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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.
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.
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.
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.
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.
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.
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.
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.
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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There is another phenomenon that we can talk about that you're probably familiar with called capillary action. Capillary action is that thing that you see when you have a little bit of water in the bottom of your cup.
The water in the straw has somehow risen above the level of this water; it's somehow defying gravity. It might not come as a surprise to you based on what we've already been talking about in this lesson, but hydrogen bonding is responsible.
We talked about hydrogen bonding linking the molecules together and giving the molecules more strength. The forces that are holding the water molecules together are called cohesion. So water molecules combine with water molecules, but water molecules are polar and they can also bind to other polar things. The water molecules can also bind to plastic. If I have the plastic of the straw, I can have hydrogen bonds forming between the water molecules and the plastic of the straw. These partial charges are the things that are interacting between the water molecules are the plastic. This is called adhesion.
Between cohesion and adhesion, that provides enough force to drive some of the water up the straw against the forces of gravity. By the same token, what our shirt scientists have done is they've created fabric which is going to have tiny little crevasses in it. These crevasses are going to cause capillary action on our water molecules, and it's going to basically suck the water molecules off our skin into these little crevasses, and then allow it to evaporate off the top of the shirt. Pretty ingenious, huh?
These are just a few of the really interesting properties of water that hydrogen bonds confer upon water.
Hydrogen bonds, as you can see, can explain a lot of the special properties that make water a really important part of life on earth. The ordered, unbroken hydrogen bonds in ice cause water molecules to be farther apart than they would be in the liquid state. This resulting lowered density of ice relative to water explains why it floats.
Specific heat is the amount of energy required to raise the temperature of one gram of substance one degree Celsius. Because it takes extra energy to break hydrogen bonds between water molecules, water has a high specific heat.
The polar nature of water molecules causes them to stick together. This is known as cohesion. Similarly, water molecules can also form hydrogen bonds with other polar molecules, and this is known as adhesion. Together, cohesion and adhesion explain capillary action, which is the ability of water to rise against the forces of gravity in a small tube.
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