Hey there, this is Eric Simon and welcome to another MP3 Tutor session. Today’s topic is water—a substance that is often taken for granted but has remarkable properties that are essential for life. Water and life are so tied together that biologists find it almost impossible to imagine life without water. Unlike other planets in our solar system, the surface of the Earth is covered with liquid water. In fact, the presence of water is one of the main reasons that Earth is habitable. As you listen to this MP3, NASA has two robot explorers on Mars. Their mission is to search for evidence of liquid water on that planet. Such evidence would be essential to establish the possibility that life might have once existed on Mars. By the way, both rovers did find evidence of liquid water on the surface of ancient Mars, but evidence of life has not yet been found.
What is it about water that makes it so essential for life? As with so many other things in biology, the function of water is a consequence of its structure. Taking a minute to understand the structure of water will make it much easier for you to understand why water acts in so many special ways.
Probably the first chemical formula you learned was H2O, the formula for water. H2O is a shorthand method of representing that each water molecule is composed of two atoms of hydrogen and one atom of oxygen. One molecule of water has two covalent bonds that join each hydrogen atom to the oxygen atom. Remember that a covalent bond consists of a pair of shared electrons. An oxygen atom has a greater affinity or attraction for electrons than a hydrogen atom does. Therefore, when a hydrogen atom and an oxygen atom share a pair of electrons in a chemical bond, the electrons spend more time around the oxygen nucleus than they do around the hydrogen nucleus. This seems like such a trivial point but it has big consequences. The unequal sharing of electrons creates two separate areas of electrical charge. Around the oxygen atom the molecule is partially negatively charged with more electrons, more of the time. Around the hydrogen atoms, the molecule is partially positively charged because the electrons spend less time there. This type of bond is called a polar covalent bond and water is a polar molecule—a molecule with two “poles”—one slightly negative and the other slightly positive. In nature, opposite charges attract. So the positive pole of one water molecule is attracted to the negative pole of another water. In liquid or solid water, the water molecules arrange themselves to take advantage of this attraction between opposite charges. This attraction has a name: hydrogen bonds. Hydrogen bonds tend to make water sticky, sticking to each other and to other molecules. It is this “stickiness” that underlies the versatile properties of water.
Water sticks to all sorts of things, making them wet—cotton jeans, soil, your hair. When water sticks to some other substance, like cotton, it is called adhesion. Water also sticks to itself, as evidenced by a drop of water dripping from a faucet or falling from a cloud. This property also has a name: cohesion. Cohesion is the tendency of water molecules to stick to each other.
Now that we understand the basic structure of water, let’s talk about four special properties of water. They are:
- its ability to moderate temperature,
- the fact that ice floats,
- its ability to dissolve substances, and
- surface tension.
We’ll relate each of these properties back to the sticky structure of water that we just described.
The first property of water that we are going to discuss is its ability to moderate temperature. Unlike most common substances, water can absorb a lot of energy with only small changes in temperature. Here’s a mind experiment that makes this point: In your imagination, place a pan of water on the stove and turn the burner to high. Wait 30 seconds. Which would more likely burn you at this point: the pan made from steel or the water inside it? You can probably guess that the pan reaches burning temperatures much faster than the water does. This demonstrates that water resists changes in temperature more than steel does.
Water’s ability to absorb heat is the result of the sticky nature of water. Before the temperature can increase in water some energy is absorbed to overcome the stickiness—to separate water molecules from each other. Energy is therefore put into breaking the hydrogen bonds between waters rather than raising the temperature.
A consequence of this property is that water can reduce temperature swings for the planet as a whole and also in specific environments near large bodies of water. Oceans have remarkable abilities to keep the air temperature around them from changing a lot. For example, in Seattle or Boston, which are both along seacoasts, the temperatures rarely get above 100 degrees Fahrenheit or below zero. On the other hand, cities in the U.S. heartland, such as Minneapolis or Omaha, experience these temperatures on a yearly basis. Without the huge oceans, Earth’s temperatures would not be compatible with life, as we know it.
The second property of water that we’ll discuss today is the fact that solid water is less dense than liquid water. In other words, ice floats. Do you realize how unique this is? For almost all known substances, the solid form is denser than the liquid form. If you were to make frozen gasoline cubes, for example, the solid gasoline would sink to the bottom of a glass of gasoline.
Solid water, or ice, is different because water molecules at freezing temperatures align with each other in a precise arrangement to take advantage of the maximum possible hydrogen bonds. Liquid water, which is not so precisely arranged, is denser than ice. This is almost impossible to describe without a picture so be sure to check out the illustrations in your book and on the website.
Why is the fact that ice floats so important to life? Imagine what would happen if ice were more dense than liquid water. During the winter, a layer of ice would form on the top of a lake where it is exposed to the cold air. This ice would sink to the bottom, and more ice would form on top, which would also sink, and so on. Soon enough, the entire lake would be frozen solid from top to bottom and no life could survive the winter. Instead, because ice floats, a thin layer of ice insulates the rest of a lake, allowing aquatic life to survive the winter. Think about that the next time you are clinking ice cubes in your soft drink.
The image of your soft drink brings us to water’s third property. Consider what your soft drink is: it’s a solution of sugars or sugar substitutes, flavoring, and carbonation. This makes the point that water is a very versatile solvent; that is, water can dissolve many other substances. In fact, water is sometimes called the “universal solvent.” Your cells are essentially small packets of water that contain a complex solution of molecules dissolved within the water. Substances that dissolve in water or are attracted by water are called hydrophilic. And, guess what? This property is also the result of water “sticking” to hydrophilic substances. Of course, not all substances dissolve in water. Two substances you are familiar with that do not dissolve in water are waxes and oils. Water beads up on waxy surfaces and the oil in salad dressing rises above the watery vinegar. Substances that do not dissolve in water or repel water are called hydrophobic. Both hydrophilic and hydrophobic substances can be found in living systems.
The fourth property of water that I’ll talk about today is surface tension. You’ve probably noticed that liquid water seems to have a skin on top. If you are careful, you can suspend a paper clip on top of this skin. This skin is a result of surface tension created when individual water molecules stick to each other. The stickiness creates water droplets or a layer at the surface of a pool or pond of water. This forms a boundary layer that is stout enough to serve as a surface habitat for small insects, snails, and other small organisms.
Simple enough, right? Let’s recap the topic of water:
- Water molecules form sticky hydrogen bonds due to their polar covalent bonds.
- The stickiness of water molecules to each other and to other substances is responsible for properties such as temperature moderation, floating ice, dissolving substances, and surface tension.
I hope that this MP3 Tutor has helped you to understand the properties of water. Next time you drink a glass of water show a little appreciation for such a versatile and unique substance. And, good luck with the rest of your studying!
