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!
