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
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