Station 1 Surface Tension & Adhesion

Transcription

1 Station 1 Surface Tension & Adhesion Water has a simple molecular structure: H2O. Each molecule of water is made up of two atoms of hydrogen connected to one atom of oxygen. The way that these atoms are made, and the way that they join into a molecule of water, make the water molecule into a miniature magnet. Water is thus called "polar." Have you ever noticed that magnets like to stick together? So do water molecules. This is called cohesion. These little magnets also like to stick together when they are on a liquid's surface, which is called surface tension, and is the reason that a raindrop tends to be round. This also explains why water beads up when poured on a smooth surface like glass. Atoms are most stable when they have a particular configuration of their outer shells, a concept which will be discussed in chemistry. These configurations explain why hydrogen in water takes on a partial positive charge and why oxygen takes on a partial negative charge. These partial charges cause water molecules to stick to each other like magnets. The stickiness (known as cohesion) in this particular case is due to hydrogen bonding. In this case, hydrogen bonding involves the attraction between the positively charged hydrogen atom of one water molecule and the negatively charged oxygen atom of another water molecule. (As no electrons are actually shared however, hydrogen bonds are much weaker than covalent bonds - they easily break and easily form again). Hydrogen bonds hold water molecules together so tightly that the water s surface acts like a membrane. A water strider, like the one pictured on the left, can walk on the water surface without breaking through.

2 Station 2 The Climbing Property of Water Ever wonder how a tree as tall as a redwood can get water all the way from its roots to its top leaves? Water is pretty heavy, yet the redwood tree moves thousands of gallons of water (that's 8,000 pounds, or 4 tons) up into its canopy every day, and it does it without doing any work. That's pretty amazing! Here is how it works. Water molecules love some other molecules and hate other molecules. Have you ever noticed that magnets like to stick to other metals? So do water molecules. When water molecules stick to other molecules that are also little magnets, it is called adhesion. This explains why it is easy to clean up spilled water with a paper towel: the water molecule's little magnets like to stick to the cellulose molecules of the paper, which are also like little magnets. Water molecules will stick to any other molecules that are like little magnets (polar), but do not like to get involved with any molecules that hate little magnets (nonpolar), like oil. Oil and water don't mix, right? That is why you have to shake the salad dressing real hard before you pour it: the oil molecules hate the water little magnets. Paper towels are made out of trees, and trees are made out of cellulose. The leaves make molecules of sugar out of sunlight, water, and carbon dioxide, then combine the sugars into huge sugar chains. These sugar chain molecules are called cellulose. This name comes from the fact that the plant material is made of cells, and the "-ose" word ending means "sugar". Cellulose is also a great magnet (polar molecule), so water sticks to cellulose just like a magnet to the refrigerator door! Water Molecules Now that we know a bit about water molecules, let's look at how water acts in a little tube. Water loves to rise in a tube bonded to paper towel

3 Meniscus shown in blue water molecules want to stick together and stick to side Have you ever noticed that water in a glass tends to hug the sides and even stick up above the water's surface? The edge of the water that sticks up above the water's surface is called a meniscus. When you put a small tube into water, the water likes to stick to each side, with a meniscus on each side. If the tube is so skinny that the meniscus on one side can touch the meniscus on the other side, the water will rise up the tube (each meniscus wants to go up the side, and they chase each other). This is called capillary action. The redwood tree's trunk is made up of millions of little bitty tubes (xylem), and these tubes are made of cellulose. The water molecules like to stick together and like to stick to the walls of the tubes of cellulose, so they rise up the tubes by capillary action. All the way to the top! The water pressure decreases as it rises up the tree. This is because the capillary action is fighting the weight of the water. Although the xylem tube is very thin, and therefore the weight of the water is very low, it is not zero. Eventually, the effects of gravity on the water starts to equal the effects of capillary action. Scientists have found that the pressure inside the xylem decreases with the height of the tree, and similarly, the size of the redwood leaves decreases with the decrease in pressure. (See an excellent article in the San Francisco Chronicle, the source of the illustration to the right.) Scientists Dr. George W. Koch of Northern Arizona University, Dr. Gregory M. Jennings of Humboldt State in Arcata, California, and Dr. Stephen D. Davis of Pepperdine University in Malibu, California, have studied the water pressure inside a coast redwood. They studied the correlations among the tree's height, its internal water pressure, leaf size, photosynthesis and other factors. Dr. Koch and his colleagues have concluded that no existing species of tree can grow higher than 130 meters (427 feet). Redwood Tree and Leave Size The leaves are smaller at the top of the tree (graphic modified from the SF Chronicle, see reference in text to left)

4 Station 3 Polarity of Water Oil is a hydrophobic or water hating molecule, so called because its chemical structure does not allow the formation of hydrogen bonds. Therefore, oil does NOT dissolve in water. When mixed, the two substances form separate layers, and because oil is less dense, it sits on top of water. This characteristic behavior of water and oil is crucial, among other things, in the structure of the cell membrane. Let s look at the cell membrane and see how that membrane keeps all of the pieces inside. When you think about a membrane, imagine it is like a big plastic bag with some tiny holes. That bag holds all of the cell pieces and fluids inside the cell and keeps any nasty things outside the cell. The holes are there to let some things move in and out of the cell. The cell membrane is NOT one solid piece. Compounds called proteins and phospholipids make up most of the cell membrane. The phospholipids make the basic bag. The phospholipids are in a shape like a head and a tail. The heads like water (hydrophilic) and the tails do not like water (hydrophobic). The tails bump up against each other and the heads are out facing the watery area surrounding the cell. The two layers of cells are called the bilayer.

5 Station 4 Water as Universal Solvent A solvent is a substance that dissolves, or breaks apart, another substance (known as a solute). Whether a substance will dissolve in a solvent depends upon its polarity. Polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes. Water is not only a good solvent, it is the best. Because of its high polarity, water is called the universal solvent. It dissolves more different substances than any other solvent known. This is because so many other molecules are ionic or polar, and their electrical charges make them attracted to the water molecules, causing them to stay in solution. Thus we find that water dissolves many kinds of salts and sugars, many proteins, and a variety of hormones that dissolve in our blood (since blood is mostly water) and regulate various life processes. Even nonpolar molecules dissolve in water to some extent if they are small. Thus enough oxygen dissolves to allow fish and other aquatic animals to survive, and enough carbon dioxide dissolves to enable algae and many plants to live underwater.

6 Station 5 Water s High Specific Heat and Heat of Vaporization Water is also unusual in being able to absorb a lot of heat energy without having its temperature increase by very much. It is said to have high specific heat. An amount of heat that will raise the temperature of a container of water by 10 degrees will raise the temperature of an equal weight of alcohol by 20 degrees and an equal weight of iron by 94 degrees. Water molecules are held together so strongly by their hydrogen bonds that an amount of heat that will get other molecules moving much faster will not speed up water molecules much at all. This property of water helps to reduce temperature fluctuations in the animal of plant body (homeostasis), and it also makes for mild climates in the vicinity of large bodies of water. Heat of vaporization is the amount of heat energy required to evaporate a given weight of a liquid. Water has a very high heat of vaporization, which means that it takes a lot of heat to evaporate just a little water. This keeps water in many more lakes and ponds during the summer than would be the case if water had a lower heat of vaporization. Heat of fusion is the heat energy that must be removed from a given weight of water in order to freeze it. Water s relatively high heat of fusion means that it takes much longer for lakes and streams to freeze in the winter, allowing living things more time to adjust to the change.