Properties of Water - Online Tutor, Practice Problems & Exam Prep

Water (H₂O) is a polar molecule with a bent geometry, allowing it to form up to four hydrogen bonds, which contribute to its unique properties such as high boiling and melting points, high heat capacity, and low density when frozen. Water acts as a biological solvent, effectively dissolving electrolytes and polar substances due to its high dielectric constant. In contrast, methane (CH₄) has significantly lower boiling and melting points, demonstrating the importance of hydrogen bonding in water's behavior and its role in supporting life.

Unique Properties of Water

In this video, we're going to talk about the medium of life, water. So you guys know that the chemical formula of water is H₂O, so there's 1 oxygen atom and 2 hydrogen atoms. And you guys already know that water is a polar molecule, which of course means that it has polar covalent bonds. And we know that water has 2 polar covalent bonds. Now, the molecular geometry of water is a bent geometry. So water is a polar bent molecule. So if we took a look at our water molecule down below, notice that each of the hydrogens 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 bent geometry. So these 2 hydrogens are not 180 degrees apart from one another, and in fact, the bond angle between these 2 hydrogens is 104.5° degrees. And so what that means is that this is a bent molecule and that the dipole moments, so if we were to draw the dipole moments of each polar bond here, the dipole moments go in these particular directions, and so they don't cancel each other out. Whereas, if this were a linear molecule and the hydrogens were 180 degrees apart, the dipole moments would be going in opposite and equal directions, which would make it a nonpolar 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 2 lone pairs of electrons, and you can see that by these 4 black dots that are on the oxygen. And so each of these, 2 dots here create a separate lone pair. So we've got 2 lone pairs, and together the lone pairs as well as these other characteristics that we talked about allow each water molecule to form 4 hydrogen bonds, up to 4 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. These unique properties include a high boiling point and melting point shown by these up arrows, as well as a high heat capacity and high heat of vaporization. And so recall, heat 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 4 of these properties here are very, very high for water, and that's very unique. And we'll be able to appreciate this more in some of our later videos when we compare water to 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 to ice, 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 4 hydrogen bonds. And so it can form, 1 hydrogen bond with each of the of its hydrogens, and the oxygen can form 2 hydrogen bonds because it has 2 lone pairs. And so you can see that one water molecule can form up to 4 hydrogen bonds and that applies to all the water molecules. And so you can imagine that in a, a solution of a bunch of water, all 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 is referring 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 are not water and these are going to be polar or charged objects. And so you can see that's true here. So here this interaction where water is interacting with another object that's not water is adhesion. And so, in our next video, we're going to talk about water solubility. So I'll see you guys in that video.

Water Solubility

So recall from your previous chemistry courses that solubility is the property of a solute to be dissolved by a solvent. And recall that a solvent is in high concentration and does the dissolving, whereas a solute is in small concentrations and gets dissolved by the solvent. And water is actually the biological solvent, so it does the dissolving for life. Water interacts with other polar substances and dissolves electrolytes. And recall that electrolytes are simply molecules that dissociate or break apart to form ions, and ions are atoms that have charges. The ions that form can create dipole-dipole interactions with water molecules. Electrolytes that are dissolved in water are called hydrated electrolytes. Hydrated electrolytes are surrounded by a shell or layer of water molecules that surrounds the electrolytes, and that layer of 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 typical table salt. When we put sodium chloride in water, it becomes hydrated electrolytes. 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 hydrogens are partially positive, and that creates a dipole-dipole interaction shown by all of these dotted red lines. Now, over here with the sodium ion which is positively charged, notice that the hydrogens are facing away from the sodium atom unlike the chlorine atom over here. Instead, the oxygen atoms of the water molecules are facing towards the sodium atom because the oxygen is partially negative and the sodium has a positive charge, so that creates dipole-dipole interactions.

You can see here we have a hydration shell, which is the layer of water molecules surrounding the ion, and here we have another hydration shell. That essentially diminishes the electrostatic interactions between the chlorine and sodium. It's almost as if they're not even attracted to one another because they have all these dipole-dipole interactions with water that stabilize these charges. And so, there's actually a measurement for a solvent's ability to dissolve, and that's called the dielectric constant. Water has a high dielectric 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. We'll talk more about our high dielectric constant in our next video. Now, because water has a high dielectric 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.

In our example down here, we have a water molecule and a polar molecule here with a carbonyl group. It's important to note that molecules themselves will increase in solubility as long as they have more polar groups and less nonpolar groups. Here we have a water molecule, and again, it's going to have a partial negative charge, and the hydrogens are going to have a partial positive charge because of its polar bonds. The polar carbonyl group here is also going to have a partial negative oxygen. The carbon here is going to be partially positive. Notice that the partially positive hydrogen on the water molecule is going to interact with the partially negative oxygen on the carbonyl group of this other molecule, and this creates an intermolecular interaction, more specifically a dipole-dipole interaction. Even more specifically, this is a hydrogen bond. So, hydrogen interacting with an electronegative atom on another molecule. And again, this is all review from our previous videos. 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, we aim to give you a greater appreciation for water's unique properties. We know that the chemical formula of water is H₂O, but recall that the molar mass of water is going to be 18.02 grams per mole, whereas the molar mass of methane is 16.04 grams per mole. So, they only differ by about 2 grams 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 0 degrees Celsius. If we compare that directly to methane, we'll see that the boiling points and melting points drastically differ. Methane has a boiling point of -161.5 degrees Celsius and a melting point of -182 degrees Celsius. These are much lower boiling and melting points in comparison to the very high boiling and melting points of water. Again, in our previous videos, we said that water has a high boiling and melting point, and this brings it to life a bit so you can see the comparisons.

Now, water also has a high heat capacity. 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 by 1 degree Kelvin than it does the amount of energy to raise the temperature of methane by 1 degree Kelvin. This is due to the hydrogen bonds that form. All of these hydrogen bonds give water a high boiling point, a high melting point, high heat capacity, and a high heat of vaporization. Here, we can see that the heat of vaporization of water requires more energy to vaporize a water molecule at its boiling point than it does the amount of energy to vaporize methane. What's also very interesting is liquid density; we know that water has a density of 1.0 grams per milliliter. When liquid water freezes into ice, the density actually decreases. It goes down. Whereas with methane, notice that the density, going from liquid to solid, actually increases. It goes from 0.2 to 0.52, and that goes up. This is typical of most substances. So this is almost like an anomaly for water where it decreases in density when it freezes. This is very interesting to note and is really important because this means that ice is able to float on liquid water, and that acts as an insulator for life when temperatures start to get really cold. You can imagine if you have water, say that's water, and water is going to freeze from top to bottom. So here, we have ice at the top, whereas we still have liquid water at the bottom of our pool. This ice acts as an insulator to keep the liquid below it warmer so that it doesn't freeze, and that's very important.

Now, again, we talked about the dielectric constant in our previous videos, and we know that this has to do with the ability of a solvent to be a good solvent, to essentially dissolve things. The dielectric constant of water is about 47 times larger than the dielectric constant of methane. So, water is a much better solvent and much better at dissolving than methane is. 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.

Here’s what students ask on this topic:

What are the unique properties of water due to hydrogen bonding?

Water's unique properties stem from its ability to form hydrogen bonds. These properties include a high boiling point and melting point, high heat capacity, and high heat of vaporization. The hydrogen bonds also contribute to water's high surface tension, allowing it to form droplets and enabling capillary action. Additionally, water's density decreases upon freezing, causing ice to float, which insulates aquatic life in cold environments. These hydrogen bonds make water an excellent solvent for polar substances and electrolytes, forming hydration shells around ions.

Why does water have a high dielectric constant?

Water has a high dielectric constant due to its polar nature and ability to form hydrogen bonds. The dielectric constant measures a solvent's ability to reduce the electrostatic interactions between dissolved ions. Water's polarity allows it to surround ions with hydration shells, effectively diminishing the electrostatic forces between them. This high dielectric constant makes water an excellent solvent for ionic and polar substances, facilitating various biological processes by dissolving proteins, carbohydrates, and nucleic acids.

How does the density of water change upon freezing?

Unlike most substances, water's density decreases upon freezing. This anomaly occurs because water molecules form a crystalline structure in ice, which is less dense than liquid water. The hydrogen bonds in ice create an open hexagonal lattice, causing the molecules to be spaced further apart. As a result, ice floats on liquid water. This property is crucial for aquatic life, as the ice layer on top of bodies of water insulates the liquid below, preventing it from freezing solid and allowing organisms to survive in cold temperatures.

What is the significance of water's high heat capacity?

Water's high heat capacity is significant because it allows water to absorb and store large amounts of heat energy with minimal temperature change. This property helps regulate Earth's climate and maintain stable temperatures in aquatic environments, providing a conducive habitat for life. In biological systems, water's high heat capacity helps buffer organisms against rapid temperature fluctuations, ensuring proper functioning of metabolic processes. This thermal stability is essential for maintaining homeostasis in living organisms.

How do cohesion and adhesion contribute to water's properties?

Cohesion and adhesion are crucial for water's unique properties. Cohesion refers to the attraction between water molecules due to hydrogen bonding, leading to high surface tension. This allows water to form droplets and enables capillary action, which is vital for transporting water in plants. Adhesion is the attraction between water molecules and other polar or charged substances. This property allows water to cling to surfaces, aiding in processes like nutrient absorption in plants and the formation of hydration shells around ions, enhancing water's solvent capabilities.