Transcript Properties of Water - Mrs. Sugden's Classes

Water – Chapter 3
Water is a POLAR colvalent
molecule
• Oxygen exerts a greater pull over the shared
electrons and they move a little closer to the
oxygen.
Check this out
• Water has a variety of unusual properties
because of attractions between these polar
molecules.
– The slightly negative regions of one molecule are
attracted to the slightly positive regions of nearby
molecules, forming a hydrogen bond.
– Each water molecule
can form hydrogen
bonds with up to
four neighbors.
HYDROGEN BONDS
• The attraction between
water molecules are
called hydrogen bonds.
• The hydrogen bonds
joining water molecules
are weak, about 1/20th as
strong as covalent bonds.
• They are pretty weak
alone, but their great
numbers give them
strength (like the tiny
teeth in a zipper).
• They form, break, and
reform with great
frequency.
Organisms Depend on Cohesion
Hydrogen bonds hold the substance
together, a phenomenon called cohesion
Water is also Adhesive
• Water will stick to other surfaces.
Cohesion and Adhesion
Contribute to:
Capillary Action
• Capillary Actionwater will move up
xylem against gravity
Cohesion and Adhesion also
contribute to:
• Surface Tension
• Water has a greater
surface tension than
most other liquids
because hydrogen
bonds among surface
water molecules resist
stretching or breaking
the surface.
• Cohesion forms drops, surface tension
keeps them sperical and adhesion sticks
them to the leaf!
The Argyroneta aquatica
Thermal Properties of Water
• Water stabilizes air
temperatures by absorbing
heat from warmer air and
releasing heat to cooler air.
• Water can absorb or release
relatively large amounts of
heat with only a slight
change in its own
temperature due to its
HIGH SPECIFIC HEAT
CAPACITY.
Celsius Scale at Sea Level
100oC
Water boils
37oC
23oC
Human body
temperature
Room temperature
0oC
Water freezes
Specific Heat is the amount of heat that must be
absorbed or lost for one gram of a substance to
change its temperature by 1oC.
Three-fourths of the earth is
covered by water. The water
serves as a large heat sink
responsible for:
1. Prevention of
temperature fluctuations
that are outside the
range suitable for life.
2. Coastal areas having a
mild climate
3. A stable marine
environment
Water is a Coolant
Because it takes a lot of energy for
water to evaporate it is a GREAT
coolant! It removes a lot of heat from a
system!
• It moderates Earth’s
climate
• It stabilizes
temperatures in
aquatic ecosystems
• It prevents
organisms from
overheating
It is also useful for animals who
use water or mud to cool off
during a hot day!
Density of Water
• Most dense at 4oC
• Contracts until 4oC
• Expands from 4oC to
0oC
The density of water:
1. Prevents water from freezing from the bottom up.
2. Ice forms on the surface first. The freezing of the
water releases heat to the water below creating
insulation.
3. Makes transition between season less abrupt.
4. Supports the mass of large and small organisms.
– When water reaches 0oC, water becomes locked into
a crystalline lattice with each molecule bonded to to
the maximum of four partners.
– As ice starts to melt, some of the hydrogen bonds
break and some water molecules can slip closer
together than they can while in the ice state.
– Ice is about 10% less dense than water at 4oC.
Water is a transport medium
• Water dissolves nutrients, gasses and waste
products that can be carried in the circulatory
system of animals, the xylem and phloem of
plants.
The human body is 70% water!
Water is a Solvent for Life
• Water is a good solvent because it is a
POLAR molecule and it will dissolve polar
solutes easily.
• Polar attractions cause water molecules to
surround and isolate the solute molecules.
• Dissolved particles are able to move around
and diffuse making more likely they will
collide leading to a reaction.
• Metabolic reactions take place in water.
Ex) Sodium Chloride in water
All metabolic reactions occur in
solutions
• There are two important
quantitative proprieties of solutions:
1. Concentration
2. pH
Concentration of a Solution
• Molecular weight – sum of the weights of all atoms in
a molecule
• Mole – amount of a substance that has a mass in
grams numerically equivalent to its molecular
weight.
• Avogadro’s number – 6.02 X 1023
– A mole of one substance has the same number of molecules
as a mole of any other substance.
Molarity
The concentration of
a material in solution
is called its molarity
or M.
A one molar (1M)
solution has one mole
of a substance
dissolved in one liter
(L) of solvent,
typically water.
Therefore the units of
molarity of M =
mols/L.
Dissociation of Water Molecules
• Occasionally, a hydrogen atom shared by two
water molecules shifts from one molecule to the
other.
– The hydrogen atom leaves its electron behind and is
transferred as a single proton - a hydrogen ion (H+).
– The water molecule that lost a proton is now a
hydroxide ion (OH-).
– The water
molecule with
the extra proton
is a hydronium
ion (H3O+).
• A simpler way to view this process is
that a water molecule dissociates into
a hydrogen ion and a hydroxide ion:
– H2O <=> H+ + OH• This reaction is reversible.
• At equilibrium the concentration of
water molecules greatly exceeds that
of H+ and OH-.
• In pure water only one water molecule
in every 554 million is dissociated.
• At equilibrium, the concentration of
H+ or OH- is 10-7M (25°C) .
Acids and Bases
• An acid is a substance that
increases the hydrogen ion
concentration in a solution.
• Any substance that reduces the
hydrogen ion concentration in a
solution is a base.
– Some bases reduce H+ directly by
accepting hydrogen ions.
• Strong acids and bases complete
dissociate in water.
• Weak acids and bases dissociate
only partially and reversibly.
pH Scale
• The pH scale in any aqueous solution :
– [ H+ ] [OH-] = 10-14
• Measures the degree of acidity (0 – 14)
• Most biologic fluids are in the pH range
from 6 – 8
• Each pH unit represents a tenfold
difference (scale is logarithmic)
– A small change in pH actually indicates a
substantial change in H+ and OHconcentrations.
Buffers
• A substance that eliminates large sudden
changes in pH.
• Buffers help organisms maintain the pH of
body fluids within the narrow range
necessary for life.
– Are combinations of H+ acceptors and
donors forms in a solution of weak acids
or bases
– Work by accepting H+ from solutions
when they are in excess and by donating
H+ when they have been depleted.
Acid Precipitation
• Rain, snow or fog with more strongly acidic than
pH of 5.6
• West Virginia has recorded 1.5
• East Tennessee reported 4.2 in 2000
– Lowers pH of soil which affects mineral
solubility – decline of forests
– Lower pH of lakes and ponds – In the
Western Adirondack Mountains, there are
lakes with a pH <5 that have no fish.