Transcript Properties of Water

Properties of Water
What is water?
Water is H2O
One molecule of
oxygen and two
molecules of
hydrogen.
But notice that the two hydrogen
atoms are not on opposite sides of the
oxygen. They are bunched together on
the same side.
This means that one
end of the molecule
is different than the
other end.
The oxygen end of the molecule has a
negative charge and.......................
the hydrogen end
has a positive
charge.
This makes water
a POLAR molecule.
One pole is positive
and the other is
negative.
Because opposites attract, the positive end of one
water molecule is attracted to the negative end of
any nearby water molecule.
That’s why water
droplets stick to
one another.
Water is cohesive. It sticks to itself.
Water sticks to itself forming
droplets!
Other molecules may also be polar
so water molecules stick to them....
Some molecules are non-polar.
Non-polar molecules won’t
mix with water.
That’s why oil and water
will separate even if you
try to mix them together.
Water does a very strange thing.
It expands when it freezes at 0° C.
Because it expands it gets less dense.
Because it gets less dense, ice floats!
Because ice floats there are ice burgs!
Chasing Ice
What would happen to lakes in the
winter if water didn’t expend when it
freezes? How would life on Earth be
different?
In winter the air gets cold.
The water at the surface, in
contact with this air freezes.
Because the ice is less dense
than the water below it floats
on the surface.
Since ice is a good insulator it
protects the lake water below
from the cold air.
The lake never freezes solid. Life in the lake survives the winter.
If water didn’t expand when it froze, ice would
be more dense and would sink.
More water at the
surface would freeze
and sink until......
The lake would freeze solid
And everything in
the lake would die.
Water has been called the
UNIVERSAL SOLVENT.
What is a solvent?
A solvent is a material, usually a liquid, in which
other materials dissolve.
If you want to clean a
dirty paint brush you
buy a paint solvent
that dissolves the old
paint.
Water dissolves so many different
substances that it is called the
universal solvent.
YOU are a big, walking
solution of trillions of
molecules all dissolved
in water!
That’s why............
Astronomers who are searching the
universe for alien life forms always say
FOLLOW THE WATER!!!
Where there’s Liquid
water......
There’s life!!!
Water dissolves substances as
it flows over and through the land
on its way to the sea.
Any soluble soluble
substance, such as
salt, will be dissolved
and will end up in
the ocean.
This is why the seas
are salty.
Does that mean that the oceans
used to be less salty than they are
today?
Why does the salt accumulate
in the oceans?
Water can evaporate leaving the salt behind or
Water can freeze leaving the salt behind
The only way salt can leave the ocean is if
the entire ocean evaporates..................
What else is dissolved in sea
water besides salt?
Lots of things like gypsum which is used to make
wallboard and plaster,
and calcium carbonate which forms limestone,
chalk, and marble.
How does limestone and chalk form?
Clams, oysters, and other organisms extract the
calcium carbonate from the seawater to make
their shells. When they die their shells pile up
and form limestone which may become marble.
Who cares?
Limestone and marble are mined for commercial
use but more important is the billions of tons of
carbon that remains in the oceans and does not
get into the air where it would cause global
warming on a MASSIVE scale.
Specific Heat
Water has the highest specific heat of any
common substance.
Specific heat refers to the amount of energy
water needs to absorb before it changes
temperature.
It takes a LOT of heat energy to heat water so
water heats up and cools down VERY SLOWLY
What difference does the specific
heat of water make in my wonderful
and exciting life?
Locations near
the oceans have
cooler summers
and warmer
winters than
inland locations.
Large bodies of water
eliminate extremes of temperature!
This affects
the growing
season and what
crops can be
grown in a region.
Water has Surface Tension
Surface tension is the tendency of the surface
of a liquid that allows it to resist an external force.
Remember that water molecules are cohesive.....
they stick together.
The water molecules IN the
liquid are attracted equally in
every direction so all the forces
acting on them cancel each
out.
But the forces acting on the
molecules on the surface are
unequal.
Surface molecules are pulled
inward towards the center.
This creates a force, or tension
This tension pulls the water
into a spherical shape or
droplet.
Water striders are insects that can walk on water.
They can do this because their mass is distributed
over a wide area and because their legs are nonpolar. They repell water.
The force of
gravity is less than
the force needed
to break the
surface tension.
Water shows Capillarity
(aka capillary action)
Capillarity is the movement of water molecules
due to the attraction of those molecules to the
surface they are on.
Remember, water molecules are
polar. One end of a molecule is
positive, the other negative.
These charges are attracted
to charges on a surface such as
a tube which pulls the water up.
The attraction between
water molecules and
paper fibers pulls the
water up and into the
paper.
Capillarity in the xylem tubes
of plants helps to explain how
plants can “pull” water up
to their leaves against the
force of gravity.
If water didn’t have this property
the tallest plant on Earth would be
only a few inches tall and
giant sequoias (as
well as all other
trees)....
wouldn’t exist
Gases dissolve in water!
This includes oxygen (not to be confused with the
oxygen that is part of the water molecule).
Dissolved oxygen can be easily extracted!
Fish do it all day long.
So could you if you had
gills.
Carbon dioxide (CO2 ) also
dissolves in water.
When carbon dioxide dissolves in water it forms
Carbonic Acid, H2CO3
You are probably more familiar with carbonic
acid as seltzer or club soda. It is a very weak
acid but it does react with.......
Calcium carbonate which forms limestone,
marble, and chalk.
Since all rainwater contains dissolved carbon
dioxide, all rainwater is really a weak acid.
When the carbonic acid rain reacts with limestone it converts the limestone to carbon
dioxide (bad for global warming) and creates
CAVES
So what is so special about water?
1) It is polar and because it’s polar it......
2) Dissolves other substances such as salt, and
calcium carbonate.
3) Has surface tension which results in capillarity.
4) Gases, including oxygen, dissolve in water.
5) It dissolves CO2 forming carbonic acid.
6) It has a high specific heat which means it
changes temperature, up or down, very slowly.
7) It expands when it freezes so that ice is less
dense than liquid water.
8) When it evaporates or freezes the substances
that may be dissolved in it, salt for example,
remain behind.
9) In its solid form, ice, it is a very good insulator.
How would YOUR life be different
if any of these properties changed?