So, in the last article, things probably got a little dense as we continued to journey through the wonderful world of chemical bonds and how that pertains to what makes water such a big deal. Sorry about that, but it had to be done. Chemical bonds and the making and breaking of them are just too important to all of the sciences not to take the time to get familiar with them. We’ve touched on bonds before, and we will come back to them again and again as we go forward, too.
At the end of the last article, I left you with a container of water, where the water molecules are not randomly arranged, because of the “polar” nature of the molecule, where the oxygen side of the molecule has a little bit of a negative charge and the side of the molecule where the hydrogen atoms are is a little bit positive. It might not seem like that big a deal, but this characteristic of water is pretty much the secret to life. I guess that makes it a pretty big deal, after all.
The attractions between water molecules are called “hydrogen bonds”. They aren’t real bonds—they are more like magnetic attractions based on the opposite charges on the “tops” and “bottoms” of adjacent water molecules. An individual hydrogen bond is very weak, but if there are millions or billions of them, like there are between all the molecules in a cup of water, they can, collectively, be very strong. The collective strength of hydrogen bonds in water is best seen when you pour water into a glass up to the very brim. You can keep pouring water into the glass until it forms a little “hump” at the brim. Pour in just a little more after that, and it will run down the side, but before that, it will stand up above the brim just a little bit. The ability of water to do that is due to a property called “surface tension”. It’s also why drops of water on a table are little hemispheres instead of just flat, round dots and why water bugs can walk on the surface of a pond. Surface tension results from the attraction of the water molecules for each other, which is called “cohesion”. In the case of the water in the glass, the attraction of the water molecules trying to stay together is actually stronger than the pull of gravity trying to pull the water down the side of the glass. Once you put in a little too much, gravity overcomes the cohesion among the molecules, due to hydrogen bonding, and the water will spill. For the water bug, the surface tension of the water is greater than the force the bug’s legs put on the water, so it stands on top of the water—it isn’t floating, it’s literally walking on top of the water. If you look at a picture of a water bug on the water, you can see little dimples in the surface of the water where each of the bug’s legs touch. That’s because the water molecules are holding together with more force than the bug’s foot is exerting on it.
Source: Creative Commons
Another very important effect of this property of water has to do with how plants can grow tall like they do. We all know plants need water. I’ll talk about why they need water in another article when we get to photosynthesis, which is how plants turn water, carbon dioxide and sunlight into energy and growth. Anyway, plants need water. Most plants absorb water through their roots, as we all know. Most of the water, however, is actually used in the leaves (for photosynthesis). How does the water get from the roots, which may be many feet underground, up to the leaves, which may be 100 or more feet above the ground? A hint: it’s not pumps or suction, and they don’t absorb water (at least very little) through the leaves.
Water gets up and out to the leaves because of hydrogen bonding. You may be familiar with a process called “capillary action”. Most likely, you’ve seen it when someone is taking a blood sample and they take that tiny glass tube and touch it to a drop of blood. The blood rises in the tube. You also might see something similar if you ever pour water into a vessel with a narrow neck or suck liquid into a glass syringe. If you look at the very top of the liquid in the neck of the vessel or in the syringe, it’s not flat. It actually looks like the liquid is sort of climbing up the side of the tube at the edges, causing a little dimple at the top of the liquid, where the liquid in the middle is lower than at the edges. Both of these effects are caused by hydrogen bonding, where the water molecules near the edge of the tube are attracted to the glass, which is called “adhesion” (it’s usually glass or something similar. These effects don’t happen in plastic tubes because the water molecules aren’t attracted to the most plastics like they are to glass). Cohesion is when something sticks to itself (like water molecules causing surface tension). Adhesion is when something sticks to something else (like the water molecules being attracted to the glass sides of a capillary tube).
Source: Dr. Keith Haywood, USGS
Capillary action is how water gets from the roots of a plant up to the leaves. Trees, ferns, grasses, flowers and shrubs are all what are called “vascular plants”. Only plants like mosses and lichens are not vascular, and you might notice that mosses and such lie pretty much flat on whatever they are growing on. Vascular plants all share the characteristics of having stems (or trunks) that contains many tiny little tubes. You can easily see them in the “veins” you see in leaves. Water can travel all the way from the roots to the leaves at the top of the tallest tree, due to capillary action, moving slowly through the tubes. Until plants evolved to be vascular, they never got very tall, because water couldn’t get out to the leaves (or needles or fronds or whatever, where the photosynthesis takes place).
The hydrogen bonding between water molecules also make water relatively hard to heat up or cool off. This is called a “high heat capacity”. What we call “heat” is really just energy. When you “heat” something, you are just adding energy to it. Adding energy causes the molecules of whatever you are heating to vibrate faster. As the molecules vibrate more quickly, they also tend to move a little further apart, which is why most things expand when they get hot, like a bridge deck in the sun. That is why bridges have expansion joints—so they can get longer when they heat up and get shorter (contract) when they get colder. As things cool off, their molecules vibrate more slowly and mover closer together. Anyway, because of hydrogen bonding between water molecules, it takes a lot of energy to make the molecules vibrate faster, so it is hard to heat water up. Likewise, once it gets hot, water tends to cool relatively slowly. This is really important for lots of reasons. One is that it makes the climate of Earth livable. If you have ever spent time in a desert, you understand why that is so. The temperature in a desert can often range over 70 or 80 degrees in the course of a normal day, going from 100 degrees or more in the daytime to 20 or 30 degrees at night, and the temperature swings from one extreme to the other in about an hour or two after sunrise and after sunset. The reason is that there is very little moisture in the desert, either in the air (very low humidity) or in the ground. Because there is little moisture in the air, radiant energy from the sun penetrates the atmosphere very easily, heating the land. The land, because it, too, contains very little moisture, heats up quickly. When the sun goes down, the land (soil, sand, rocks, etc.) cools off very quickly and it gets cold. If there is a lot of water around, however, the climate tends to be much more moderate. This is one big reason why people like to live in places like Hawaii. Water heats up much more slowly than the land when the sun rises. Since the water is absorbing (slowly) so much energy from the sun, the land heats up more slowly. Also, since as we all know, hot air rises while cool air sinks, the air above the land rises as it warms, and this causes a breeze to blow from the cooler water in toward the land in the mornings. In the evenings, you get the opposite—after the sun goes down, the water holds heat, keeping things warmer as the land cools. You also get a breeze blowing from the land to the water, making for those wonderful evenings. Those are local phenomena—they are felt near the coast and so on. The presence of so much water on Earth also moderates the climate everywhere. If we didn’t have our oceans, our temperature range would be much like that of the Moon. When the sun hits the waterless Moon, it heats up to about 250 degrees. When the sun sets on the Moon, the temperature drops to about -250 degrees. It is hard to imagine any living creature that could survive a temperature range of 500 degrees. So, the high heat capacity of water makes the temperature range on most of the Earth suitable for life.
Related to the high heat capacity of water is the high “heat of vaporization” of water. This means that water is a liquid over a really broad temperature range, from 32 to 212 degrees Fahrenheit (0 to 100 degrees in the much more sensible metric measurement system). Because of this broad “liquid phase” between freezing and boiling, water exists as a liquid over much of the Earth’s surface most of the time. We have all heard about water being necessary for life and all that, but it is actually LIQUID water that is necessary. Water ice is found all over the solar system—on Mars, comets, some of the moons of Jupiter, for example– but liquid water is much harder to find, and none of it is on the surface. The Earth is in what is called the “Goldilocks Zone”, meaning that our temperature is just right for liquid water. Any closer to the sun and it would be too hot and our water would have evaporated away long ago. Any further from the sun and our water would all be ice. The fact that we have liquid water just laying around on the surface of our planet, in oceans, lakes and so on is very rare, indeed, and one of the reasons that life has developed on Earth. Water just falls from the sky on this planet. It’s easy to get tired of rain or even to resent it, in cases of floods that seem to be happening somewhere in the US all the time, or when it falls at the wrong time and ruins a crop or rains out a wedding, but you should really consider how absolutely amazing it is that the most valuable thing in the universe, in terms of its importance for life, just falls from our sky.
It is so easy to see the wonder of nature, if you just look and appreciate the many things that happen around you, every day. A little understanding of what is going on is great for perspective! We will finish up with our exploration of the properties of water in the next article.