The Science of Water in the
Living World
Polarity and Hydrogen Bonding
Water is a polar molecule.
• Polar Molecule: a molecule that has
a slightly positive side and a slightly
negative end.
– The are more electrons
surrounding the oxygen atom
than the hydrogen atoms.
– Oxygen atom has a slightly
negative charge.
– Hydrogen atoms has a slightly
positive charge.
Polarity and Hydrogen Bonding
• Hydrogen Bonds: A weak bond that forms between the
positive end of one water molecule and the negative end
of another water molecule.
• How is a hydrogen bond different than a covalent bond?
Properties of Water
• Adhesion: a water molecule
forms a hydrogen bond to a
surface or an object.
Polarity and Hydrogen Bonding
Properties of Water
• Cohesion: a water molecule forms a hydrogen bond with
another water molecule.
– Cohesion makes water
have high surface tension –
it is difficult to break the
surface of a body of water.
– Cohesion makes water a
viscous liquid – it is difficult
to move through.
Polarity and Hydrogen Bonding
Capillary Action: waters
ability to flow in narrow
spaces against the force of
gravity because of
adhesion and cohesion.
Examples:
• Water flowing into the
spores of a sponge
• Water flowing into plant
roots and up the plant
stem.
• Groundwater moving up
through soil.
Polarity and Hydrogen Bonding
Water can mix other polar/ionic molecules.
• Polar molecules can be referred to as hydrophilic
(water loving)
• Solute – the substance being dissolved
• Solvent – the substance in which the solute is
dissolved.
• Ex: Salt (solute) dissolves in water (solvent).
Polarity and Hydrogen Bonding
• Salt crystals dissolve in water
– The negative end of the water molecules are attracted
to the positive ions in salt.
– The positive end of the water molecules are attracted
to the negative ions in salt.
– Water molecule surround each ion and keep them
separated.
Chemical Properties of Water
• Water does not mix with nonpolar molecules.
– Non polar molecules can be referred to as
hydrophobic (water fearing).
– Water (polar) and oil (nonpolar) do not mix.
Heat and Water
Water molecules are in constant motion. The higher the
temperature the faster the molecules are moving.
The Three States of Water:
• Gas (vapor): When a water molecule moves fast enough it
breaks free of all hydrogen bonds and becomes a gas.
– Water molecules are spread much further apart than in
liquid water.
– Increasing temperature, increases the rate of evaporation.
Heat and Water
The Three States of Water:
• Liquid: Hydrogen bonds hold most of the water molecules
together in small groups.
– Hydrogen bonds are
constantly being broken and
reformed.
– Cold Water: Is more dense
because molecules move
slower and packed in
tighter.
– Warm Water: Is less dense
because molecules move
faster and spread out.
Heat and Water
The Three States of Water:
• Ice: when water freezes, the molecules move so slowly
that hydrogen bonding locks each molecule in place
forming a crystal.
– In ice, the water molecules
are further apart than in
liquid water.
– Ice is less dense than liquid
water.
– Why is it important that ice
floats in the ocean?
Heat and Water
Heat Capacity: The amount of heat needed to raise the
temperature of a substance.
• Water has the highest heat capacity of any natural
substance.
• Heat energy must
be used to break
hydrogen bonds
between water
molecules before
the molecules are
able to move faster
and increase
temperature.
Heat and Water
• It takes a long time and a lot of energy for water to heat
up or cool down.
• Water’s high heat capacity protects organisms from
drastic changes in temperature.
• A relatively constant
water temperature
helps organisms
maintain
homeostasis: an
organism’s ability to
maintain constant
and stable internal
conditions.
pH of Ocean Water
Acids and The
Bases
Acid, Bases and pH
• Water molecules sometimes split apart to from ions.
• What is an ion?
• Pure water is neutral it has a equal number of
positive and negative ions
Acids and Bases
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Acids
Have more H+ ions
than OH- ions in
solution.
pH less than 7
The lower the
number, the stronger
the acid.
Ex: HCl is a strong
acid produced by the
stomach to digest
food.
Bases
• Produces more OHions than H+ in
solution
• pH greater than 7
• The higher the
number the stronger
the base.
• Ex: NaOH is a basic
(alkaline) solution
used in soap making.
Acids and Bases
pH: (power of Hydrogen) as in Hydrogen ions
• Measures the H+ concentration in solution: [H+]
• It is a log scale, so it increases by powers of ten.
• Ex: pH 2 = [H+] of 0.01M, pH 3 = [H+] of 0.001M
• Ex: A solution with a pH of 2 is ten times more acidic then
a solution with a pH of 3.
Acids and Bases
Buffer: A solution that prevents
sharp and sudden changes in pH.
• Many land animals have buffers
in their blood that maintain a
constant pH.
• Ocean water acts as a buffer and
resists changes in pH.
• If the pH of an organism changes,
it will affect the chemical
reactions inside their cells.
• Buffers help organisms maintain
homeostasis with respect to pH.
Acids and Bases
1. According to the scale, what
substance is neutral?
2. What solution has an [H+] of
0.0001M?
3. Which substance produce the
most OH- ions in solution?
4. How many more times basic is
tomato juice compared to pure
water.
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