Hello marine biology students.
In this video we're going to be learning about
some of the unique properties of water and
seeing how life would be different living
in water as opposed to life here on land.
[Intro Music]
So let's talk about some of the properties
of water and why it might impact organisms
living within it.
In the water molecule itself, H2O, there are
two atoms of hydrogen and one atom of oxygen
covalently bonded together.
While this molecule itself is quite simple,
water is unique in many ways.
Each water molecule has a slight positive
and negative electrical charge and this is
due to polar covalent bonds.
Polar covalent bonds do not share electrons
evenly and so there's an electrical charge
imbalance on either side of that covalent
bond.
The positive charges are near the hydrogen
atoms and the negative charges exist around
the oxygen atoms.
And this is because oxygen has a higher electronegativity
or stronger pull on those shared electrons
the electrons spend more time around the oxygen
and since electrons have a negative charge,
we have that partial negative charge near
that oxygen.
So here in this diagram of water molecules
we can see the water molecules themselves
composed of oxygen and hydrogen.
We can see the partial charges and then also
notice that there is attraction between separate
water molecules based on those electrical
charge differences.
These attractions are called hydrogen bonds.
So again, due to these slight electrical charges,
water molecules are attracted to each other.
The negative charge of one molecule is attracted
to the positive charge of the other molecules.
This attraction of one water molecule to another
is known as hydrogen bonding.
So, it's important to realize that hydrogen
bonds are not the covalent bonds holding the
hydrogen to the oxygen, instead hydrogen bonds
are the attractions between two separate water
molecules.
When we look at the group of water molecules
together, water in its liquid form consists
of many water molecules having hydrogen bonds
with their neighbors.
And these hydrogen bonds, they can break and
reform and transfer from one to the other.
So, the individual water molecules can move
in relation to each other.
If these water molecules are moving fast enough,
if enough kinetic energy is added to them,
they can actually move so quickly that they
can break the hydrogen bonds and break free
of their connections of neighboring water
molecules, and in that case the water has
evaporated.
It has gone from a liquid form into a vapor
form.
So water can be found in three states, and
it's the hydrogen bonds that end up impacting
these states of water.
Hydrogen bonds help keep water molecules as
a cohesive group at most temperatures found
on Earth.
This is the reason why we have liquid water.
Water can be found in three different phases
of matter on the surface of the Earth.
Water can be found in its liquid form, in
its gas form, which is water vapor and also
as a solid in the form of ice.
Ice is the solid form of water that is caused
by reducing the kinetic energy of the molecule.
As the temperature is lowered, the hydrogen
bonds lock into place, preventing water molecules
from moving in relation to each other.
This also holds the water molecules a bit
further from each other than in liquid water.
An interesting property of seawater is that
it has a lower freezing point then fresh water
and as sea water begins freezing it, actually
releases a concentrated salty brine resulting
in sea ice having less dissolved solute than
regular seawater.
We will see that this also plays a role in
driving ocean circulation.
Now, the gas form of water is known as water
vapor and humidity
is the amount of water vapor that's in the
air.
Water vapor is formed when molecules of water
escape the hydrogen bonds holding them together.
And in this way, they become airborne.
This process, called evaporation, increases
with increasing temperatures.
Water is the only substance on Earth that
naturally exists in all three states, solid
as ice, liquid as liquid water, and gas as
water vapor.
The concept we're going to visit many times
over as we discuss water is the concept of
density.
At lower temperatures, water molecules are
closer to each other than at warmer temperatures.
In sea water at 75 degrees, the molecules
are further apart than with the same water
at 35 degrees.
When water molecules are closer together,
the substance is said to have a greater density.
Substances with higher density are heavier
than those with lower density when the same
volume is present.
Cold water therefore sinks underneath the
warmer water.
Colder water also holds more oxygen 
then the same volume of warm water, a crucial
factor for organisms living at the ocean floor.
Even though colder water is more dense than
warmer water, this changes when the water
gets cold enough to freeze.
Ice is less dense than liquid water due to
the distance between the water molecules increasing
as the hydrogen bonds lock into place.
This is why ice floats.
This is important for organisms living in
areas where freezing temperatures
are common, such as in the Arctic Ocean or
in Antarctica.
This is also key to life on planet Earth as
we know.
Now, this may sound like a bit of a stretch,
but if water were to sink after it froze,
that layer of ice would sink crushing all
life below it and it would settle at the bottom.
So it really would prevent life from surviving
underneath ice.
If ice did not float, a body of water would
also freeze from the bottom up, or in layers
of freezing and sinking.
Eventually the whole body of water would freeze.
Even though we think of it as being cold,
ice is an excellent insulator.
Since ice floats, the floating ice creates
a barrier between the cold air temperature
and the water below the ice, keeping it from
freezing.
In addition to hydrogen bonds, water has other
unique chemical properties.
One is a high latent heat of melting.
The latent heat of melting is the amount of
energy needed to change water in state from
ice into liquid or conversely the amount of
heat that needs to be taken away to convert
liquid water into ice.
Water has a higher latent heat of melting
than many other commonly occurring substances.
This means that ice melts at a relatively
high temperature and it absorbs a great deal
of heat as it melts.
Water also absorbs a great deal of heat before
its temperature rises.
This property is known as heat capacity 
and it is defined as the amount of heat required
to raise a substance’s temperature by a
given amount.
Water has a very high heat capacity, making
it resistant to easily changing temperature.
This high heat capacity is significant for
marine organisms
because it means that they are not subjected
to the wide temperature ranges so often seen
on land.
An exception is very shallow water, like tide
pools, which warm quickly due to the relatively
small volumes of water and can subject certain
marine organisms to sudden drastic temperature
changes.
A great deal of heat is also required for
evaporation to occur.
The amount of heat required for a substance
to evaporate is known as the latent heat of
evaporation or the latent heat of vaporization.
Water has a very high latent heat of evaporation.
This means that it takes a lot of energy for
liquid water to become water vapor without
changing its temperature.
When water is changing its states of matter,
its temperature does not change until the
state change is complete.
The extra energy goes to the state change
instead of temperature change, so here in
this graph we can see the temperature of water
over time with a constant amount of heat being
added to it.
So, we see that as the ice starts warming
up, starts being warmed, there's a pause in
temperature change as all the ice melts into
liquid water.
Once the water is liquid, the temperature
increases again until it hits a level where
that liquid water is being converted into
gas.
This is also why boiling water reaches a maximum
temperature and will not go higher unless
pressure is increased.
That'll be the end of this video, but we'll
have a bit more to discuss about the properties
of water in the next video.
Thanks for your attention.
See you then.
