The Nature of Matter
Properties of Water
The Water Molecule
•
How does the structure of water
contribute to its unique properties?
•
Found as a liquid state over most
of Earth’s surface
•
Neutral (10 protons and 10
electrons)
•
Has polarity (oxygen’s 8 protons
attract electrons more strongly
than the single protons of water’s
two hydrogens)
The Water Molecule
•
A molecule in which the charges are
unevenly distributed is said to be
“polar” (a little like a magnet with
two poles)
•
The partial charges on a polar
molecule are written in parentheses
(-) or (+)
•
Because of these opposite charges,
polar molecules attract one another
•
This is a hydrogen bond - attraction
between a hydrogen atom with a
partial positive charge and another
atom with a partial negative charge
•
The most common
elements involved are
oxygen, nitrogen, and
fluorine
Hydrogen Bonds
•
Not as strong as ionic or covalent bonds
•
Because water is polar it is able to form multiple hydrogen
bonds, which account for many of its special properties
which include:
•
Expands slightly when frozen
•
Makes ice less dense than liquid water
•
Can dissolve many substances
Cohesion
•
Cohesion is the attraction between
molecules of the same substance
•
A single water molecule can be
involved in as many as FOUR
hydrogen bonds at the same time
•
Causes water molecules to be
drawn together
•
Drops of water form beads on a
smooth surface because of this
•
Produces surface tension
Adhesion
•
Adhesion is an attraction between
molecules of different substances
•
Graduated cylinder - when you try
to read it the water dips slightly in
the center because of adhesion
•
Adhesion between water and glass
molecules is strong than cohesion
between water molecules
Adhesion
•
Adhesion between water and glass
also causes water to rise in a narrow
tube against the force of gravity
•
This is called capillary action
•
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
Heat Capacity
•
Another result of the many hydrogen bonds of water is that it
takes up a large amount of heat energy to cause those molecules
to move faster, which raises the temperature of the water
•
Water’s heat capacity, the amount of energy required to raise its
temperature, is relatively high
•
This allows large bodies of water (oceans and lakes) to absorb
large amounts of heat with only small changes in temperature
•
This protects the organisms that live in these bodies of water from
drastic changes in temperature
•
Water absorbs the heat produced by cell processes, regulating
the temperature of the cell
Solutions and Suspensions
•
Water is not always pure
•
It is often found as part of a mixture (material composed
of two or more elements or compounds that are
physically mixed together but not chemically combined)
•
Examples:
•
•
Salt and pepper stirred together
•
Gases that make up our atmosphere (oxygen,
nitrogen, carbon dioxide, etc.)
Mixtures that can be made with water are solutions and
suspensions
Solutions
•
If a crystal of salt is placed in a glass of warm water,
sodium and chloride ions on the surface of the crystal are
attracted to the polar water molecules
•
Ions break away from the crystal and are surrounded by
water
•
The ions gradually become dispersed in the water forming
a type of mixture called a solution
•
All components of a solution are evenly distributed
throughout
Solutions
•
In a saltwater solution, the salt is the solute (the substance
that is dissolved) and the water is the solvent (the
substance in which the solute dissolves)
•
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.
•
Greatest solvent on Earth!
•
When it has dissolved all of the solute it can, it is saturated
Suspensions
•
Some materials do NOT dissolve 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 non dissolved material are known as
suspensions
•
Some of the most important biological fluids are both solutions and
suspensions
•
Blood is mostly water
•
the water in blood contains many dissolved compounds
•
blood also contains cells and other undissolved particles that remain
in suspension
Acids, Bases, and pH
•
Why is it important for cells to buffer solutions against
rapid changes in pH?
•
Water molecules sometimes split apart to form ions
•
This reaction can be summarized by a chemical
equation in which double arrows are used to show that
the reaction can occur in either direction
Acids, Bases, and pH
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In pure water, about 1 water molecule in 550 million splits
to form ions in this way
•
The # of hydrogen ions = the # of hydroxide ions, so water
is neutral
Acids, Bases, and pH
•
Chemists devised a measurement system called the pH
scale to indicate the concentration of H+ ions in solution
•
pH scale ranges from 0-14
•
pH 7 is neutral (H+ ions = OH- ions)
•
Pure water has a pH of 7
•
Solutions with a pH below 7 are called acidic because
there are more H+ ions than OH- ions
•
Solutions with a pH above 7 are called basic because
there are more OH- ions than H+ ions
Acids, Bases, and pH
•
Each step on the pH
scale represents a factor
of 10
•
A liter of a solution with a
pH of 4 has 10 times as
many H+ ions as a
solution with a pH of 5
Acids
•
Where do all of the extra
hydrogen ions in a low-pH
solution come from?
•
They come from acids!
•
An acid is any
compound that forms
hydrogen ions (H+) in
solution
•
Strong acids have pH
between 1 and 3
•
Hydrochloric Acid (HCl)
produced by the
stomach helps digest
food (strong acid)
Bases
•
A base is a
compound that
produces hydroxide
ions (OH-) in solution
•
Strong bases such
as lye (NaOH) used
in soapmaking, tend
to have pH values
ranging from 11-14
Buffers
•
The pH of fluids in most cells in our bodies must be kept
between 6.5 and 7.5
•
If the pH is lower or higher, it will affect the chemical
reactions that take place in cells
•
Controlling pH is important for homeostasis (keeping
internal environment relatively stable)
•
One way to control pH is through dissolved compounds
called buffers
•
Buffers are weak acids and bases that can react with strong
acids and bases to prevent sharp, sudden changes in pH
Buffers
•
Blood, for example, has a
normal pH of 7.4
•
Sudden changes in blood pH
are usually prevented by a
number of chemical buffers
(bicarbonate and phosphate
ions)
•
Buffers dissolved in life’s
fluids play an important role in
maintaining homeostasis in
organisms