Properties of Water - Video Tutorials & Practice Problems

Unique Properties of Water

in this video, we're gonna talk about the medium of life water. So you guys know that the chemical formula of water is H 20 So there's one oxygen atom and two hydrogen atoms. And you guys already know that water is a polar molecule, which of course, means that it has polar covalin bonds. And we know that water has to polar covalin bonds. Now the molecular geometry of water is a bent geometry, So water is a polar bent molecule. So we took a look at our water molecule down below. Notice that each of the hydrogen has a partial positive charge due to the polar bonds. And then the oxygen atom has a partial negative charge due to the polar bonds and again notice that the geometry of the water molecule is a bet. Geometry. So these two hydrogen are not 180 degrees apart from one another, and in fact, the bond angle between these two hydrogen is 104. degrees. And so what that means is that this is a bent molecule and that the dipole moment. So if we would have draw the dipole moments of each polar bond here, the dipole moments ago in these particular directions, and so they don't cancel each other out. Whereas if this were a linear molecule in the hydrogen, is were 180 degrees apart. The dipole moments would be going in opposite and equal directions, which would make it a non polar molecule. However, that's not the case here, because the bonds are at 104.5 degrees apart from one another, and that makes their dipole moment a polar molecule. And so another thing that's important to note is that water has to lone pairs of electrons, and you can see that by these four black dots that are on the oxygen. And so each of these two dots here create a separate lone pair. So we've got to lone pairs and together the lone pairs, as well as these other characteristics that we talked about, allow each water molecule to form four hydrogen bonds upto four hydrogen bonds with neighboring molecules. And so it's really the abundance and the strength of these hydrogen bonds that give water all of its unique properties, and these unique properties include a high boiling point and melting point shown by these up arrows as well as, Ah, high heat capacity and high heat of vaporization. And so recall he capacity is simply the amount of energy that's needed to raise the temperature of the water. One unit of temperature and then the heat of vaporization is the amount of energy that's needed to vaporize the water from liquid to gas when the liquid is at its boiling temperature. And so both of these are all four of these properties here are very, very high for water, and that's very unique. And we'll be able to appreciate this Maurine some of our later videos when we compare water, tow, methane and so another very interesting property of water is that its density actually decreases when it freezes from liquid to solid ice. And so that's because of the crystal formations that form. And this is not usual of most substances. Most substances when they freeze from liquid to solid. Their density increases, however, with liquid water. When it freezes from liquid toe heist solid ice, the density decreases Now. Water also has a very strong surface tension, and that has to do with its ability to have cohesive and adhesive properties. So let's take a look at our example below and notice that we have a single water molecule here that's forming four hydrogen bonds. And so it conform. One hydrogen bond with each of the if it's hydrogen and the oxygen conform to hydrogen bonds because it has two lone pairs, and so you can see that one water molecule conform up to four hydrogen bonds, and that applies toe all the water molecules. And so you can imagine that in, ah, a solution of a bunch of water. All of these water molecules form an abundance of hydrogen bonds, and it's all of these hydrogen bonds that give water its unique properties. Now, over here, what you'll notice is that we're distinguishing between cohesion and adhesion. Now cohesion has to do with the ability of water molecules to cling on and interact with each other. And so you can see here cohesion, preferring to one water molecule over here interacting with another neighboring water molecule, and that is called cohesion Now. Adhesion, on the other hand, is when water molecules interact with other substances that air, not water, and these were gonna be polar or charged objects. And so you can see that's true here. So here this interaction, where waters interacting with another object that's not water is adhesion. And so, in our next video, we're gonna talk about water soluble ity, so I'll see you guys in that video.

Water Solubility

so recall from your previous chemistry courses that Sally ability is the property of a salute to be dissolved by a solvent and recall that a solvent is in high concentration and does the dissolving, whereas a salute is in small concentrations and gets dissolved by the solvent and water is actually the biological solvent. So it does the dissolving for life and water interacts with other polar substances and dissolves electrolytes and recall that electrolytes are simply molecules that disassociate or break apart. Toe form ions and ions are atoms that have charges and thes ions that form. They can create dipole dipole interactions with water molecules and so electrolytes that air dissolved in water are called hydrated. Electrolytes and hydrated electrolytes are surrounded with a shell or layer of water molecules that again surrounds the electrolytes, and that layer water molecules is called a hydration shell. Now, in our example below, what you'll see is that we've got sodium chloride, which is our typical table salt. And when we put sodium chloride and water, it becomes hydrated electrolytes, so you can see the chlorine atom here is negatively charged, and the hydrogen atoms of all the water molecules are facing the negatively charged chlorine because the hydrogen are partially positive, and that creates a dipole dipole interactions shown by all of these dotted red lines. Now over here with the sodium ion, which is positively charged, notice that the hydrogen zehr facing away from the sodium Adam, unlike the chlorine atom over here. So instead, the oxygen atoms of the water molecules are facing towards the sodium atom. And that's because the oxygen's partially negative and the sodium is a positive charge. So that creates dipole dipole interactions. And so you can see that here we have a hydration shell, which is a the layer of water molecules surrounding the ion. And here we have another hydration shell, so that essentially diminishes the electrostatic interactions between chlorine and sodium. It's almost as if they're not even attracted to one another, because they have all of these, uh, dipole dipole interactions with water that air stabilizing thes charges. And so there's actually a measurement for a solvents ability to dissolve. And that's called the dye electric constant, the dye electric constant and so water has a high die electric constant, which means that it has a high ability to dissolve other substances and to essentially diminish the electrostatic interactions between electrolytes that have been dissolved. And we'll talk more about our high die electric constant in our next video. Now, because water has a high die electric constant, it makes it perfect for dissolving proteins, carbohydrates and nucleic acids. But we already know from our previous videos that water is not good for dissolving lipids because lipids are hydrophobic. So, in our example, down here, what we have is a water molecule and a polar molecule here with this Carbonnel polar group. And so it's important to note that, uh, molecules themselves will increase in soluble ity as long as they have mawr, polar groups and less non polar groups. So here we have a water molecule, and again it's gonna have a partial negative charge. And the hydrogen, they're gonna have a partial positive charge because of its polar bonds. Now, the Polar Carbonnel Group here is also gonna have a partial negative oxygen. The carbon here is gonna be partially positive. And so I notice that the partially positive hydrogen on the water molecule is gonna interact with the partially negative oxygen on the Carbonnel Group of this other molecule, and this creates an inter molecular interaction. More specifically, it's a dipole dipole interaction, and more specifically than that, this is a hydrogen bond. So hydrogen interacting with electro negative Adam on another water on another molecule. And so again, this is all review from our previous videos. And so in our next video, we're going to directly compare water with a molecule of similar molecular weight and size methane, so I'll see you guys in that video.

Water vs. Methane

in this video, we're going to directly compare water with a molecule of similar molecular weight and size methane, and so not that you need to memorize all of these values below. But when we compare the values of water to the values methane, hopefully that will give you a greater appreciation for water's unique properties. And so we know that the chemical formula water is H 20 but recall that the chemical formula of methane is CH four. So methane has a carbon instead of an oxygen and two more hydrogen atoms. And so also noticed that Moeller Mass of water is going to be 18.2 g per mole, whereas methane Mueller masses 16.4 g per mole. So they only differ by about 2 g per mole, which is not a big difference. And these two molecules really do have a similar molecular weight. Now the boiling point of water is 100 degrees Celsius, and the melting point of water is zero degrees Celsius. Now, if we compare that directly to methane, what we'll see is that the boiling points in melting points drastically differ. Methane has a boiling point of negative 161. degrees Celsius and a melting point of negative 182 degrees Celsius. So these are much, much, much lower boiling and melting points in comparison to the very high boiling and melting points of water. And so again and our previous videos, we said that water has a high boiling and melting point, and this kind of brings it toe life a little bit so you can see the comparisons now. Water also has a high heat capacity, so you can see that the heat capacity of water is about double the heat capacity of methane. So it takes about double the amount of energy to raise the temperature of water one degree Kalfin than it does the amount of energy to raise the temperature of methane one degree Calvin. And so this is due to the hydrogen bonds that form. And so all of these hydrogen bonds make water. Um, give water high boiling point. Ah, high melting point, high heat capacity and a high heat of vaporization. So here we can see that the heat of vaporization is about five times greater than the heat of vaporization off methane, so it takes about five times more Thea Mount of energy to vaporize a water molecule when it's at its boiling point than it does, uh, the amount of energy to vaporize methane. So what's also very interesting is liquid density we know has a density of 1.0 g per milliliter. And so when ice liquid water freezes into ice, the density actually decreases. It goes down, whereas with methane noticed that the density ah, going from liquid to solid here it actually increases. And so it goes from 0.2 to 0.52 and that goes up. And that's usual. This is typical of most substances, so this is almost like an anomaly for water, where it decreases in density when it freezes. So that's very interesting to note. And this is really important because this means that ice is ableto float on liquid water and that acts as an insulator for life when temperatures start to get really cold. So you can imagine if you have water, say that's water, and then water is going to freeze from top to bottom. So here we have ice is going to be at the top, whereas we still have liquid water at the bottom of our pool. So this ice here acts as an insulator to keep the liquid below it warmer so that it doesn't freeze. And that's very, very important. Now again, we talked about Die electric constant in our previous videos, and we know that this has to do with the ability of a solve it to be. A good solvent toe essentially dissolved things. So the higher the dye electric constant, the better it is that dissolving things and the dye electric constant of water is about 47 times larger than the dialectic constant of methane. So that is much, much better. Water is a much, much better solvent and much better dissolving than methane is. And so this concludes our comparison of water and methane, and I'll see you guys in our practice videos

Water stuck to the glass window shield of a car is an example of what?

• A. High Surface Tension
• B. Adhesion
• C. Cohesion
• D. High Heat Capacity

Rank the following compounds according to increasing water solubility:

i) CH₃-CH₂-CH₂-CH₃

ii) CH₃-CH₂-O-CH₂-CH₃

iii) CH₃-CH₂-OH

iv) CH₃-OH

• A. i < iii < iv < ii
• B. i < ii < iv < iii
• C. iii < iv < ii < i
• D. i < ii < iii < iv
• E. None of the above are correct.