Water
The Single most abundant compound
in most living things
Properties of Water
• Water:
– Expands upon freezing
– Has adhesion properties (capillary action)
– Has cohesion properties due to surface tension
– Has a high specific heat
– Has a high heat of vaporization
– Has a high heat of fusion
– Is the greatest solvent on Earth
Expansion upon Freezing
• Water is a liquid across most of the earth
• Water expands as it freezes (ice is less dense
than liquid water = ice floats on water)
• Fish and aquatic life can survive under ice
Polarity
• Oxygen has 8 protons in its nucleus. Hydrogen
only has 1 proton in its nucleus. Oxygen has
much stronger attraction for electrons than
hydrogen
• Greater probability of finding the shared
electrons near the oxygen atom than the
hydrogen atom (covalent bonding)
Polarity
Thus, the oxygen end of the molecule has a partial negative
charge while the hydrogen end of the molecule has a partial
positive charge.
Hydrogen Bonds
• Because of the partial
charges, polar molecules
such as water can attract
each other
• Hydrogen bonds form
between hydrogen atoms
of one water molecule
and oxygen atoms of
another water molecule
Cohesion / Surface Tension
• Cohesion is an attraction
between molecules of
the same substance.
• Water is extremely
cohesive due to H
bonding.
Adhesion / Capillary Action
• Adhesion is an attraction
between molecules of
different substances
• Adhesion is why water is
drawn out of the roots of
plants and up into its
stems and leaves
Meniscus of water occurs because the adhesion between water
molecules and glass molecules is stronger than the cohesion
between water molecules – called capillary action
High Specific Heat
• Specific heat means the ability of
a substance to raise 1 gram of
body mass by 1°C.
• Water has high specific heat
because a lot of energy is
required to break H bonds
• Important because water buffers
organisms' internal temperatures
High Heat of Vaporization
• Water evaporates only at
a high heat.
• It resists evaporating
because H bonds must be
broken to change states
from liquid to gas
• Allows you to cool from
sweating
High Heat of Fusion
• Amount of energy
needed to change
state from solid to
liquid
• Also due to breaking
H bonds
• Resists melting. Keeps
environment cool
Solutions
• A mixture is composed of
two or more elements or
compounds that are
physically mixed but not
chemically combined (salt
and pepper)
• Two types of mixtures can
be made with water:
• Solutions
• Suspensions
Water as a Solvent
• All of the components of a solution are evenly
distributed throughout.
• In a salt-water solution, salt is the solute – the
substance that is dissolved
• Water is a solvent – the substance in which
the solute dissolves.
• Water’s polarity gives it the ability to dissolve
ionic compounds and polar molecules.
• Water is the greatest solvent on earth.
Solutions and Suspensions
– When a crystal of table salt is placed in warm
water, sodium and chloride ions are attracted to
the polar water molecules.
Cl -
Cl Na+
Na+
Water
Water
Solutions and Suspensions
– Ions break away from the crystal and are
surrounded by water molecules.
Cl -
Solutions and Suspensions
– The ions gradually become dispersed in the water,
forming a solution.
Solutions and Suspensions
• Suspensions are mixtures
of water and
nondissolved materials.
• Blood that circulates
through your body is
composed of suspended
cells in a liquid plasma
matrix
Acids, Bases, and pH
• A water molecule can react to form hydrogen
and hydroxide ions.
• Water is neutral because the number of
positive hydrogen ions (H+) produced is equal
to the number of negative hydroxide ions
(OH-) produced.
• The pH scale indicates the concentration of H+
ions in solution. The pH scale ranges from 0 to
14.
The pH Scale
• At a pH of 7, the
concentration of H+
ions and OH- ions is
equal.
• Pure water has a
pH of 7.
Sea water
Human blood
Pure water
Normal rainfall
Milk
The pH Scale
Solutions with a pH
below 7 are called
acidic because they
have more H+ ions
than OH- ions.
The lower the pH, the
greater the acidity.
Acid rain
Tomato juice
Lemon juice
Stomach acids
The pH Scale
Solutions with a pH
above 7 are called
basic because they
have more OH- ions
than H+ ions.
The higher the pH, the
more basic the
solution.
Oven cleaner
Bleach
Ammonia solution
Soap
Acids
• An acid is any compound which
forms H+ ions in solution.
• Acidic pH is below 7, therefore
has more H+ ions that pure
water
• Examples include: acetic acid
(vinegar), gastric juice (stomach
acid), sulfuric acid (in car
batteries)
Bases
• A base is a compound that produces OH- ions
in solution.
• Basic, or alkaline, solutions contain fewer H+
ions that pure water, and have a pH above 7
• Basic examples include: baking soda (sodium
bicarbonate), ammonia (NH3), lye (NaOH),
milk of magnesia
Buffers
• The pH of fluids within most cells of the
human body is kept between 6.5 and 7.5
• If pH is lower or higher if will affect chemical
reactions. Therefore, pH balance is an
important part of homeostasis.
• Buffers are weak acids of bases that react with
strong acids or bases to prevent sudden sharp
changes in pH.