Properties of Water

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  • 0:06 Hydrogen Bonding Makes…
  • 1:13 Density
  • 2:33 Properties of Ice and…
  • 4:25 States of Matter and…
  • 5:49 The Effect of Water on…
  • 7:37 Cohesion, Adhesion and…
  • 9:53 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
Why does ice float? Why can water rise on its own against gravity in a small tube? Find out how these mysterious properties of water can be explained by hydrogen bonds.

Water has a lot of really interesting properties. Why does ice float when most other solids sink? Why is it that coastal cities tend to be a little bit more temperate than inland ones? How do fancy athletic shirts help remove sweat so quickly? We can talk about these questions in terms of hydrogen bonds.

Hydrogen Bonding Makes Water Unique

We've learned that hydrogen bonds can form between atoms with a partial charge. So if I have an atom here with a partial positive charge, and I have an atom over here with a partial negative charge, you can form a hydrogen bond between those two atoms, linking these two molecules. Since water is a molecule with partially charged atoms, water can form a hydrogen bond with other water molecules. And this property is what gives water a lot of its unique attributes that help make life possible.

Diagram of a hydrogen bond between two molecules
Hydrogen Bonds


The first question that I posed was why does ice float when most other solids sink? If I have a cup of water here and I was to put an ice cube in it, it's going to float. Alternatively though, if I was to take a tiny grain of salt and stick it into the cup of water, it would immediately sink to the bottom of the cup even though it's much smaller than this larger ice cube.

So why does that happen? Well, what we have to talk about to understand how the ice cube is working is something called density. Density is a measure of the mass of an object per unit volume. This is going to give us a sense of how much mass - or how much matter - is occupying the given volume. In terms of most solids, the solid is going to have more density than the liquid, in this case, water. The thing with more density is going to sink; it's going to go toward the bottom. The things with less density are going to float or move up relative to the water.

Properties of Ice and Liquid Water

What's different about ice is hydrogen bonds. Water can form hydrogen bonds, and in a liquid state, these bonds are constantly forming and breaking. Now, you're going to have multiple hydrogen bonds forming between different molecules and because they're constantly forming and breaking, that's giving water its more fluid nature. As the temperature of the liquid water lowers, it's going to start to form ice.

What's going to happen is the water molecules are going to become more ordered because every hydrogen bond that can form is going to form. Because these hydrogen bonds are all forming at the same time, the water molecules are becoming more ordered; they're forming a crystalline structure.

Illustration of a crystalline structure
Crystalline Structure

Because of this linkage, the water molecules are in this rigid structure, and this rigid structure is holding them farther apart than they would be if they had been in the liquid state. Because they are being held farther apart, they're occupying more volume, and because they're occupying more volume - if we remember our density formula, we said density was equal to mass divided by volume - if I've increase the volume but kept the mass the same (because we've increased this volume down here) the overall density has decreased because the denominator has increased but the numerator has stayed the same. So because this density is lower, ice is going to float on water.

States of Matter and Specific Heat

Another question I brought up at the beginning of the lesson was why coastal cities are more temperate than inland cities in general. To answer this question, we have to think about another aspect of water.

It takes a certain amount of energy to change the physical states of matter. So for instance here, we can talk about ice. We know it takes a certain amount of energy to melt that ice and turn it into water. By the same token, it takes a certain amount of energy, again, that you have to add to the water to be able to boil it, to turn it into steam. Now this is true about all substances, but what's different, again, about water - we also have to remember that water is linked together by hydrogen bonds. So we need to input energy into the system to break these hydrogen bonds in addition to the energy needed to change the state of matter.

We talk about the energy that's needed to raise the temperature of one gram of a substance one degree Celsius: the specific heat. We then can say that water has a high specific heat.

The Effect of Water on Climate Temperature

This is the reason why bodies of water can help cool the surrounding area. So if I have my nice body of water here and I have my surrounding land, it's going to take extra energy to heat this air over here because energy is also being sucked into this water to heat this water that's close to this land. So this area that's close to the land is going to experience a more temperate climate because of this water that's insulating it from hot air, whereas somewhere farther inland, that insulation isn't happening.

Why We Sweat

The last question that I posed was how those athletic shirts help keep you dry. To answer this question, let's first take a minute to think about why our bodies even sweat. Based on what we've discussed so far about water, how do you think sweating benefits humans? Take a minute to think about that for a second.

We've already talked about how water has a high specific heat and we've also talked about how that impacts the amount of energy it takes to change water from a liquid to a gas. So if we have a sweat droplet sitting on top of our skin, it's going to take a lot of energy from the surrounding tissue to be able to evaporate that sweat. By removing this energy from the tissue, it has cooled you down. This is a reason why our bodies sweat: to cool down while we're exercising.

Cohesion, Adhesion and Capillary Action

Now that we know why we're sweating, let's see how these brilliant shirt scientists designed these shirts to help us out.

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