Okay, in this recording we're going to dive a little bit more into the significance of those polar bonds and the hydrogen bonds they create, specifically in the context of water and why water is such an important substance for life. So here's an outline of what we're about to go over. So certain characteristics of water. First, all of that hydrogen bonding due to the polar covalent bonds in an individual water molecule, when you put a bunch of these things together, you create lots and lots of hydrogen bonds.
That has a pretty significant effect on the characteristics of the substance as a whole. And we'll go through kind of each of these, how hydrogen bonding influences them, and then what their significance is in a biological context. And then we'll talk about The idea of self-ionization in water, what that is, and how it relates to pH or a substance being basic or acidic. A big part of kind of staying alive, being a living thing, is about thermoregulation or regulating temperature. And water is particularly helpful in terms of temperature regulation.
Let's talk about why. First, we need to understand a couple things about thermodynamics. We'll learn more about thermodynamics in a few units. For now, all I need you to understand is chemical bonds store energy. In order to break a chemical bond, that requires energy.
When the bond breaks, the energy that was stored in it gets released. As heat. Okay.
So breaking bonds requires energy. When it breaks, energy gets released as heat. Heat gets released.
So to boil water, that would mean breaking all of these hydrogen bonds. Because if the hydrogen bonds stay intact, then the water molecules are going to stay together. In order to... boil water take it from a liquid to a gas you need to separate all of those water molecules by breaking all those hydrogen bonds so let's go back to this first piece here right breaking bonds requires energy so breaking all those hydrogen bonds requires a lot of energy as a result the energy needed to boil water is relatively high The this term specific heat is the amount of energy needed to raise the temperature of something.
So the specific heat of water is really high because it requires a lot of energy to raise its temperature because of all of those hydrogen bonds that it needs to break. Heat of vaporization is the is how much is the temperature needed to boil water to go from liquid to gas. So water has a high specific heat, a high heat of. vaporization.
Now let's talk about the significance of water's high specific heat in two examples. One for the environment and two for our own thermoregulation. So water has high specific heat.
Something that does not have a high specific heat is most of what is on land, rocks, dirt, the ground. It doesn't take a whole lot. of energy to heat that up.
That's why on a hot day, if you go outside and you touch like, say, metal or a rock or something that's been in the sun, that's going to be really hot. Whereas if you put your hand in some water, it'll be nice and cool. If you're in an area of the world where there is a lot of water and the air temperature is really warm then the water will be able to absorb some of that heat so if we look at like the like west coast of the u.s south kind of southwest right la pacific ocean san diego the coastal cities are pretty temperate right they're warm for sure but if we compare the average temperature of a coastal city to Cities that are only a little bit more inland. So the air temperature. is relatively the same.
Like the amount of heat in the air is relatively the same in these areas. The difference is, in cities like LA and San Diego, the water is able to absorb a lot of that heat, take it out of the air, and therefore cool the surrounding land. Whereas out in these more desert areas, where there's not a lot of water, the land, the rocks...
Those things have a low specific heat. They're not as good at storing and extracting the heat from the air as the water is. And as a result, you get very, very hot temperatures during the day.
Now at night, when it starts to cool off and you're not getting direct sunlight, the water is going to slowly cool. And as a result of that water slowly cooling, it helps keep the air warm. Whereas in the, so basically all of that heat that was stored during the day slowly gets released, right? So the water cools, heat gets released from the water because the air gets cooler. And as a result, that heat is leaving the water because of that second piece of thermodynamics I talked about in the last slide.
And it keeps the... average temperature throughout the day in these coastal cities, you know, relatively mild. Whereas out in these desert areas, it's super, super hot during the day.
But during night, when the sun goes away, and that's not actively heating the air, there's no, the land, the rocks, they weren't storing that heat like the water was. And so they're not able to release that heat at night. As a result, these areas might get much much cooler at night and so you see much more drastic um changes in temperature in non-coastal areas typically because of because of water's high specific heat and it's the same principle for regulating body temperature right our bodies are mostly water and as a result we can go outside in a hot hot day and our body temperature in in your temperature won't change dramatically because we're made up of so much water Another thing related to thermoregulation is the concept of evaporative cooling.
So when those hydrogen bonds break, remember, they release heat. They release energy in the form of heat. This is why sweating is an effective form of cooling our body.
The sweat goes onto our skin and the water in that sweat starts to evaporate. All of those hydrogen bonds... start to break. All of the energy stored in those hydrogen bonds gets released and it gets released as heat. So that heat leaves our skin and as a result our body cools down.
So boiling water is about breaking those hydrogen bonds. Freezing water is about stabilizing those hydrogen bonds. When water turns to ice, you create more hydrogen bonding than when water is a liquid. The hydrogen bonds make a less dense structure than the liquid.
As a result, ice floats. So because these hydrogen bonds create spacing between the molecules that isn't there in liquid or in a gas, ice is less dense. Therefore, when ice is in water, it will float.
significance of this huge impact um on the environment number one if we think of you know fresh bodies of water they will freeze and the ice will float to the top as a result that layer of ice on top will insulate the water below it it'll it'll keep it um from freezing all the way through if it's deep enough and because that ice is freezing on top you instead of on the bottom. We create just a layer of ice instead of the whole body of water freezing. And you can imagine if you live in an area like the Northeast US, where it gets cold enough for water to freeze in the winter. If water froze from the bottom, then the whole body of water would freeze, and anything living in that water would have to adapt to find ways to live through their little icy season. Because water floats and you get a layer of ice on top of the water, those organisms living in the water don't need to make that change.
Also, the ice itself, the surface ice, serves as a habitat for some animals, like in, you know, parts of the world near closer to the poles that are really cold. If you ever seen any kind of nature documentary, you see a lot of animals, you know, live on those large sheets of ice like, you know, penguins, polar bears, seals, things like that. Another, you know, why, you know, when we talk about climate change, we often talk about those. um habitats being affected first uh because it's it's directly related to that ice disappearing and melting so an individual hydrogen molecule sorry an individual water molecule can form up to four hydrogen bonds which is a lot and like i said individual hydrogen bond relatively weak multiple hydrogen bonds creates a you cumulatively, collectively, pretty strong force.
So all of those hydrogen bonds together are actually strong enough to create, through the processes of cohesion and adhesion, are able to do something pretty special. So cohesion is the ability of water molecules to stick to each other. And this is what creates surface tension in water.
This is why, you know, little critters can walk on top of water. You can see the kind of like dimpling of the water. it doesn't completely give way.
There's some force, there's some like surface tension that's able to support the weight of this insect. This is also why you can fill a glass of water up a little bit over the lip of the glass and it'll form that dome. All of those water molecules are sticking together because of all the hydrogen bonding.
If you were to try to pour, you know, when you all turn 21, of course, and not before, if you try to pour... alcohol into a glass and get it to the rim it'll immediately spill over alcohol is non made up of non-polar covalent bonds so it doesn't have that same cohesion and therefore you don't get that surface tension adhesion is water molecules ability to stick to other polar molecules so these dipoles can also attract dipoles of other molecules not just other water molecules uh as a result uh this is why you get like you can form like water droplets um if you've ever seen like condensation on a glass for example those water droplets they're sticking to the glass this is especially important for plant physiology if you think about trees they gather all their water and nutrients from their roots but most of their activity metabolically speaking is happening in the leaves So they need that water to be transported up against gravity. And the reason they're able to do that is because the droplets will form and stick to the insides of the walls of the vessels in the trees.
That's what you're seeing here. We have, you know, what is essentially like the equivalent of a blood vessel, but for a tree carrying water because these water molecules are sticking to the walls of that vessel. And. therefore and they're also sticking to the water molecule ahead of it so therefore it's collectively pulled up through the roots through the trunk and to the leaves that need it against the force of gravity water is often called the solvent of life let's talk about what that word means so a solute is something that devolves into a solvent and the result is a solution So water being the solvent means that solutes can dissolve into it. Specifically, things that are going to dissolve well in water are other polar molecules.
Often, these molecules are called hydrophilic, whereas nonpolar molecules, or hydrophobic, water-phobic fearing, do not dissolve well. And so that's what you're seeing down here. A hydrophilic... or polar molecule gets dropped into water all of the dipoles in that polar molecule are happy to interact with all of the dipoles created by the water molecules As a result, these hydrophilic molecules are happy to separate and start interacting with the surrounding water molecules. A hydrophobic or nonpolar molecule doesn't have those dipoles.
As a result, they don't want to interact with these charged polar water molecules. This is why things like fats, like oils, do not dissolve well in water. They don't have those dipoles. they don't have the charges that will attract the water molecules.
As a result, they will stay in little kind of droplets and not want to completely separate in the water, unlike the hydrophilic molecules will. So moving on a bit from the hydrogen bonding, but staying with the idea of water as a solvent, let's talk about ionization of water. When water exists, a very small fraction of it, a very small piece of it, tends to do something called self-ionization.
What that means is instead of having two water molecules, the dipole, the attractive force created by the polar bonding, will be so strong, say like this oxygen is so highly negatively charged, that it happens to attract the hydrogen of a nearby water molecule. As a result, instead of having two H2Os, we get one H3O and one OH-. This is called self-ionization because we've created ions.
We've gone from two polar molecules with no, you know, they have the partial charge but not a full charge, and we've created two ions. The H3O is called the hydronium ion, and the OH-is called the hydroxide ion. So depending on the presence of that hydronium ion, water is going to have a pH. This term aqueous solutions, this means something where water is the solvent.
Acids or acidic molecules are going to increase the concentration of that hydronium ion. Often. Instead of writing out the full H3O+, we just use the shorthand H+. So if something goes into water and increases the amount of H+, that makes the water more acidic. If something goes into water and it decreases the H+, that'll make it more basic.
An example would be something that creates more hydronium, sorry, more hydroxide ions. This OH minus is just dying to attract an H plus. As a result, it'll scoop up any free H pluses.
And because we measure pH by taking the negative log of the concentration of the hydronium ion, more hydronium ion means a lower number because we're taking the negative. So the lower pH values are acidic or the higher pH values are basic. Pure water is completely neutral and then adding anything to that water can adjust it to be more acidic or more basic.
And we'll talk more about why these molecules can do that in class. Just like regulating our temperature is important, regulating pH is also very important for living things. One way our body does that is through the use of buffer solutions. Buffer solutions resist changes, dramatic changes in pH. So if we have just water and we add an acid to it, it's going to become acidic very quickly. Whereas if we have water plus a buffer, adding an acid to it won't dramatically change the pH. And same situation with the base.
Just water becomes very basic, the buffer, small change. One common buffer solution in our body. is carbonic acid. When carbon dioxide mixes with water, it creates H2CO3, or carbonic acid. This H2CO3, when broken down, creates two ions, the H plus ion and the HCO3 minus ion.
So if a base were to be added to this solution, then this H2CO3 could easily break down and create more H+. to neutralize that base. Whereas if an acid is added to the solution, this HCO3-could happily pick up that hydronium ion, right, could reduce the amount of hydrogen by moving back into this direction. So whether a base or an acid is added, this carbonic acid molecule can resist dramatic changes, unlike just water, which can't switch back and forth as quickly. This concept gives us some insight into effects of climate change.
So as we know, CO2 emissions into the air is one of the big causes of climate change. And in a specific instance here is how it can affect ocean ecosystems. When that carbon dioxide absorbs into the water, it creates that carbonic acid. The carbonic acid, although it acts as a buffer, it is a, you know, acid in itself, and it'll create that hydrogen, hydronium ion, or the H plus ion, and make the water more acidic. That H plus ion will be attracted to a negatively charged, the molecule is CO3 2 minus, so it has negative charge, it'll attract that H plus.
and become hc03 minus um that hc03 minus will attract uh calcium to form caco3 and by kind of soaking up absorbing that calcium normally what that free calcium would be used for is to help stabilize coral reef structures and if instead that calcium is being, you know, preoccupied, for lack of a better term, then the coral reef can't build its skeleton. And that's how you get bleaching of the coral reefs. It means the water is getting more acidic because there's more carbon dioxide in the area.