Welcome back to BOGObiology!
In this video, we'll be discussing the properties of water.
Today, we’ll be examining water’s structure and polarity, its tendency to form hydrogen
bonds, and the high specific heat, cohesion, adhesion, surface tension and capillary action
that all result from these hydrogen bonds.
All of these properties are related, and it all begins with the structure of the water
molecule.
Water Structure & Polarity: The water molecule is shaped like the letter
V, with an oxygen atom in the center and two hydrogen atoms joined to it by single covalent
bonds.
Both oxygen and hydrogen contribute electrons to these covalent bonds, but the distribution
is not equal.
Let’s zoom in on a pair of atoms that like to form covalent bonds.
You probably learned that a covalent bond looks like this in secondary school, but this
is an idealized and simplified image.
In reality, the electrons are constantly whizzing around and changing position, but overall
the electrons in the covalent bonds of a water molecule spend more time close to oxygen than
they do to hydrogen.
This is because oxygen has a higher tendency to attract electrons, a property that we call
“electronegativity” . This unequal electron sharing caused by the electronegativity differences
is known as a “dipole moment”.
Since electrons have a negative charge and they spend a larger part of their time near
the oxygen, it gives the oxygen a partial negative charge, and the hydrogens a partial
positive charge.
We designate a partial positive or negative charge with these symbols.
When a molecule has these uneven electron distributions and therefore regions that are
more positive and regions that are more negative, we say a molecule is “polar”.
The greater the difference between the electronegativity of the atoms in a bond, the more polar the
chemical bond is.
Because water has these charged regions and is polar, it can do a lot of important things,
which we’ll now get into, the first (and most crucial) being hydrogen bonds.
Hydrogen Bonds: Oppositely charged parts of water molecules
are attracted to one another (negative oxygen and positive hydrogen) in what we call hydrogen
bonds.
The attractions between water molecules can also be called electrostatic forces.
It’s very important to remember that water really, really, REALLY likes to form hydrogen
bonds.
One water molecule can form up to four hydrogen bonds at a time.
In liquid water, the hydrogen bonds are weak and last a very, very short time, just a tiny
fraction of a second before they reform in another configuration.
This constant breaking, rearranging and reforming of hydrogen bonds accounts for many of water’s
unique properties, including its heat capacity, cohesion, adhesion, surface tension and capillary
action.
In short, if your teacher asks you any question similar to “Why does water do ________?”
it’s very likely the correct answer starts with “Because water really loves to form
hydrogen bonds...”.
You’ll hear me repeat this phrase a lot in the rest of this video.
Specific Heat Capacity: Thanks to its hydrogen bonds, water can absorb
a very large amount of heat energy without changing into a gas.
Water has a high specific heat capacity, which means it needs a lot of heat in order to raise
the temperature of one gram of water by one degree celsius.
This is why it takes ages (and a lot of heat from the stove) to get a pot of cold water
to boil.
When water does eventually heat up, the heat energy causes hydrogen bonds to break and
the molecules to move around freely.
When it cools, the bonds re-form and release energy.
Water’s specific heat is essential for maintaining homeostasis.
Blood is a very watery liquid, so it retains many properties of water.
Warm-blooded or endothermic animals use the flow of blood to more evenly distribute heat
around their bodies.
This is why your body temperature resists instantly crashing while playing in the snow,
and skyrocketing when sitting on a hot beach.
In short, because water really loves to form hydrogen bonds, it takes a LOT of heat energy
to overcome this tendency, break the bonds and make water warm up.
Cohesion: Water has a tendency to stick to itself because
it easily forms these hydrogen bonds.
At any given point in time, a large percentage of water molecules are engaging in hydrogen
bonds with several of their neighbors.
The process of water molecules constantly bonding, breaking up and re-bonding with one
another via hydrogen bonds is what holds water together.
We call this tendency of water to stick to itself “cohesion”.
Because water really loves to form hydrogen bonds, its molecules stick together.
This allows insects (and certain other folks) to “walk” on the surface.
Surface tension is closely related to cohesion.
Usually, each water molecule is bonded to four other water molecules most of the time
in a tetrahedral shape, and the force is the same in all directions.
However, at the surface of the water, the outer molecules have fewer adjacent water
molecules to bond to.
So, instead they reinforce the bonds with the molecules next to them.
Because there are forces pulling the outermost molecules downwards and to the side, but not
upwards, it creates a stronger layer on the surface, and a net force that pulls the molecules
inwards.
Because water really loves to form hydrogen bonds, the top layer is reinforced, creating
surface tension.
Adhesion: Water also has a tendency to stick to other
polar objects, in addition to sticking to itself.
We call this behavior “adhesion”.
The combination of cohesion and adhesion is what causes water to form a rounded “bead”
that rolls around on a waxy surface like a leaf, or a non-stick pan, rather than spreading
out into a puddle.
Because water really loves to form hydrogen bonds, the water adheres weakly to the waxy
surface, but strongly to itself, forming a rounded water bead.
Capillary Action: The combination of cohesion, adhesion and
surface tension creates a final phenomenon called capillary action.
Capillary action is when a liquid flows through a narrow space without the assistance of any
external forces, such as gravity.
Capillary action occurs when adhesion to the walls of a vessel is stronger than the cohesion
between the water molecules.
The liquid will stick to the walls of the container, and then pull other molecules up
behind it, then climb higher again, sort of like a rock climber.
If the diameter of the tube is too big, the molecules cannot climb the walls and create
a flow.
Instead, they climb as high as they can before gravity overpowers the adhesive and cohesive
forces and stops the liquid moving any higher.
This competition between forces results in a “dip” in the center of the liquid, known
as a “meniscus”.
Capillary action is important in biology as well as in medicine.
It is what allows water to move upwards through a plant, resisting the pull of gravity.
As water evaporates from the leaves, they pull the lower molecules upwards behind them.
Capillary action is also a key part of how a glucometer measures a diabetic patient’s
blood sugar.
Blood is very watery, so it retains some of the same characteristics as a water molecule.
Test strips contain a tiny tube, which uses capillary action to quickly draw a drop of
blood upwards into a hand-held machine for testing.
We used these all the time when I was an EMT for the fire department, and capillary action
is increasingly being harnessed for medical testing and diagnostics in the emerging field
of microfluidics.
To summarize, because water really loves to form hydrogen bonds, it can flow through a
narrow space without the assistance of outside forces (and play an essential role in medicine).
That wraps up our discussion of the properties of water.
If you’d like to see the sources I consulted to make this video, I’ve left links to them
at the very bottom of the video description, as well as a ready-made APA citation for this
video.
Thanks again for watching and please remember to like, comment and subscribe!