water solution, table salt is the solute—the
substance that is dissolved.
Water is the solvent—the substance in which the
solute dissolves.
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2-2 Properties of Water
Solutions and Suspensions
When salt is placed in water it separates into
sodium and chloride ions.
1. The sodium and chloride ions are attracted to
the polar water molecules.
2. The ions get surrounded by water molecules
thus creating a solution
Cl
-
Cl Na+
Na+
Water
Water
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2-2 Properties of Water
Solutions and Suspensions
Water's polarity gives it the ability to dissolve both
ionic compounds and other polar molecules, such as
sugar. Thus, water is the greatest solvent on Earth.
However, some compounds will NOT dissolve
in water. These are called nonpolar compounds.
If you try to dissolve oil in water, it will separate
out because:
1. Water molecules are attracted to each other
2. Nonpolar oil molecules will be shoved apart
as water molecules come together.
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2-2 Properties of Water
Solutions and Suspensions
Suspensions
Some materials do not dissolve when placed in water
but separate into pieces so small that they do not
settle out.
The movement of water molecules keeps the small
particles suspended.
Such mixtures of water and nondissolved material
are known as suspensions.
Examples: jello, gravy
Blood is both a solution of dissolved solutes like
sugar and a suspension of cells & other substances
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2-2 Properties of Water
Acids, Bases, and pH
Acids, Bases, and pH
A water molecule can break apart to form hydrogen
(H+) and hydroxide ions (OH-) . This is called
dissociation.
Water is neutral because the number of H+ ions
produced is equal to the number of OH- produced.
An acid is any compound that forms H+ ions in
solution. Acids turn pH paper red.
A base is a compound that produces OH- ions in
solution. Bases turn pH paper blue.
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2-2 Properties of Water
Acids, Bases, and pH
The pH scale
Chemists devised a measurement system called
the pH scale to indicate the concentration of H+
ions in solution.
The pH scale ranges from 0 to 14.
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2-2 Properties of Water
Acids, Bases, and pH
The pH Scale
At a pH of 7, the
concentration of H+
ions and OH- ions is
equal.
Sea water
Pure water has a pH
of 7.
Human blood
Pure water
Milk
Normal rainfall
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2-2 Properties of Water
Solutions with a pH
below 7 are called acidic
because they have more
H+ ions than OH- ions.
Acids, Bases, and pH
The pH Scale
The lower the pH, the
greater the acidity.
Strong acids have a pH
between 1 and 3.
Hydrochloric acid in
stomach is a strong acid.
Acid rain
Tomato juice
Lemon juice
Stomach acids
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2-2 Properties of Water
Acids, Bases, and pH
The pH Scale
Solutions with a pH
above 7 are called basic
because they have more
OH- ions than H+ ions.
Oven cleaner
Bleach
Ammonia solution
Soap
The higher the pH, the
more basic the solution.
Strong bases, such as
oven cleaner, tend to
have pH values ranging
from 11 to 14.
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2-2 Properties of Water
Acids, Bases, and pH
Buffers
The pH of the fluids within most cells in the human
body must generally be kept between 6.5 and 7.5.
If the pH is lower or higher, it will affect the
chemical reactions that take place within the cells.
One of the ways that the body controls pH is
through dissolved compounds called buffers.
Buffers are weak acids or bases that can react
with strong acids or bases to prevent sharp,
sudden changes in pH.
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2-2
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2-2
A molecule in which the charges are unevenly
distributed is called a
a. polar molecule.
b. cohesive molecule.
c. hydrogen molecule.
d. covalent molecule.
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2-2
A dissolved substance is called a
a. solvent.
b. solution.
c. solute.
d. Suspension.
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2-2
A compound that produces hydroxide ions in
solution is called a(an)
a. base.
b. buffer.
c. acid.
d. salt.
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2-2
Hydrogen bonds between water molecules
result from
a. adhesion between water molecules.
b. magnetic attractions between water
molecules.
c. uneven electron distribution in each water
molecule.
d. ionic bonds in the water molecule.
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2-2
On a pH scale, a value of 2 means that the
solution has
a. equal concentrations of H+ and OH- ions.
b. the same concentration of H+ ions as pure
water.
c. higher concentration of H+ than in pure
water.
d. lower concentration of H+ than in pure water.
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